GIS-based Analysis and Visualization of the Groundwater Changes in Minqin Oasis

Zhenghua Yang LanZhou Geotechnical Investigation and Surveying

Institute 730030, China e-mail: yang.zhh@126.com

Haowen Yan* GIS Department, Lanzhou Jiaotong University

Lanzhou 730070, China e-mail: haowen2010@gmail.com

Yanping Liu GIS Department, Lanzhou Jiaotong University

Lanzhou 730070, China e-mail: 870564628@qq.com

Abstract—Groundwater depth refers to the distance from the ground surface to the groundwater surface and groundwater level refers to the elevation of the groundwater surface. The changes of groundwater depth and groundwater level are both the most directive reaction to the changes of the groundwater resources. This paper combines the Ordinary Kriging Interpolation method with the techniques from geographic information systems to analyze the dynamic changes of the groundwater depth and the groundwater level in Minqin Oasis in Januarys between 1998 and 2010. The results show that: (1) the groundwater depth is greater in the south while it is less in the north. It has declined year by year and the annual mean decline rate was 0.55m/a. The decline range has been gradually decreasing from the southwest to the northeast and the area with bigger groundwater depth has been becoming bigger and bigger due to the increasing of human activities in the Oasis. (2) The groundwater level is higher in the southwest whereas lower in the northeast and it has been continuously decreasing year by year. Meanwhile, the area of low level region has been gradually expanding from the northeast to the southwest.

Keywords-kriging interpolation; geographic information system; groundwate; MinQin Oasis

I. INTRODUCTION Minqin Oasis in Gansu province is located in the

downstream of Shiyang River to the east of Hexi Corridor[1]. It plays an extremely important role in hindering the meeting and invading the south of Tengger Desert and the Badain Jaran Desert. With the development of social science and technology and the rapid development of industry, agriculture in recent decades, the upstream and midstream areas have brutally taken use of the surface water resources. This has made the surface water into Minqin Oasis be greatly reduced, resulting in the contradiction between the water supply and the demand in the Oasis water day by day: the quantity of the groundwater being used has increased year by year; the overexploiting of the water is being exacerbated, and the water level is declining greatly.

The decline of the groundwater level has leaded to a sharp rise in the salinity of the groundwater, and the vegetation is degenerating and dying, and the process of the soil salinization and desertification has speeded up. In deed, the Oasis has begun to shrink. The unbalanced ecological system in Minqin Oasis has triggered a series of ecological environmental problems, and the extent and scope of the hazards has expanded gradually, which has attracted the attention of the researchers in the communities of geography and environment home and abroad. Therefore, it is of great importance to analyze the dynamic changing regular of the groundwater in Minqin Oasis; because not only does it provides scientific evidences for exploring and using the ground water reasonably, but also it has great realistic instructive functions on improving and protecting the local groundwater sources.

This study uses the Ordinary Kriging interpolation method in processing the data of groundwater depth during 1998 to 2010 for the purpose of getting the space-time evolution law of the groundwater depth, aiming at providing reliable evidences for reasonably managing， and efficiently utilizing the groundwater resources in the Oasis.

II. STUDIED AREA Minqin Oasis lies in the middle part of Minqin county

(102°45'E-103°55'E and 38°20 'N-39°10'N[2]). Red Cliff Mountain and Agoura Hills are in the south of the Oasis, The Oasis is surrounded by Tengger Desert and Badain Jaran Desert from its east, west and north respectively. It is one of the typical desert oases in China. In its south, the Oasis along the Shiyang River is connected with Liangzhou District. The most of agriculture and industry of the Minqin County focuses on the Minqin oasis [3]. The whole area of oasis is about 2,400 square kilometers [4], which accounts for about 15% the area of the whole county, including 13 townships (Fig.1).

This work was supported by the InnovativeTeam of the Ministry of Education (IRT0966)

Figure1. The location of Minqin Oasis.

Minqin oasis is in the temperate continental desert climate zone. It is hot in summer and cold in winter, and sufficient in sunlight; however, the temperature difference between day and night is very large. It is scarce of rainfall but the evaporation is intensive. Because of the appropriate climate conditions, it is especially suitable for the crops to grow[5]. Strong winds, flying sand and sandstorms occur frequently in spring.

III. DATA USED IN THE STUDY The data used in this study was collected from the Water

Affairs Bureau of Minqin County, including the positioning coordinates of seventy monitoring wells and the average observed values of the groundwater depth from the seventy monitoring wells in Januarys from 1998 to 2010.The coordinate system of the positioning data is Beijing Geodetic Coordinate System 1954 and the elevation system is the 1985 National Height Datum. The projection of the data is Gauss-Kruger Projection. The groundwater monitoring wells in oasis is evenly distributed (Fig.2).

Figure2. The location of groundwater monitoring wells

in Minqin Oasis.

