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SATELLITE WATER MONITORING AND FLOW FORECASTING SYSTEM FOR THE YELLOW RIVER BASIN Sino-Dutch Cooperation Project ORET 02/09-CN00069 Scientific Final Report December 2008

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  • SATELLITE WATER MONITORING AND FLOW FORECASTING SYSTEM

    FOR THE YELLOW RIVER BASIN

    Sino-Dutch Cooperation Project ORET 02/09-CN00069 Scientific Final Report

    December 2008

  • Title page

    3

    SATELLITE WATER MONITORING AND FLOW FORECASTING SYSTEM

    FOR THE YELLOW RIVER BASIN

    Sino-Dutch Cooperation Project ORET 02/09-CN00069 Scientific Final Report

    December 2008

    Authors

    Andries Rosema, Marjolein de Weirdt and Steven Foppes EARS, Kanaalweg 1, 2628 EB Delft, Netherlands. Email: [email protected]

    Raymond Venneker and Shreedar Maskey UNESCO-IHE Institute for Water Education, Westvest 7, 2611 AX, Delft, Netherlands.

    Hongqi Shang, Songchang Ren, Feng Sun, Yangbo Sun, Falu Zheng, Yunpeng Xue, Zhongqun Yuan and Hui Pang. Bureau of Science, Technology and Foreign Affairs, Yellow River Conservancy Commission, Ministry of Water Resources, PR. China, No.11, Jinshui Road, Zhengzhou 450003, China

    Bastiaan Bink and Xiaobo Wu Hofung Limited, The Hague, Netherlands and Beijing, China. Email: [email protected]

    Yuanze Gu, Weimin Zhao, Chunqing Wang, Xiaowei Liu, Suqiu Rao, Dong Dai, Yong Zhang, Liye Wen, Dongling Chen, Yanyan Di, Shuhui Qiu, Qingzhai Wang, Liuzhu Zhang, Jifeng Liu, Longqing Liu, Li Xie, Ronggang Zhang, Jian Yang, Yawei Zhang, Meng Luo, Bo Hou, Lai Zhao, Lihua Zhu, Xiaodong Chen and Tequn Yang. Hydrology Bureau, Yellow River Conservancy Commission, Ministry of Water Resources, P.R. China, No.12 Chengbe East Road, Zhengzhou 450004, China. Chengyang Lu, Gensheng Liu, Xijun Guo and Deyan Du. Upper Hydrology Bureau of YRCC, No. 157 Wudu Road, Lanzhou 730030, China.

    Xiaoying He, Xinwu Tu and Wenjuan Sun. Sanmenxia Hydrology Bureau of YRCC, No.7 Hepingxiduan, Sanmenxia 472000, China

    Scientific final report of the project Establishment of a Satellite Based Water Monitoring and Flow Forecasting System in the Yellow River Basin, commissioned by the Yellow River Conservancy Commission to a consortium consisting of EARS Earth Environment Monitoring BV, UNESCO-IHE Institute for Water Education and Hofung Ltd. The project was co-funded by the Yellow River Conservancy Commission and the Government of the Kingdom of the Netherlands through Grant Agreement CN200400105 related to ORET project 02/09 – CN00069. This report may be referred to as: Rosema, A; De Weirdt, M; Foppes, S; Venneker, R; Maskey, S; Gu, Y; Zhao, W; Wang, C; Liu, X; Rao, S; Dai, D; Zhang, Y; Wen, L; Chen, D; Di, Y; Qiu, S; Wang, Q; Zhang, L; Liu, J; Liu, L; Xie, L; Zhang, R; Yang, J; Zhang, Y; Luo, M; Hou, B; Zhao, L; Zhu, L; Chen, X; Yang, T; Shang, H; Ren, S; Sun, F; Sun, Y; Zheng, F; Xue, Y; Yuan, Z; Pang, H; Lu, C; Liu, G; Guo,X; Du, D; He, X; Tu , X; Sun, W; Bink, B; Wu, X. (2008) “Satellite Monitoring and Flow Forecasting System for the Yellow River Basin”, Scientific final report of ORET project 02/09-CN00069, EARS, Delft, the Netherlands, 144 pg, December 2008. Cover: Yellow River at Tangke

  • Satellite Water Monitoring and Flow Forecasting System for the Yellow River

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    Acknowledgement This project has been approved and supported by Chinese Ministry of Water Resources, Chinese Ministry of Finance and the Yellow River Conservancy Commission. We especially thank Yongfu Zheng of the Ministry of Finance, Jianming Liu, Zhiguang Liu, Xingjun Yu, Ge Li, Hai Jin, Qingping Zhu, Mengzhuo Guo and Yubo Shi of the Ministry of Water Resources, Zijiang Huang, Xiaoyan Liu, Yuguo Niu, Hanxia Yang, Hongyue Zhang, Shuili Tian, Shiqing Huo and Long Wang of the Yellow River Organisation. We are grateful for their interest and support during project initiation, development and implementation. The authors also wish to thank the many people from the hydrology stations at Jingchuan, Tangneihai, Jungong Jun and Tangke for their work on the establishment and maintenance of the Large Aperture Scintillometer Systems, as well as for reading and forwarding the measuring data. We thank Wouter Meijninger of Kipp & Zonen in Delft for his support in questions related to the LAS data processing. We are also grateful to Rongzhang Wu of the China National Satellite Meteorological Centre, Yun Bai of the Shinetek Company, Guang Zhu and Qian Qian of CITC CMC International Tendering Corporation for their support in different phases of the project. We thank Dave van den Nieuwenhof, Huub Lavooij, Albert de Haas and Liqun Li of Royal Dutch Embassy in Beijing for their interest and support. The project would not have been possible without the grant received from the Government of the Netherlands, through the ORET organization. We are grateful for the fair and proper settlement of all administrative, financial and contractual matters related to this project.

  • Contents

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    CONTENTS Section Title Page 1 INTRODUCTION 7 1.1 China’s water resources problems 7 1.2 Flooding 8 1.3 Water shortages 8 1.4 Need for basin wide water resources monitoring and management 9 1.5 Sino-Dutch water monitoring and flow forecasting project 10 1.6 Project objectives 10 1.7 Project deliverables 11 1.8 Project approach 11 1.8.1 Development phase 12 1.8.2 Implementation and testing phase 13 1.8.3 Demonstration phase 14 1.9 Project impact 15 1.10 References 16 2 THE YELLOW RIIVER TARGET AREAS 17 2.1 The source area of the Yellow River 17 2.1.1 Hydrological observations in the source area 18 2.1.2 Information acquisition and transmission 22 2.1.3 Hydrological forecasting 22 2.2 The lower Weihe River 23 2.2.1 Hydrological observations 24 2.2.2 Information acquisition and transmission 25 2.2.3 Flood forecasting 25 3 ENERGY AND WATER BALANCE MONITORING SYSTEM 29 3.1 System components 31 3.1.1 Pre-processing 31 3.1.2 Precipitation mapping 32 3.1.3 Energy balance monitoring 33 3.1.4 Snow and snowmelt 42 3.1.5 Drought monitoring 43 3.2 LAS measurements 45 3.2.1 LAS theory 45 3.2.2 LAS equipment and installation 47 3.2.3 LAS measuring sites 46 3.2.4. Data collection 46 3.2.5 Data processing 47 3.2.6 LAS results 48 3.3 EWBMS software system 52 3.3.1 Satellite data reception and pre-processing 53 3.3.2 Rain gauge data reception and pre-processing 54 3.3.3 Precipitation module 55 3.3.4 Energy balance module 55 3.3.5 Freeze/Thaw module 58 3.3.6 Drought monitoring system 58 3.3.7 Processing information data base 62 3.3.8 Imageshow-2 analysis tool 62 3.4 Catchment drought monitoring system 64 3.4.1 Climatic drought 64 3.4.2 Hydrological drought 65 3.4.3 Agricultural drought 67 3.5 Validation of EWBMS products 69

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    3.5.1 Validation of precipitation 69 3.5.2 Validation of air temperature 74 3.5.3 Validation of net radiation 77 3.5.4 Validation of sensible heat flux 79 3.5.5. Validation of catchment water budget 81 3.6 References 85 4 LARGE SCALE HYDROLOGICAL MODEL 87 4.1 Technical reference 87 4.1.1 Land component transport 88 4.1.2 River routing 90 4.1.3 Land-river coupling 91 4.1.4 Forecasting of river flows 92 4.2 System implementation 93 4.2.1 Software components 93 4.2.2 User interface 94 4.3 Upper Yellow River Water Resources Forecasting System 96 4.3.1 Description of the data requirements 96 4.3.2 Description of the terrain data 96 4.4 Weihe basin High Water Forecasting System 98 4.4.1 Description of the data requirements 98 4.4.2 Description of the terrain data 98 4.5 Evaluation of the simulation results 99 4.5.1 Validation data 99 4.5.2 WFRS validation results 101 4.5.3 HWFS validation results 103 4.5.4 Discussion 105 4.6 References 108 5 SYSTEM IMPLEMENTATION AT YRCC 111 5.1 System set-up 111 5.1.1 Satellite receiving and processing system 111 5.1.2 Computer network 113 5.1.3 Data base 115 5.1.4 Organization and operation 117 5.1.5 LAS station, data collection and processing 118 5.2 Catchment monitoring bulletin 118 5.2.1 Reporting flood and drought information 118 5.2.2 Bulletin contents 119 5.3 Catchment monitoring website 119 5.3.1 Target users 119 5.3.2 Website design and structure 120 6 CONCLUSIONS, OUTLOOK AND RECOMMENDATIONS 123 ANNEX 1: LAS STATIONS INFORMATION 127 ANNEX 2: CATCHMENT MONITORING BULLETIN 131

  • Chapter 1 - Introduction

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    1 INTRODUCTION This document is the final report of the project “Satellite Based Water Monitoring and Flow Forecasting System in the Yellow River Basin”. This Sino-Dutch project was funded by the Chinese and Dutch Government. The Dutch funding contribution was provided through the ORET program, a program that supports export transactions that are relevant for social economic development and for the environment, but are not feasible in a commercial sense. After signature of the contract in November 2003 and the Grant agreement in May 2004, the project started in June 2004. The project was completed in the last month of 2008.