IV. ANALYSIS OF THE GROUNDWATER DEPTH CHANGES The Ordinary Kriging Interpolation method of

Geostatistical Analysis in ArcGIS [6]is used to interpolate the average observed values of the groundwater depth in the Januarys from 1998 to 2010, and the results are as follows:

• In 1998, the average depth of the groundwater was 10.57m, which was greater in the southern part and

less in the northern part. There was a big groundwater funnel (the No.1 funnel) at the joint of Suwu Township, Dangba Township and Datan Township, whose area was 91.13 square kilometers. At the center of the No.1 funnel, the groundwater depth was 17.06-18.77m. Meanwhile, at the junction of Honshaliang Township and Quanshan Township, there also formed a groundwater funnel (the No.2 funnel), at the centre of which the groundwater depth was 14.22-15.72m. In 1998, the deepest groundwater depth in oasis was 18.77m, and the shallowest was 5.64m, and the area where the groundwater depth was lower than 15m has accounted for 73% of the Oasis (Fig.3).

Figure3. The distribution map of the groundwater depth

in January, 1998.

• In 2010, the mean groundwater depth in the Oasis has declined to 17.15m. Because people’s living standard has been improved, and the population has increased greatly and the industry has developed dramatically, the water consumption in the urban area of the Oasis increased rapidly, and the center of the No.1 funnel has been moved to the middle of the county, located in the junction of Suwu Township and Sanlei Township. At the center of the No.1 funnel, the groundwater depth was 28.62-31.43m. At the same time, the center of the No.2 funnel has been moved to Hongshaliang Township and its groundwater depth was also 28.62-31.43m; In Xiqu Township, there has been a funnel again (the No.3 funnel), and the groundwater depth of whose was 21.59-24.45m. In 2010, the deepest groundwater depth was 31.43m in oasis, and the shallowest was 6.00m. The area where the groundwater depth was above 15m has accounted for 86.03% of the Oasis. As see from Fig.4, there will have been a confluent trend between the No.1 funnel and the No.2 funnel.

Figure4. The distribution map of the groundwater depth

in January, 2010.

• From 1998 to 2010, the groundwater depth has declined by 6.58m from the 10.57m to the 17.15m, whose annual mean decline rate was 0.55m/a. During the past 13 years, it can be seen from Fig.5 that the decline range of the groundwater depth was relatively larger in the joint of Suwu Township, Dongba Township, Shuangcike Township and Datan Township in the south, where the groundwater depth was during 9m and 10m; In the north, the decline range of the groundwater depth was relatively smaller at those places where the Tengger Desert adjoins to Shoucheng Township and Donghu Township respectively, and the groundwater depth was during 4m and 5m.

Figure5. The decline range of the groundwater depth between

1998 and 2010.

V. ANALYSIS OF THE GROUNDWATER LEVER CHANGES Converting the average observed values of the

groundwater depth in Minqin County in Januarys from 1998 to 2010 to the values of the groundwater level respectively, and using the Ordinary Kriging Interpolation method of Geostatistical Analysis in ArcGIS to interpolate the values of the groundwater Level. The results have been shown as in the Figure 6 and Figure 7.

Figure6. The distribution map of the groundwater absolute elevation

in January, 1998.

Figure7. The distribution map of the groundwater absolute elevation

in January, 2010.

As see from Fig.6 and Fig.7: overall, the oasis groundwater level presented highly in the southwest, but lowly in the northeast. In Xuebai Township, the groundwater level was the highest, but in the Donghu Township and Xiqu Township, it was the lowest. In 1998, the highest groundwater level in oasis was 1405m and the lowest was 1302m, and the difference of them was 103m; In 2010, the highest groundwater level in oasis was 1403m and the lowest was 1289m, and the difference of them was 103m. All of those are mainly related to the terrain of oasis. Over the past 13 years, the oasis groundwater level has been continuously decreasing year by year. Meanwhile, the area of low level region has been gradually expanding from the northeast to the southwest.

In recent years, because of the excessive exploitation and utilization of the groundwater in Oasis, the mineralization of groundwater has rised continuously and the soil salinization has become more and more serious. Those conditions are particularly serious in Lake District which includes Xiqu Township, Donghu Township and Shoucheng Township. At present, the groundwater in the Lake District has not been appropriate for the irrigation and drinking for the local and there has been 30 thousand people immigrating to outside, becoming the ecological refugees [7].

VI. CONCLUSION This paper uses Geostatistical Analysis method to

simulate the features of the temporal and space variations in the groundwater depth and the groundwater level. The results basically reflect the local actual situation. On the whole, the spatial distribution of the groundwater depth and the groundwater level in the Oasis were nonuniform. The groundwater depth has declined dramatically from 1998 to 2010 and its decline range has been decreasing gradually from the southwest to the northeast. From 1998 to 2010, the oasis groundwater level has been continuously decreasing year by year. The reasons for the results are as follows: it mainly caused by the decrease of the quantity of the upstream inflow and the excessive exploitation of the groundwater resources in the Oasis. Therefore, the urgent business for alleviating the crisis of the groundwater resources in the Oasis is to increase the quantity of the upstream and reduce the exploitation of the groundwater.

ACKNOWLEDGMENT Yanping Liu thanks the Water Affairs Bureau of Minqin

County for providing the data used in the experiments.

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