    1.1 China’s water resources problems.

    Water is one of the most important issues in relation to China's development. With a growing population and a booming economy the water demand is increasing steadily. At the same time water availability – especially in the north of the country - is limited and characterized by a highly uneven distribution geographically and seasonally. The Yellow river (Huanghe) is, after the Yangtze, the second largest river in China. The river basin is situated in the arid, semi-arid and sub-humid zones, which zones are characterized by relatively low but highly variable rainfall. The average annual run-of is about 58 billion m3. The lower reach of the Yellow river runs through a relatively narrow corridor towards the sea. By consequence the Yellow river basin has only a short coast line. While water availability in the Yellow river catchment is highly irregular, water demand is steadily increasing. China’s impressive economic growth and improving living standards - alongside a still increasing population - put pressure on water resources. Urban areas, industry, agriculture and nature are all competing for a share of the precious natural resource. Also some large cities, which are situated outside of the catchment (e.g. Tianjin), depend on water from the river. Due to water intake from the Yellow river for industry, agriculture and residential use, the flow tends to dry up in the lower reach during the summer period. However, in case of high precipitation the risk of flooding looms in the lower reach, where the riverbed runs elevated high above the land. Growth and development are held back or become ‘non sustainable’ in water shortage areas, resulting in damage to local economy and nature. This puts more and more pressure on decision making concerning the allocation and development of water resources. A detailed assessment of water resources in the Yellow river is currently carried out every 10 years. But in view of the climatic variability, more frequent assessments are highly desirable. Monitoring (measuring time series) is currently restricted to precipitation and river flow at a limited number of locations. These measurements suffer from a lack of overview. A new satellite based water resources monitoring and flow forecasting technology has been implemented to help addressing this problem. There are two neighbouring basins, that of the Hai and Huai river respectively. The Yellow (Huanghe), the Hai and Huai river basins are together referred to as the 3-H basins. 40% of the Chinese population lives here. The 3-H basins are the breadbaskets of China and produce 67% of its wheat, 44% of its corn and 72% of its millet. In addition they produce 65% of its peanuts, 64% of its sunflower and 42% of its cotton. At the same time these basins have only 10% of China's water resources.

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    Water resources issues in the 3-H basins are related. Excess water, if available, may be transported from the Yellow river to the densely populated areas along the coast in particularly the Hai River basin (Beijing, Tianjin). The whole region is regularly hit by disasters of both water scarcity and excess.

    1.2 Flooding

    The Yellow river carries a huge amount of sediments, originating from the Loess Plateau in the upper and middle reaches. 75 % of the sediments is deposited in the lower reaches and the river estuary. As a result the river floor is rising 5-10 centimetre per year, and the levees in the floodplain had to be rebuilt 4 times during the second half of the 20th century: in 1950, 1955, 1964 and 1977. Costs are approximately 2 Billion US$ each time. The river water level is now up to 10 meter above the surrounding land. The combination of this phenomenon, with the sometimes-high precipitation in the middle reach, creates a high risk of flooding. Since 600 BC, dike bursts occurred 1590 times and the river changed its course 26 times. Very serious floods occurred in the 1930's. A major flood in 1933 caused more than 50 dike bursts. 18000 people were killed and 3.6 million ha of farmland was damaged. In 1938 another flood inundated 27 counties and caused 3.4 million victims. According to the report Agenda for Water Sector Strategy for North China (Worldbank 2001) flood losses in the Yellow river basin increased from 1500 RMB/ha in the 1950's to 9000 RMB/ha in the 1990's. In 1998 losses were in the order of 1% of the GDP. In the Yellow river basin the provinces most affected were Henan and Shandong, followed by Shanxi and Shaanxi. The total flood prone area of the Yellow river is 118000 km2. 71 million people are living in this area. An estimation of losses in case of complete flooding of this area, based on the fore mentioned figure of 9000 RMB/ha, would then be 100 billion RMB or 13 billion US$. According to the document "An overview of Chinese water issues" (Unknown 1997) the losses due to flooding in China were 20 billion US$ in 1990 and 10 billion $ in 1996. Very severe flooding occurred in 1998. These floods killed 3500 people, damaged 7 million houses and submerged 250.000 km2 of farmland. Total damage amounted to 30 billion US$ nation wide. In these events 250,000 km2 of land was inundated.

    1.3 Water shortages Since the 1980's water shortages in the 3-H basins have been growing in magnitude and frequency of occurrence. This has created severe economic losses. Water demand in 2000 was 169 billion m3 and exceeds the total supply, which is 132 billion m3 per year (table 1.1). Shortages are expected to grow from 37 billion m3 to 56 billion m3 in 2050 if no measures are taken (Worldbank 2001). Current demands and shortages for these basins are presented in following table.

  • Chapter 1 - Introduction

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    Table 1.1: Water demand, supply and shortages in the 3-H basins (billion m3/yr) Huanghe Huai Hai Total Demand 47 72 50 169 Supply total 37 55 32 124 Shortage 10 17 18 37 • domestic + industry 2 4 4 5 • agriculture 8 13 14 32

    Roughly 80% of all water is used in agriculture, and within this sector the use for irrigation is far dominant. The water demand structure in the Yellow river basin is as follows. Table 1.2: Water demand in the Yellow river basin Category Share (%) Urban life 3 Urban industry 12 Rural life 2 Rural industry 2 Irrigation 76 Livestock 1 Fisheries / Pasture 3

    As a result of surface water shortages, there is an increasing reliance on groundwater. Groundwater extraction in the 3H basins is about 50 billion m3 per year and about 13 billion m3 in the Yellow river basin alone. Table 1.3: Utilization of ground water in the 3H basins (billion m3/yr) Huanghe Huai Hai Total Groundwater extraction 13 16 22 51

    In many areas ground water resources are over exploited and ground water tables are falling as much as several meter per year. As a result the cost of ground water extraction is growing. Other effects are: saline water intrusion in the coastal areas (now covering an area of 142 km2) and ground subsidence. The urban areas of Tianjin and Beijing suffer from subsidence, causing settlement of structures, bridge collapse, storm water drainage problems and reduction in flood protection. Ground subsidence has caused 1.4 billion RMB damages to structures during the 1990's The earlier mentioned Worldbank study also estimates the economic value of water for different sectors. In agriculture, predominantly irrigation, the value is 0.8-1.6 RMB/m2, or 1.2 RMB/m3 on average. For domestic and industrial use the value varies between 3 and 6 RMB/m3.

    1.4 Need for basin wide water resources monitoring and management Provinces in the upper reach of the Yellow river have been overusing the available water, which has lead to shortages and even drying up of the river in downstream areas. In 1997 the irrigation rate in the upper reach was 12000 m3/ha on average, while 3700 m3 was used in the middle reach and 4500 m3 in the lower reach of the river. According to Changming Liu (2000) average gross irrigation water use in the north-east of China was 8400 m3/ha. This is about 4 times the water required for a single crop. The problem of water shortage versus overuse has caused intense

  • Satellite Water Monitoring and Flow Forecasting System for the Yellow River

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    conflicts between political entities. Therefore a basin wide control of the limited water resources has become essential. China's Agenda for Water Sector Strategy for North China recommends that: the River Basin Councils are to be charged with, and given the necessary legislative support for: (a) Determining water resources allocations (surface and groundwater) for the provinces, (b) Developing policies and programs to promote sustainable water resources management, particularly with respect to flood control and drought relief, ground water management, water resources protection and pollution control, promotion of increased water use (especially irrigation) efficiency, and comprehensive basin development planning. The Yellow River Conservancy Commission is the first river commission that has been charged with such far reaching tasks. It is clear however, that proper decision making in relation to these tasks, requires a lot of information on river flow and water resources on the one side, and water needs on the other side. On the demand side water needs in agriculture is by far the largest and most important part. It is also the most difficult demand to assess, because of the variability of “natural” water supply by precipitation. In dry years, water needs for irrigation will be much larger than in more wet years.

    1.5 Sino-Dutch water monitoring and flow forecasting project The Sino-Dutch project “Satellite Based Water Monitoring and Flow Forecasting System in the Yellow River Basin” has developed and implemented an operational water balance monitoring and flow forecasting system for the Yellow river basin. The main components of this system are the “Energy and Water Balance Monitoring System” (EWBMS) developed by the Dutch remote sensing company EARS and the Large Scale Hydrological Model (LSHM) developed by UNESCO-IHE, both in Delft, Netherlands. Based on these technological components the following dedicated subsystems have been developed and implemented for the Yellow River basin: • Flow Forecasting system in the upper reach of the Yellow river • Flow and high water forecasting system for the Weihe tributary. • Drought monitoring system for the entire Yellow river basin. The system is to become a major tool in the hands of the Yellow River Conservancy Commission. It will help to carry out tasks with respect to (1) the management of water resources in the Yellow river basin, (2) flood forecasting and early warning, and (3) the monitoring and early warning of drought.

    1.6 Project objectives The satellite based water monitoring and flow forecasting project has been carried out with the following objectives: • To develop a system for energy and water balance monitoring. • To develop a system for drought monitoring and early warning. • To develop a prototype system for flow forecasting in the Upper Reach. • To develop a prototype system flow and flood forecasting in the Weihe. • To calibrate, test and improve these systems. • To implement these systems at the YRCC premises. • To train the partners in understanding and effectively using these systems. • To assist the YRCC in monitoring of water resources and drought.

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    1.7 Project deliverables In course of project the partners have provided the following deliverables to YRCC: • Two FY2c geostationary satellite receiving systems. • EARS Energy and Water Balance Monitoring System (EWBMS), providing daily

    data fields of: surface temperature, 1.5 m air temperature, global and net radiation, actual and potential evapotranspiration, rainfall, snow height and effective precipitation.

    • 4 Surface flux measuring systems consisting of a Large Aperture Scintillometer (LAS), a CNR1 net radiometer and a data logger.

    • Drought Monitoring and early warning System (DMS) for the entire basin. • Water Resources Forecasting System for the Upper Reach (WRFS). • Flood Forecasting System for the Weihe (HWFS). • EWBMS, DMS, WRFS and HWFS methodology description reports. • EWBMS, DMS, WRFS and HWFS user manuals. • A project final report (this document). • Half yearly project progress reports. • 17 man-years of technical assistance. • 30 man-months of training in understanding, use and application of the

    monitoring system and its technological components. • 36 man-months of research fellowships, to carry out joint research and

    development. 1.8 Project approach

    The project has been carried out in 3 phases. The 1st phase, the system development phase has been used to design the various systems, to resolve a number of technical and methodological questions, to develop the proto-types and to train the Chinese partners in understanding the backgrounds and potential of the technology. The first phase took about 2 years. The 2nd phase is for implementation and testing. The prototype monitoring systems were installed at the YRCC premises. Sites for the surface flux measuring systems have been selected and the LAS, radiation, temperature and wind sensors were installed; three on the Qinghai plateau in the upper reach, and one in the Loess plateau area. The flux data measured with these systems have been used to validate the satellite based systems and to optimize their performance. For validation also data from regular weather and flow measuring stations have been used. This phase has also taken at least two years with considerable time overlap with the 1st and 3rd phase. The 3rd phase is the demonstration phase. With a total duration of also 2 years, the system has been run by YRCC in a semi-operational way. Water resources, water level and drought information have been generated operationally. Monthly river flow bulletins have been developed and published. A website has been developed too inform a larger public. The activities carried out in the different phases are briefly described hereafter.

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    1.8.1 Development phase Satellite data receiving system

    At the beginning of the project two PC based receiving systems for the geostationary satellite data have been selected and were purchased from the Beijing based company Shinetek. One system has been implemented in Zhengzhou and the second one in Lanzhou. They are both receiving the Chinese geostationary meteorological satellite FengYun-2c, and serve as mutual back-up. FY2 is one of the most operational satellite systems. Its follow-up has already been launched and in case of unexpected failure can fast replace the current satellite. With the two receiving systems visual and thermal infrared images are received every hour. The data are stored on hard disk until being further processing once every day.

    EWBMS adaptation The EWBMS has been adapted to the needs of YRCC. User requirements have been discussed early in the project phase on the basis of a report describing the current methodology. On the basis of this report discussions were held and modifications were agreed on in the interest of the present application of the EWBMS system. The most important user requirements that were agreed for implementation are the following two: 1) Generation of all basic data fields on a daily basis in stead of 10 daily. 10 daily data products are also generated, but they are shifting averages, i.e. each day a 10 daily average is produced on the basis of the last 10 days. 2) Extension of the EWBMS software so as to take care of precipitation at below zero temperatures, the storage of snow during winter, and melting of snow in spring. This in view of the climatic conditions in the upper reaches of the river. The second modification was a considerable effort and required the development of the theoretical framework and a special algorithm. Drought monitoring system For the drought monitoring system (DMS) it is proposed to use the Climatic Moisture Index as proposed in the framework of the UN Convention to Combat Desertification

    CMI = P / EP (1.1)

    Where P is the precipitation, EP the potential evapotranspiration and CMI the Climatic Moisture Index, a parameter which indicates climatic drought. More directly related to the drought conditions at the ground surface is the "Soil Moisture Index" (SMI), which may be defined as SMI = E / EP (1.2) Where E denotes the actual evapotranspiration. In both indices the potential evapotranspiration is not one of the basic products. The potential evapotranspiration can be derived in several ways. The "EARS method" estimates the potential evapotranspiration as 0.8 times the net radiation. The factor 0.8 is derived by approximation from the Penman-Monteith equation. A second approach is to estimate the potential evapotranspiration from the satellite observed air temperatures using the Thornthwaite formula. Other drought products may be developed according to the YRCC needs, for example Number of Dry Days, etc.

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    Water Resources Forecasting System (WRFS) for the upper reach

    The WRFS has been developed as a grid-based modelling system, which can assimilate the precipitation, snow melt and evaporation data from the EWBMS at the time-scale of 1 day, without the need to aggregate on the spatial scale. In this phase, the distributed water balance model has been developed and the components have been fit into the structure arising from the geometrical terrain arrangement. The initial model parameterisation of other hydrological characteristics was carried out. The data transfer from the EWBMS to the water resources forecasting model component was developed as separate module, which enhances the capability of independent incremental upgrading during later stages of the project. During definition of the required key model parameters to be incorporated, and during construction of the model, extensive use of information and experience from YRCC has been incorporated through intense collaboration. Similarly, data requirements and procedures for calibration and validation of the model components were jointly defined. The possibilities and limitations for calibration and validation, however, depend to a large degree on existing historical data records. Part of the procedure consisted of a sensitivity assessment for the individual parameters. Flood Forecasting System (HWFS) for the lower Weihe River

    A flood forecasting systems has been developed, initially by extending the presently available flow routing tools used by the YRCC. This requires establishing a coupling with a grid-based model structure, to be built as a separate module, which is capable to the EWBMS generated data in a similar fashion as for the WRFS component. Starting from pilot sub-catchment, progressive improvements have been introduced and tested, finally covering the entire Weihe catchment. Similarly to the development of the WRFS, procedures, data requirements and selection of data records for the calibration and validation phase have been established and carried out. YRCC participated actively in the development of the flood forecasting system and contributed essential information and knowledge of the area and its hydrological behaviour.

    Training

    A detailed training program including satellite reception, energy and water balance processing, drought monitoring, water resources forecasting and high water level forecasting has been prepared during the development phase of the project.

    1.8.2 Implementation and testing phase

    EWBMS and drought monitoring system At the beginning of the second year the Energy and Water Balance Monitoring system and the Drought Monitoring System were implemented at the YRCC premises in Zhengzhou, in addition to the satellite data receiving system. Also rain gauge data reception ran smoothly by that time. The EWBMS system was made operational in such a way that it could produce the basic products, such as precipitation, melt water, evapotranspiration, radiation and air temperature on a daily basis. Dekadal or monthly drought monitoring products (CMS, DMS) could be generated as well. YRCC operators were trained in using the system and in generating the products.

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    The first part of this phase was used to test and validate the quality of the distributed data products. For testing of the data products the following methods were used.

    1) Precipitation data will were tested by means of the Jack-knifing method. 2) Radiation data were tested by comparison with radiometer data; mainly for the net radiometers installed together with the LAS systems (see point 3) 3) Sensible heat flux data were tested by means of Large Aperture Scintillometer (LAS) systems. 4 systems have been installed, 3 in the Upper Reach and 1 in the Loess plateau area, two different climatic regions of the Yellow river basin. 4) Actual evapotranspiration can be considered validated if the radiation and sensible heat flux are validated and calibrated (see 2 and 3). 5) 1.5-meter air temperatures derived from satellite will be tested with measured air temperatures available from existing weather stations. 6) In addition the water balance (rainfall minus actual evapotranspiration) has been tested by comparing this quantity for a period of one or more years with the discharge measured at the basin outlet. By comparison of the data derived from the satellite and those measured on the ground the EWBMS have been tested. Deviations were studied and the system was improved wherever deficiencies in the models could be identified. Having calibrated the basic satellite data products, also the drought monitoring indices (CMI and SMI), which are ratio's of the previous fluxes, can be considered reliable. At the end of this phase a Validation and Calibration Report has been generated. Water Resources and High Water Level Forecasting Systems (WRFS and HWFS) During this phase, the WRFS and HWFS have been implemented, tested and assessed at YRCC. A first appraisal of their functioning was done on the basis of comparing test results against hydrological response observations and measurements from field studies and measuring sites, i.e. validation. Where necessary, improvements were made to the individual modules and alternative solutions to certain were carried through. The HWFS component was extended to comprise the full area of the sub-basin. Training was conducted in order to familiarise YRCC staff with the systems operation.

    1.8.3 Demonstration phase

    During the demonstration phase the EWBMS, the drought monitoring system (DMS), the Water Resources Forecasting System (WRFS) for the upper river, and the Flood Forecasting System (HWFS), for the Weihe River, have been run in an operational way. Products were generated and provided to end-users. A satellite monitoring bulletin and a website were developed for this purpose. At the end of the demonstration phase the project has been assessed by means of a validation workshop.

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    EWBMS and Drought Monitoring System (DMS)

    The EWBMS and drought monitoring system were demonstrated operationally during the last two years of the project. A drought monitoring and early warning bulletin for the Yellow River basin was developed and will be published regularly. Drought related products will be generated and published in this bulletin, for example: rainfall and the soil moisture index (= relative evapotranspiration), sub-catchment water balances, etc. Other drought products, such as the cumulative number of dry days, may be added. Water Resources and High Water Level Forecasting Systems (WRFS and HWFS) Based on monitoring of the results and evaluation of the performance of both systems since the implementation and testing phase, the systems implementation was finalized and the documentation completed. Particular attention will be paid to ensure that the improvements arising from the systems performance assessment, in combination with those from initial calibration and validation are implemented. Training of system operators at the YRCC Hydrological Bureau continued, so as to assure that they master the technology well and the products become established tools in support of operations duly carried out by the YRCC.

    1.9 Project impact

    Water budget monitoring in the upper reach and forecasting of the rivers base flow leads to earlier knowledge of the amount of water that is available for use in the middle reach of the river and will enable a more rational and sustainable water allocation to users. Lack of information has so far prohibited this. The main water intake along the river is for irrigation and it has been noted that water quota for irrigation have been far too high. This has adversely affected users in the middle and lower reaches of the river. For an equitable water distribution the system provides early information on river run-off and drought, i.e. actual water needs. This will allow water managers sufficient time to prepare rational water distribution plans based on actual water supply and needs. Water resources forecasting The system is expected to bring an overall increase of water use efficiency, which is conservatively estimated at 1%. Given a total water use for irrigation in the Yellow river basin of 27 billion m3 per year, and a related average economic value of water of 1.2 RMB/m3, the estimated yearly social economic value that can be attributed to this part of the system may then be estimated at 0.01*1.2*27.109 RMB/year or 324 million RMB/year.

    Flood forecasting

    The flood forecasting system implemented for the Weihe tributary provides run-off and related river level forecasts on a daily basis. These forecasts are based on satellite based daily distributed data fields of precipitation and evapotranspiration which cover the whole sub-catchment. The system will help to provide improved predictions of high water levels and to timely alert authorities and the population to take flood protection measures.

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    According to the Worldbank (2001), flood losses in the Yellow River basin increased from 1500 RMB/ha in the 1950’s to 9000 RMB in the 1990’s. The total flood prone area is about 118000 km2. Based on these figures the potential costs of a complete flooding could nowadays be estimated at 120 billion RMB. If such a flood occurs once every century and by improved high water forecasting 10% of the damage could be prevented, than the social economic value of the system would be 120 million RMB per year. Drought monitoring Another important functionality of the EWBMS is drought and desertification monitoring. Such information has significant meaning in relation to food security and food trade, ass well as in relation to land degradation. The drought monitoring system may be used to monitor and forecast the production of crop and grass lands. Crop yield forecasts help to optimize food market performance and reduce price fluctuations. According to Hayami and Peterson (1972) improved market monitoring and reduced price fluctuations increase population welfare. They also allow realizing better prices in food trade. Finally the system can help to reduce desertification damage. The total benefits of drought monitoring and early warning for China would be about 3 billion RMB per year, or about 300 million RMD in the Yellow river area. Summarizing it may be concluded that the impact and social economic returns of the project can be very high, in the order of 700 million RMB per year.

    1.10 References

    Hayami, Y and Peterson, W. (1972) “Social Returns to Public Information Services: Statistical Reporting of US Farm Commodities”, The American Economic Review, Vol. 62, 1972, pp 119-130.

    Worldbank (2001) “Agenda for the Water Sector Strategy for North China”, Worldbank report 22040-CHA, April 2, 2001.

    Unknown (1997) “An Overview of Chinese Water Issues”, China Environment Series, 10 September 1997, pp 46-48. Changming Liu (2000) “Water Resources Development in the First Half of the 21th Century in China”, 2nd World Water Forum, China Water Session, pp 1-16), March 2000.

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    2 THE YELLOW RIVER TARGET AREAS 2.1 The source area of the Yellow River

    The upper reach of the Yellow River covers the area above Tangnaihai hydrological station on the main stream. The drainage area of the upper reach covers 121972 km2,

    located between 95°00′ and 103°30′ E and from 32°19 to 36°08′ N. The river length to the upstream source is 1552.4 km. With an altitude over 3000 m, this area has a low air density, with oxygen content between 0.166 and 0.186 g/m3. There are many mountains in the source area, such as Bayankala, Animaqin and Min Mountain, and there are scattered plains, rivers, basins and hills. The mountain summits over 4000 m are bare, while the lower slopes are covered with grassland. The glacier of the Animaqin covers an area of 191.95 km2. Its melting water makes up 1.0% of the runoff at Tangnaihai.

    Temperature and Ice The annual temperature is below zero and only July and August are frost-free. The difference between yearly minimum and maximum temperature amounts to 75 oC. The recorded lowest temperature is –53 oC. The warmest period generally falls in August, the coldest in January. The following table provide an overview of the ice situation in the upper reach Table 2.1: Characteristics of river ice in the YR source area Reach Ice flooded from Fully frozen Melting from Above Jimai October D3 January March D3 Mengtang-Maqu November D3 December March D3 Jungong-Tangnaihai November D1-2 March D1-2

    D=dekad, period of 10 days

    Figure 2.1: The project focal areas: the Yellow River Upper Reach and the lower reach of the Weihe River

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    Rainfall and Evaporation The average annual rainfall in the river source area is 474.6 mm. The rainfall in the area of Hongyuan, Ruoergai,Maqu and Jiuzhi is in the range of 500-800 mm, while rainfall, while rainfall in the reaches from Maqu to Jungong is much lower: 250-400 mm. 91% of the rain falls from May to October. Incidental rain falls between October and May. Above Maduo rain is scattered throughout the year. Pan evaporation is between 1200 and 2300 mm/yr and declines from north to south.

    River runoff The average annual rainfall on the area amounts to 69.9 billion m3. The average runoff is 20.5 billion m3, corresponding to 168 mm water depth, which on average is 35% of the rainfall. There are usually two peaks in the run-off: in July and September. Most runoff occurs between May and October (78.5%).

    2.1.1 Hydrological observations in the source area

    Figure 2.2 and table 2.2 and 2.3 provide an overview of the hydrological observations network in the source area of the Yellow River. The Development of Automatic Observation and Reporting System

    The project Automatic Observation and Reporting System for the River Source Areas of the Yellow River (here in after referred to as the River Source Project) is composed of four sub-systems: data collection system, data transmission system, monitoring and information centre, and water resources monitoring system. In consideration of the hydrological situation in the upper reaches of the Yellow River, the development of the system is mainly based on the construction of the data

    Figure 2.2: Hydrological station network in the source area

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    collection platform. The system will function only with people in charge, but without requiring their permanent presence. The data will be collected and processed in an automatic, digital, distance-observed and remote-controlled way, supported by regular inspection visits. The River Source Project has completed the rainfall collection sub-project and the construction of the sub-centre. The other projects are scheduled for construction. Considerable progress is expected to be made in the observation and reporting techniques and in modernization of the data collection. Distribution of the stations

    There are 10 hydrological stations or water level stations in the river source areas, of which 14 are all administered by the Xining Hydrological Reconnaissance Bureau of the Yellow River Conservancy Commission (YRCC). The stations are located at: Huangheyan, Jimai, Mengtang, Maqu, Jungong, and Tangnaihai station, on the mainstream, while the following stations are situated on the branches: Hanghe on the Requ, Jiuzhi on the Shakequ, Tangke on the Baihe river and Dashui on Heihe river. In addition, there are 5 entrusted rainfall stations in: Awancang, Longriba, Waqie, Maiwa and Dongqinggou. See also table 2.2.

    Items of Hydrological Observation

    The stations are responsible for the observation and reporting of the following items: - Rainfall and Evaporation: The rainfall observation instruments include the manual rainfall device and the siphon rainfall device; the evaporation observation instruments include E601 Evaporator and 20 cm Diameter Evaporator. Recording and processing of the data is manual. - Water level: Three instruments are used: vertical rule, floater stage, and ultrasonic no-touching stage. Vertical rules are used in all stations. Some stations are equipped with HW-1000 ultrasonic devices, and only a few use the floater device. Water level recording is automatic in 4 stations: Tangnaihai, Xunhua, Guide and Xiangtang. In the other 10 stations data are recorded manually. Computation and processing is also manually. - Discharge: The major way of discharge observation is by continuous measurement. Only a few stations are measured discontinuously or at regular intervals. The tools used include observation boat, running speed cableway, hanging box cableway and buoy projector. Boat observation is used at Huangheyan, Jimai, Mengtang, Maqu, Jungong, and Tangnaihai. In addition at Tangnaihai, the half-automatic running speed cableway is used to measure low and high flows. Bridge observation is adopted in Huanghe station. Rubber boats are used in Jiuzhi,Tangke and Dashui. The stations at Maqu and Tangnaihai are equipped with a buoy projector to measure flood. However, most of them were built already in the 1960s and 1970s, and the trestles and cables have expired. The same data can be collected by buoy projectors. The collection, analysis, computation, and processing of the data involved is mainly done manually. - Sediment. Sampling of silt is done manually with the help of a horizontal sampling device. The processing and analysis of the sample sediment, and the related data processing is done by hand.

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    Table 2.2 Information of the hydrological station network in the source area Coordinates No. River Station Station type Esta-

    blished (yr.m)

    Long. Lat. Drainage

    area (km2)

    Alti- tude (m)

    Observation Item

    1 Zhaling Lake Zhaling Lake water level planned 97°30′ 34°51′ 17728 water level 2 Eling Lake Eling Lake water level planned 97°45′ 35°05′ 18428 Water level 3 Yellow River Huang-

    heyan hydrology 1955.6 98°10′ 34°53′ 20930 4215 Water level,

    volume of flow, silt, evaporation

    4 Yellow River Jimai hydrology 1958.6 99°39′ 33°46′ 45019 3948 Water level, volume of flow, silt, evaporation

    5 Yellow River Mengtang hydrology 1987.8 101°03′ 33°46′ 59655 3636 Water level, volume of flow,

    rainfall 6 Yellow River Maqu hydrology 1959.1 102°05′ 33°58′ 86048 3400 Water level,

    volume of flow, silt, rainfall, evaporation

    7 Yellow River Jungong hydrology 1979.8 100°39′ 34°42′ 98414 3079 Water level, volume of flow,

    silt, rainfall, evaporation

    8 Yellow River Tang-naihai hydrology 1955.8 100°39′ 35°30′ 121972 2665 Water level, volume of flow,

    silt, rainfall 9 Yellow River Duide hydrology 1954.1 101°24′ 36°02′ 133650 2201 Water level,

    volume of flow, silt, rainfall, evaporation

    10 Yellow River Xunhua hydrology 1945.10 102°30′ 35°50′ 145459 1850 Water level, volume of flow,

    silt, rainfall 11 Requ Huanghe hydrology 1978.8 98°16′ 34°36′ 6446 4200 Water level,

    volume of flow, rainfall

    12 Shakequ Jiuzhi hydrology 1978.9 101°30′ 33°26′ 1248 3560 Water level, volume of flow,

    rainfall 13 Baihe River Tangke hydrology 1978.9 102°28′ 33°25′ 5374 3410 Water level,

    volume of flow, silt, rainfall, evaporation

    14 Heihe River Dashui hydrology 1984.6 102°16′ 33°59′ 7421 3400 Water level, volume of flow,

    rainfall 15 Datong River Xiangtang hydrology 1940.1 102°50′ 36°21′ 15126 1776 Water level,

    volume of flow, silt

    16 Huang-shui River

    Minhe hydrology 1940.1 102°48′ 36°20′ 15432 1752 Water level, volume of flow,

    silt, rainfall, evaporation

    17 Saierqu Awangcang rainfall 1977 101°42′ 33°47′ Rainfall

    18 Baihe River Longriba rainfall 1976 102°22′ 32°27′ Rainfall 19 Baihe River Waqie rainfall 1976 102°37 33°08′ Rainfall 20 Black River Maiwa rainfall 1977 102°54′ 32°03′ Rainfall 21 Qiemuqu Dongqinggo

    u rainfall 1977 99°58′ 34°32′ Rainfall

    22 Gequ Maqin rainfall planned 100°15′ 34°29′ Rainfall 23 Shakequ Jiuzhi rainfall planned 101°29′ 33°25′ Rainfall 24 Baihe River Hongyuan rainfall planned 102°34′ 32°48′ Rainfall 25 Heihe River Ruoergai rainfall planned 102°58′ 33°35′ Rainfall 26 Zequ Zeku rainfall planned 101°28′ 35°02′ Rainfall 27 Gande Xikequ rainfall planned 99°54′ 33°58′ Rainfall 28 Henan Zequ rainfall planned 101°35′ 34°45′ Rainfall

    Rem 3 additional tour surveying sections are established in Ruoergai on the mainstream, Jimai and Jiaqu on the branches.

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    Table 2.3: Data collection in the river source area Instruments and manner of observation No River Station Water level Discharge Sediment

    concentration Sediment load Rainfall Evaporation

    1 Yellow River

    Huang-heyan

    Manual vertical rule

    Conttin. in flood , discont. in non-flood season , hanging boat

    Manual , horiz. sampling device

    1year every 4 years

    Purchase from local met.eo station

    Purchase from local meteo station

    2 Yellow River

    Jimai Manual vertical rule

    Conttin. in flood , discont. in non-flood season , hanging boat

    Manual , horiz. sampling device

    1year every 4 years

    Manual., horiz. sampling device

    Manual 20cm evaporator

    3 Yellow River

    Mengtang Manual vertical rule

    Conttin. in flood , discont. in non-flood season , hanging boat

    N/A N/A Manual , rainfall device

    N/A

    4 Yellow River

    Maqu Manual vertical rule

    Continuous., hanging boat, buoy projector

    Manual , horiz. sampling device

    1 year every 4 years

    Man. & recording siphon rainfall and rainfall device

    Manual 20cm evaporator

    5 Yellow River

    Jungong Manual vertical rule

    1 year discont. every 4 years, hanging boat

    Manual , horiz. sampling device

    1 year every 4 years

    Man. & recording siphon rainfall and rainfall device

    Manual 20cm evaporator

    6 Yellow River

    Tangnaihai Record. water level, floater fluviograph, vertical rule

    Contin. , cableway with streaml. weight, hanging boat, buoy projector

    Manual , horiz. sampling device

    Boat., horizontal sampling device

    Man. & recording siphon rainfall and rainfall device

    N/A

    7 Yellow River

    Guide Record. water level, floater fluviograph, vertical rule

    Contin. , cableway with streaml. weight, hanging boat, buoy projector

    Manual , horiz. sampling device

    Boat., horizontal sampling device

    Man. & recording siphon rainfall and rainfall device

    Manual, E601 evaporator, 20CM evaporator

    8 Yellow River

    Xunhua Record. water level, floater fluviograph, vertical rule

    Contin. , cableway with streaml. weight, hanging boat, buoy projector

    Manual , horiz. sampling device

    Boat., horizontal sampling device

    Man. & recording siphon rainfall and rainfall device

    N/A

    9 Requ Yellow River

    Manual, vertical rule

    Disc. bridge observ. N/A N/A Manual, rainfall device

    N/A

    10 Shakequ Jiuzhi Manual, vertical rule

    1 year discont.. every 4 years, rubber boat

    N/A N/A N/A N/A

    11 Baihe River

    Tangke Manual, vertical rule

    Regular in flood, irregular in non-flood season, rubber boat

    Manual, horizontal sampling device

    N/A Manual, rainfall device

    Manual, 20CM evaporator

    12 Heihe River

    Dashui Manual, vertical rule

    1year observ, every 4years, rubber boat

    N/A N/A Manual, rainfall device

    N/A

    13 Datong River

    Xiangtang Record. water level, floater fluviograph, vertical rule

    Regular, cableway with streamlined weight, hanging boat, buoy projector

    Manual, horiz. sampling device

    Boat , horiz. sampling device

    N/A N/A

    15 Saierqu Awancang Manual observ. rainfall device

    16 Baihe River

    Longriba Manual observ. rainfall device

    17 Baihe River

    Waqie Manual observ. rainfall device

    18 Heihe River

    Maiwa Manual observ. rainfall device

    19 Qiermuqu Dongqing-gou

    Manual observ. rainfall device

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    2.1.2 Information acquisition and transmission

    At present, there are 3 levels of real time water regime data collection: the hydrological stations, the data collection sub-centres and the data collection centre. The sub-centre of the upper Yellow River is located in the Upper Yellow River Hydrology and Water Resources Bureau of YRCC (Lanzhou, Gansu province). The data collection centre is in Hydrology Bureau of YRCC (Zhengzhou, Henan province). All reporting stations are communicating by PSTN (Public Switched Telephone Network), GSM (Global System for Mobile Communications) or satellite. Most stations use PSTN and GSM, and part of them use GSM and satellite. Rain gauges have been realized that automatically collect and transmit the rainfall information, while hydrological stations automatically transmit the discharge information after putting them manually into the computer. More than 90% of data can be transmitted to Zhengzhou within 20 minutes, and more than 95% of the data within 30 minutes. The sub-centre of the Upper Yellow River is in charge of the real time water information transmission. The sub-centre communicates with the centre in Zhengzhou through SDH (Synchronous Digital Hierarchy) with 2Mbaud rate. The centre in Zhengzhou is in charge of the real time water information reception, transmission and decoding, and the storage of these data into the real time water information database.

    2.1.3 Hydrological forecasting

    So far, the hydrological forecasting tasks in the source area of the Yellow River mainly concern the middle and long-term runoff forecasting and flood forecasting, and which consists particularly of the following: - Before the end of April, the long term monthly runoff forecast at Tangnaihai for

    the period May-October should be made. - During the flood season (May-October), every first dekad the updated forecast of

    runoff for the next month should be made. - During the flood season, short-term flood forecasts at Tangnaihai should be made

    when heavy and large-scale rainfall may cause flood in the source area. - At the end of the flood season, a long-term runoff forecast for the non-flood

    season (November-June) should be made.

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    2.2 The lower Weihe River

    The Weihe is the largest tributary of the Yellow River. It originates from the Niaoshu Mountain in Weiyuan county in Gansu province, flows through Gansu, Ningxia and Shannxi provinces and flows into the Yellow River in Tongguan county in Shanxi Province. The total length of the main flow is 818 km and the basin area is 134800 km2. The reach from Xianyang to the outlet is the lower Weihe River. The length of this part is 211 km. The river bed slope is around 0.68-0.15‰. Down of Lintong the river is most winding. The Weihe River water system is dissymmetrically developed. On its left bank, the tributaries are long, with larger catchments, and carry more sediment. But on the right bank, the tributaries are short and steep, and carry more water and less sediment.

    According to physical and geographic conditions, the lower Weihe basin can be divided in four types: soil-tor, loess hill, loess terrace and plain region. The soil-tor region is found in the upper and middle reaches of the south tributary. It has steep slopes, abundant precipitation, dense vegetation, little water and soil loss, high runoff coefficient and easily generates runoff. The loess hill region is mostly situated on the north side of the upper and middle Shichuan River. There is little vegetation, serious water and soil loss and not easily generates runoff .The loess terrace is mainly found on the south side of the lower Shichuan river and middle reaches. Finally the plain region is situated in the vicinity of Lower Weihe River. The area is flat with fertile soil and has a lower runoff coefficient. There are many tributaries in lower Weihe. The tributaries on the north side mostly originate from the loess hill and plateau, such as the Shichuan, Jinghe and Beiluohe rivers. They have a large catchment, slow fall, high sediment load and are main sources of sediment. The Jinghe is the largest tributary. With a length of 455.1 km and a basin area of 45400 km2, it makes up 33.7% of the Weihe river basin. The Beiluohe is the second largest tributary. Its length is 680 km2 and its basin area 26900 km2, thus covering 20% of the Weihe basin. There are a number of smaller rivers on the south side: Fenghe, Zaohe, Chanbahe, Dayuhe, Xihe, Linghe, Youhe, Chishuihe, Yuxianhe, Shidihe, which originate from the Qinling Mountain. They are short, have rapid flow, great runoff and low sediment concentration. They represent major storm flood sources in lower Weihe River.

    The lower Weihe River belongs to the warm temperate zone and semi-arid and humid climate. The annual mean temperature is 6-13 oC, the annual mean precipitation 500-800 mm and the annual mean pan evaporation is 700-1000 mm. The rainfall in southern mountainous region is more than that in the valley and northern part of the basin. Rain storms occur from July to October and bring 50-60% of the yearly precipitation. Maximum precipitation mostly occurs in July and August, as a result of strong and short-duration storms. Autumn rainfall occurs from September to October, and may last 5-10 days or longer.

    Hydrological Characteristics Runoff comes mostly from the main stream and south tributaries. The mean yearly runoff is 7.96 Bm3, 7.26 Bm3 at Huaxian, 1.37 Bm3 at Zhangjiashan of the Jinghe River and 0.696 Bm3 at Zhuangtou on the Beiluohe River, respectively. The inter-annual variation amounts to a factor 10. The maximum runoff of 20.6 Bm3 occurred in 1964, while the minimum runoff, only 2.1 Bm3, took place in 1995. The yearly runoff is also not equally distributed: 60% occurs during the flood season.

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    The multi-annual average sediment runoff is 444 Mton in the lower Weihe River, 359Mton at Huaxian, 246 Mton Zhangjiashan and 85 Mton in Zhuangtou. The sediment is mainly from north tributaries, especially the Jinghe and Beiluohe rivers. They contribute 55.4% and 19.1%, respectively, to the total of the Weihe River. Most sediment transport takes place during the flood season. The flood in the lower Weihe basin comes mainly from upper Xianyang, Jinghe River and the south tributaries. Discharge and sediment concentration are high, and rise and drop steeply. The flood is of the flat type, with high runoff volume in autumn.

    `2.2.1 Hydrological observations

    The observation stations belong to Hydrology bureau of YRCC, Shannxi Hydrology Bureau (SHB) and Shannxi Sanmenxia Bureau (SSB). There are 12 reporting hydrological stations, 3 water level stations, 6 reporting rain gauge stations. Except the reporting rain gauging stations, there are 55 basic rain gauge stations, of which 33 in south of the Weihe River, and 1 basic hydrological station at Liulin in the Shishanchuan River. They are not reporting. They belong to the Shannxi Hydrology Bureau. See figure 2.3 and table 2.4 and 2.5. A tele-metering system of water level and rainfall is built since 1998 by the Shannxi Sanmenxia Bureau. The system includes 3 water level stations (Gengzhen, Jiaokou, Weinan), and 4 rain gauge stations (Quancao, Dianzi, Wulongshan, Shanghuichi). Furthermore 44 silting cross-sections from Xianyang to the outlet of the Weihe and 22 from Zhuangtou to the outlet of the Beiluohe, have been set up by the Shannxi Sanmenxia Reservoir Administrative Bureau for measuring the channel sedimentation changes caused by the Sanmenxia Reservoir.

    Figure 2.3: Reporting station network in the lower Weihe river.

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    The hydrological observations in the lower Weihe include 9 items including: precipitation, evaporation, water level, discharge, sediment concentration, sediment delivery rate, particles size analysis, water temperature and water quality. The same items except water temperature but including ice regime are observed at Xianyang and Huaxian. Precipitation is observed by using solid-storage and tipping-bucket rainfall recording every 2 hours, both at Xianyang and Huaxian station. At Xianyang evaporation data are obtained manually from the local meteorological station. The station at Huaxian has a vertical gauge, an HW-1000 ultrasonic recorder to measure the water level. Huayin station has the vertical gauge. Xianyang station has not only the vertical gauge and HW-1000 ultrasonic recorder, but also the wire weight gauge to observe water level. The measurement facilities at Xianyang station consist of a double permanent cable with ferroconcrete brackets, which serve as electrical current measuring cables. The measuring facilities at Huaxian station consist of a single cable with free-standing steel bracket lifting ship, one suspending ship, one hydrological capstan, two electric boats. Xianyang and Huaxian stations measure the sediment concentration using a horizontal-bottle sediment sampler. The sediment particle size is analysed at both stations.

    2.2.2 Information acquisition and transmission The information sub-centre for the Weihe River is located at the Sanmenxia Reservoir Hydrology and Water Resources Bureau of YRCC (Sanmenxia City, Heman Provence). The centre is again the Hydrology Bureau of YRCC in Zhengzhou. All reporting stations are communicating by PSTN (Public Switched Telephone Network), GSM (Global System for Mobile Communications) or satellite. Most of them use PSTN and GSM, and part of them use GSM and satellite. Rain gauges have been realized that automatically collect and transmit rainfall information, while hydrological stations automatically transmit discharge after manually putting the information into a computer. More than 90% of data can be transmitted to Zhengzhon within 20 minutes, and more than 95% within half an hour. The sub-centre of Sanmenxia is in charge of real time informatiom transmission. The sub-centre is communicating with the centre in Zhengzhou by SDH (Synchronous Digital Hierarchy) at 2Mbaud rate. The centre in Zhengzhou is in charge of real time water informatiom reception, transmission and decoding, and storage of the information into the real time water information database.

    2.2.3 Flood forecasting

    Huaxian is the forecasting station in the lower Weihe. The forecasting items include the discharge hydrograph, in particular the flood peak and its time of occurrence. The peak is usually over 2000 m3/s. The forecast is based on the correlation between the peaks at Lintong and Huaxian station. The parameters in the scheme are the momentaneous discharge at Huaxian and the coefficient of excess in Lintong. In addition the correlations of the flood peak at Xianyang and Zhangjiashan with that at Huaxian are developed. The routing times of each are based on the discharges at Xianyang and Zhangjiashan separately.

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    Table 2.4: Reporting stations in the lower Weihe River River name Station name Station type East Long. North Lat. Owner Rem. Weihe Xianyang Hydrology 108 42 34 19 YRCC Weihe Lintong Hydrology 109 12 34 26 SSB Weihe Huaxian Hydrology 109 46 34 35 YRCC Weihe Huayin Water level YRCC Jinghe Zhangjiashan Hydrology 108 08 34 38 SHB Beiluohe Zhuangtou Hydrology 109 50 35 03 SHB Beiluohe Nanronghua Hydrology 109 53 34 46 SSB Beiluohe Chaoyi Water level 109 52 34 46 SSB Chanhe Qinduzhen Hydrology 108 46 34 06 SHB South Juhe Gaoqiao Hydrology 108 49 34 06 SHB South Dayuhe Dayu Hydrology 109 07 34 00 SHB South Bahe Luolicun Hydrology 109 22 34 09 SHB South Bahe Maduwang Hydrology 109 09 34 14 SHB South Qishuihe Yaoxian Hydrology 108 59 34 55 SHB North Yeyuhe Chunhua Hydrology 108 35 34 47 SHB North Wangchuanhe Gepaizhen Rain gauging 109 30 33 55 SHB South Linghe Tielu Rain gauging 109 27 34 24 SHB South Shichuanhe Fuping Rain gauging 109 10 34 45 SHB North Shichuanhe Meiyuan Rain gauging 109 21 34 54 SHB North Shichuanhe Yaoqu Rain gauging 108 53 35 12 SHB North Weihe Weinan Meteorology 109 30 34 30 SHB

    The flood peak correlation forecasting is based on the antecedent rainfall, local rainfall and the coefficient of excess. The hydraulic characteristics can be included in the scheme, which is easy in use. But the channel siltation and different hydraulic characteristics of the main flow and overflow floods are not taken into account. The river channel is wide and shallow in the lower Weihe. The cross section is compound. Normal flow is through the main channel, while the high flood is overflowing. Considering the different flood characteristics and the routing rules through the main channel and overbank, a Muskingum layered routing scheme has been developed. The layered outputs are calculated with the different parameters of the main channel and the overbank, and subsequently added. The scheme parameters can be optimized when overflow occurs in order to forecast the flood peak, the time it occurs and its progression. Local inflow, however, is not considered. In fact, the flood peak correlation and the Muskingum layered routing scheme are combined. The forecast is also optimized on the basis of real-time rain and flow information so as to improve the forecasting accuracy. Currently also the national flood forecasting system (NFFS) is being studied and tested for flood forecasting in the lower Weihe River, and will become operational soon.

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    Table 2.5: Basic stations in the lower Weihe River No River name Station name Station type East long. North lat. Remark 1 Fenghe Jiwozi Rain gauging 108 50 33 52 south 2 Fenghe Qinggangshu Rain gauging 108 51 33 55 south 3 Jianhe Bianzigou Rain gauging 108 54 34 04 south 4 Taipinghe Meichang Rain gauging 108 39 33 56 south 5 Taipinghe Taipingyu Rain gauging 108 43 34 00 south 6 Gaoguanyu Xingjialing Rain gauging 108 44 33 53 south 7 Shibianyu Xianrencha Rain gauging 108 56 33 56 south 8 Shibianyu Shibianyu Rain gauging 108 57 33 59 south 9 Xiangzihe Wangqu Rain gauging 108 58 34 05 south 10 Dayuhe Banmiaozi Rain gauging 109 08 33 57 south 11 Dayuhe Xinguansi Rain gauging 109 07 33 59 south 12 Fenghe Doumen Rain gauging 108 45 34 14 south 13 Weihe Mazhuang Rain gauging 108 39 34 26 south 14 Weihe Yaodian Rain gauging 108 51 34 24 south 15 Weihe Bayuan Rain gauging 109 41 34 09 south 16 Weihe Mujiayan Rain gauging 109 32 34 11 south 17 Wukonghe Muhuguan Rain gauging 109 30 34 03 south 18 Qinghe Lanqiao Rain gauging 109 27 34 06 south 19 Wangchuanhe Yuchuan Rain gauging 109 23 33 58 south 20 Wangchuanhe Longwangmiao Rain gauging 109 20 33 54 south 21 Wangchuanhe Wangchuan Rain gauging 109 22 34 05 south 22 Wangchuanhe Gepaizhen Rain gauging 109 30 33 55 south 23 Bahe Pantaowan Rain gauging 109 14 34 13 south 24 Bahe Xiqu Rain gauging 109 05 34 18 south 25 Tangyuhe Gaobaozhen Rain gauging 109 12 34 02 south 26 Chanhe Mingdu Rain gauging 109 06 34 08 south 27 Weihe Tongyuanfang Rain gauging 109 03 34 33 south 28 Yuchuanhe Yuchuan Rain gauging 109 19 34 21 south 29 Donggou Qingcaoping Rain gauging 108 48 35 17 north 30 Juhe Miaowan Rain gauging 108 46 35 10 north 31 Qishuihe Jinsuoguan Rain gauging 109 03 35 13 north 32 Wujiahe Yunmeng Rain gauging 109 11 35 13 north 33 Qishuihe Chenlu Rain gauging 109 09 35 02 north 34 Qishuihe Huangbao Rain gauging 109 02 35 01 north 35 Qishuihe Shizhu Rain gauging 108 58 35 04 north 36 Zhaoshihe Potou Rain gauging 108 53 34 52 north 37 Shichuanhe Caocunzhen Rain gauging 109 12 34 54 north 38 Shichuanhe Guanshan Rain gauging 109 23 34 42 north 39 Yeyuhe Anziwa Rain gauging 108 35 35 01 north 40 Yeyuhe Qinhe Rain gauging 108 36 34 55 north 41 Dongyuhe Nancun Rain gauging 108 38 34 53 north 42 Yeyuhe Bujiacun Rain gauging 108 31 34 56 north 43 Yeyuhe Kouzhen Rain gauging 108 42 34 42 north 44 Yeyuhe yunyang Rain gauging 108 48 34 38 north 45 Qingyuhe Xiaoqiu Rain gauging 108 47 34 55 north 46 Qingyuhe Fangli Rain gauging 108 44 34 50 north 47 Qingyuhe Fanjiahe Rain gauging 108 54 34 44 north 48 Qingyuhe Sanyuan Rain gauging 108 56 34 37 north 49 Qingyuhe Duli Rain gauging 109 04 34 38 north 50 Linghe Jinshanzhen Rain gauging 109 23 34 17 south 51 Qiuhe Houzizhen Rain gauging 109 31 34 16 south 52 Qiuhe Chongning Rain gauging 109 35 34 23 south 53 Chishuihe Longjiawan Rain gauging 109 41 34 25 south 54 Weihe Gushi Rain gauging 109 35 34 38 south 55 Shidihe Huichi Rain gauging 109 48 34 25 south 56 Juhe Liulin hydrological 108 49 35 03 north

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    3 ENERGY AND WATER BALANCE MONITORING SYSTEM

    Meteorological satellites have been used mainly for weather analysis and forecasting. Since the 1980’s new applications, related to the energy and water balance of the earth surface, have emerged. Surface reflectance, measured in the visible wavelength band (VIS) enables the estimation of the amount of solar energy that is absorbed by the ground. Surface temperatures, measured in the thermal infrared band (TIR) enable the assessment of the partitioning of this absorbed energy between sensible and latent heat, the latter representing the evapotranspiration of water. Geostationary meteorological satellites provide thermal infrared and visible data at 3 or 5 km resolution. Polar orbiting meteorological satellites may also be used to measure planetary temperatures. But, the lower repeat coverage makes them less suitable for cloud and rainfall monitoring. The time of data capture, the large scan angles and the variable imaging geometry makes them also less valuable for energy balance monitoring In figure 3.1 the overview of the Energy and Water Balance System (EWBMS), as used in this project, is shown. Images from the geostationary meteorological FY2c and GMS satellites are received hourly. Cloud top level frequencies or "cloud durations" are determined. From the hourly full image data, composites are prepared which represent local noon and local midnight VIS and TIR values. The extracted data are then processed to quantitative, spatially continuous image maps of rainfall, radiation, sensible heat flux, temperatures and evapotranspiration. Besides the satellite images, hardly any additional input is needed. Only ground point precipitation data, used for generating the rainfall maps, are required. The actual evapotranspiration, rainfall and temperature are the inputs for the drought monitoring model, the freeze/thaw model and the Large Scale Hydrological Model (LSHM). The latter is discussed in detail in chapter 4. Theoretical backgrounds of the EWBMS and the generation of products is discussed in section 3.1: System Components. To collect validation data, use is made of four Large Aperture Scintillometers (LAS) and some additional instrumentation, which has been established for this purpose at 4 sites in the Yellow River basin. The theory and set-up of these measurements are presented in section 3.2: LAS measurements. In section 3.3 the EWBMS software system is described. All modules: pre-processing, basic product modules, application modules, analysis tools and the EWBMS processing information metadata base are discussed briefly.

    Rainfall mapping

    Clouddurations

    Local noon and midnight composites

    Energy balance

    processing

    Precipitationground

    point data

    Rainfall

    Temperature

    Evaporation

    Freeze/Thawmodel

    Hydrologicalmodel

    Flow & Floodforecasts

    Snow andSnowmelt

    Droughtmonitoring

    model

    Drought/Desert.indices

    Basic products Applications

    FY2cGMS

    Hourly VIS, TIR

    Pre-processing

    Figure 3.1: Energy and Water Balance System (EWBMS) overview.

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    Section 3.4 discusses some results of the Catchment drought monitoring system. Climatologic, hydrologic and agricultural drought indices are calculated and presented in spatially continuous maps, and an analysis of the drought situation in the Yellow River basin is made. Section 3.5, Evaluation of EWBMS results is dedicated to the validation of the EWBMS basic products. The satellite derived data, precipitation, air temperature, sensible heat flux, and global and net radiation, are compared with data from the LAS systems and other sources. Evapotranspiration is evaluated indirectly, since no such data are measured regularly on the ground. It is assumed that if the validation results of two components of the energy balance: sensible heat flux and net radiation, are satisfactory, also the remaining component, obtained by subtracting the previous two, can be trusted. A final approach to validation is by comparing the net precipitation, i.e. rainfall minus evapotranspiration, with the river discharge at the outlet of the catchment.

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    3.1 System Components 3.1.1 Pre-processing

    The pre-processing calculates cloud durations and composes local noon and local midnight images from the hourly VIS and TIR images, obtained with the satellite receiving system (section 3.3.1). The cloud duration mapping uses only the TIR images. The radiance of an observed object in the infrared spectrum, measured in counts, is directly related to the temperature of that object. The object observed from the satellite is the earth’s surface or the top of the highest clouds present. The cloud temperature is proportional to the height above the ground: a typical lapse rate is –6.5 °C per 1000 m. Based on analysis of image histograms, four cloud level classes are discriminated. The thresholds in TIR counts are converted to planetary temperatures. The corresponding temperatures and heights are shown in table 3.1. Table 3.1: Definition of cloud levels and corresponding temperatures and heights. CLOUD LEVEL TEMPERATURE RANGE HEIGHT RANGE Cold < 226 K > 10.8 km High 226 – 240 K 8.5 – 10.8 km Medium high 240 – 260 K 5.2 – 8.5 km Medium low 260 – 279 K 2.2 – 5.2 km

    For every hour a new TIR image is received, for each pixel is determined if there is a cloud present and to which cloud class the cloud belongs. The results are stored in 4 files (one for every cloud class). These files are updated every day, so daily multilevel cloud class frequencies are produced. The main input data for the energy balance are the local noon and local midnight composites, representing the situation at local noon or midnight in one image. In the pre-processing these images are composed from the hourly VIS and TIR images. For each pixel in the image, the time of local noon or midnight is calculated based on its longitude position, and expressed in GMT (Greenwich Mean Time). Then, the two hourly images closest in GMT-time to the local noon or midnight are selected. Pixel values for the composite images are calculated by interpolating the pixel values of the two selected hourly images. Both VIS and TIR local noon composites are produced, but local midnight composites are only calculated with TIR images. When one of the

    Figure 3.2: Example of a thermal infrared (left) and a visual (right) satellite image with sea mask overlay.

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    two hourly images, closest in time to the local noon or midnight of a pixel, is not present, no interpolation is carried out. Therefore, if many hourly images are missing, the composite images will show distinct lines. When 3 or more subsequent hourly images, necessary for the creation of the composite images, are missing, no composite image for that noon or midnight day is created.

    3.1.2 Precipitation mapping

    The estimation of the spatially distributed precipitation is based on two sources of information: (1) point precipitation data from meteorological stations and (2) cloud frequency data from derived from the FY2c meteorological geostationary satellite. Point data are obtained from the WMO Global Telecommunication System (GTS) and additional meteorological station data from the China Meteorological Administration (CMA). The GTS data consist of meteorological measurements from approximately eleven thousand meteorological stations spread over the globe. 95% of these measurements are available within six hours through the GTS. In China, there are about 400 meteorological stations reporting precipitation through the WMO-GTS network. CMA provides data from another 800 national stations. In the past several methods have been developed to create rainfall fields from meteorological satellite data. Well known is the so-called Cold Cloud Duration (CCD) technique, which relates the presence of very high and “cold” clouds to rain gauge measurements. Calibration is done on historical data sets. The CCD technique is only suitable to estimate convective rainfall. The method of EARS differs in two ways. First it uses four cloud levels and the so-called “temperature threshold excess” in contrast to only one cloud level. So also lower cloud levels, associated with frontal precipitation, are accounted for. Secondly, the method uses no calibration on the basis of historic data, but combines rain gauge data and cloud durations in near real time. EWBMS rainfall processing starts with the derivation of a multiple ‘local’ regression between the satellite derived cloud data and the precipitation data for each pixel that contains a rain gauge. This ‘local’ regression is based on the station under consideration and its 12 nearest neighbours. The resulting equation for station j is: Pj,est = Σ(aj,n · CDj,n) + bj (3.1) where CDn is the cloud duration (frequency) at cloud level n. The regression equation, however, is an imperfect estimator of precipitation P. Therefore at each station the residual Dj between the estimated and the observed precipitation is determined: Dj = Pj,obs – Pj,est (3.2) Subsequently, the regression coefficients aj,n, bj from (3.1) and the residual Dj from (3.2) are interpolated between 6 precipitation stations, using a weighed inverse distance method, so as to obtain the corresponding values for pixel i. The spatially distributed precipitation is finally calculated pixel by pixel with: Pi,est = Σ(ai,n · CDi,n) + bi + Di (3.3) Note that the estimated precipitation at the location of a station is always equal to the reported precipitation. In the current project, which includes considerable parts of the Tibetan plateau area, the previous technique has been extended so as to include effects of altitude. Such effects are insufficiently present in the point rainfall data,

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    because the measuring stations are usually at relatively low altitude. It is known that precipitation depends on the amount of precipitable water between the surface and the tropopause at ∼11km. Therefore precipitation can be expected to be proportional with the height or mass of the atmosphere column between the surface and the tropopause. We have investigated these options. A correction based on height gave the best result in the overall water balance of the upper Yellow river: Pi,cor = Pi,est · (Htrp – Hpix) / (Htrp – Hstat,avg) (3.4) Where Pi,cor is the corrected rainfall, Htrp the height of the tropopauze, Hpix the altitude of the pixel and Hstat,avg the average altitude of the 6 rain gauge stations involved. In this way the precipitation at high altitudes, where no meteorological stations are located, will be lower than at lower altitudes.

    Figure 3.3: The energy balance of the earth surface

    3.1.3 Energy balance monitoring

    The purpose of the energy balance monitoring component of the EWBMS is to determine the components of the surface energy balance, which reads:

    LE = In – H – E – G (3.5)

    where: LE = latent heat flux (W/m2)

    In = net radiation (W/m2) H = sensible heat flux (W/m2) E = photosynthetic electron transport (W/m2)

    G = soil heat flux (W/m2)

    Surface albedo and surface temperature are the main input data. The energy used for the evaporation of surface water (LE) equals the net radiation energy provided to the ground surface (In) minus the energy used for heating the air (H), the energy used by vegetation for photosynthetic electron transport and the energy for heating the soil (G). On a daily basis the soil heat flux can be considered zero (G≈0). Consequently, the surface energy balance can be rewritten as:

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    LE ≈ In – H – E (3.6)

    Only noon and midnight satellite images are used for the processing of energy fluxes. Fourier analysis of the daily solar cycle, a chopped cosine function, is used to relate the noon value of radiation and sensible heat flux to daily averages. As additional information the geographic coordinates and day number is required. This approach assumes that atmospheric transmission remains unchanged during the day. A correction to the daily radiation is applied based on cloud presence in the hours around noon.

    Atmospheric correction

    By calibration of the VIS and TIR infrared digital values the planetary albedo (reflectivity) and temperature are obtained, i.e. as observed through the atmosphere. However, to calculate the different components of the surface energy balance, the surface albedo and temperature are needed. To make this conversion, atmospheric corrections procedures are carried out. Absorption and scattering of solar radiation in the atmosphere cause the planetary albedo to differ from the surface albedo. Absorption in the atmosphere is mainly due to water vapour. Scattering occurs as a result of the presence of air molecules (e.g. N2, O2) and aerosols. Planetary temperatures are lower than actual surface temperatures because of the absorption and re-emission of infrared radiation by particularly water vapour. Scattering plays only a minor role. The atmospheric correction procedures have been designed such that they do not require information on atmospheric composition and stratification, but make use of reference information in the image, for example visual contrast. Image contrast will decrease with increasing atmospheric turbidity.

    For the visible band we use the global radiation transmission model of Kondratyev (1969). The model is applied to direct and diffuse solar radiation. Simultaneous differential equations are formulated for downward and upward global radiation fluxes separately. The down welling flux is direct; the up welling flux is diffuse (figure 3.4). The radiation transmission model of Kondratyev only accounts for backscatter and ignores absorption of radiation in the atmosphere. Atmospheric absorption, however, is in the order of 10%. An absorption factor (k) was introduced in the model. In the slab δτ in figure 3.4 the global flux is modified due to backscatter and absorption. The approximate differential equations are:

    Figure 3.4: Visualisation of downward and upward radiation fluxes.

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    δI/δτ = + a.I - b.J (3.7) δJ/δτ = - c.J + d.I (3.8) where: a = (α+k) / cos(is) b = 2α c = 2(α+k) d = α/cos(is) α = Backscatter coefficient of light (≈ 0.1) k = absorption coefficient of light (≈ 0.03) is = solar zenith angle

    The differential equations are solved analytically. As a result two functions may be derived. One relates the surface albedo to the planetary albedo and the optical depth (τ). The other relates the solar radiation transmission through a cloud free atmosphere (t) to the surface albedo and the optical depth. The optical depth is an indication of the amount of optical active matter in the atmosphere.

    A = f(A’ , τ) (3.9) t = f(A , τ) (3.10)

    with: A’ = observed planetary albedo (-)

    A = surface albedo (-) τ = optical depth of the atmosphere (-)

    Figure 3.5 shows the surface albedo and absorbed solar radiation as a function of planetary albedo and optical depth. Solar radiation absorbed by the earth’s surface is defined as 1 minus the surface albedo (1-A) times the transmission through the atmosphere (t). Once the optical depth is known, equation 3.9 converts the planetary albedo to a surface albedo for each pixel. The influence of the optical depth is highest at minimum surface albedo, which is found in densely vegetated areas. To determine the daily optical depth, the first step is to determine for every pixel in the image the minimum 10-daily planetary albedo. Subsequently the “darkest” pixels with the lowest planetary albedo are obtained. These are related to a minimum surface albedo. When sufficient dense forest is present in the image, this value is typically 7%. From the minimum planetary albedo and the minimum surface albedo, the optical depth can be calculated. Having determined the optical depth, which is applied for the whole image, the planetary albedo of each pixel can be converted to a surface albedo.

    0.00

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    Figure 3.5: Surface albedo and absorbed solar radiation as a function of planetary albedo and optical depth (τ). α=0.12, is=0, k=0.03

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    The transmission model has been compared with global radiation measurements. Some tuning of the EBMS global radiation output is required to achieve a better match. For this purpose, two calibration coefficients have been introduced in (3.10):

    t = C1 * f(A, C2 * τa) (3.11)

    There are two reasons to do so. First, the transmission through the atmosphere is determined at noon. On a daily basis the effective transmission may be somewhat lower because of lower solar inclinations during most of the day. Second, the transmission of solar radiation through the atmosphere is on average less than in the visible window. A best match with observed global radiation values has been obtained with the following values C1 = 0.77 and C2 = 0.65. For the thermal infrared band a different method of atmospheric correction is used. The relation between the planetary temperature (T0') and the surface temperature (T0) is described as:

    ( ) ( )a'0m

    a0 TT)icos(

    kTT −=− (3.12)

    wth: k = atmospheric correction coefficient

    im = satellite zenith angle Ta = air temperature at the top of the atmospheric boundary layer (K)

    The air temperature at the top of the boundary layer (Ta), is obtained on the basis of a linear regression between the noon and midnight pixel temperatures, as illustrated in figure 3.6. An estimate of the air temperature is found for the case of perfect heat transfer so that T0,noon = T0,midnight = Ta. The top of the atmospheric boundary layer varies at daytime usually between one and two kilometres. A map of the air temperature at the top of the boundary layer covering the whole region is obtained by applying this method to a shifting window of 200*200 km. In order to calculate the correction coefficient, the driest pixels in the image are selected and are assumed to correspond with the condition of no evapotranspiration. For each pixel a dryness index (DI) is calculated, which is defined as follows:

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