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Government of Western AustraliaDepartment of WaterGD

Ord River Irrigation Area annual groundwater elevation

and water-table depth 1995 to 2008

Anthony Smith, Daniel Pollock and Duncan Palmer

Water for a Healthy Country Flagship Report series ISSN: 1835-095X

Australia is founding its future on science and innovation. Its national science agency, CSIRO, is a powerhouse of ideas, technologies and skills.

CSIRO initiated the National Research Flagships to address Australia’s major research challenges and opportunities. They apply large scale, long term, multidisciplinary science and aim for widespread adoption of solutions. The Flagship Collaboration Fund supports the best and brightest researchers to address these complex challenges through partnerships between CSIRO, universities, research agencies and industry.

The Water for a Healthy Country Flagship aims to achieve a tenfold increase in the economic, social and environmental benefits from water by 2025. The work contained in this report is collaboration between CSIRO and Rangelands NRM Western Australia, Department of Water Western Australia, Ord Irrigation Cooperative and Australian Government.

For more information about Water for a Healthy Country Flagship or the National Research Flagship Initiative visit www.csiro.au/org/HealthyCountry.html

Citation: Smith, A., Pollock, D. and Palmer, D., 2009. Ord River Irrigation Area annual groundwater elevation and water-table depth 1995 to 2008. CSIRO: Water for a Healthy Country National Research Flagship. Rev. A.

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CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.

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CONTENTS Acknowledgments ........................................................................................................ v

Executive Summary..................................................................................................... vi 1. Introduction ......................................................................................................... 1

1.1. This report................................................................................................................. 1

2. Contextual Information for the ORIA................................................................. 2 2.1. Physiography ............................................................................................................ 2 2.2. Climate...................................................................................................................... 2 2.3. Irrigation and drainage.............................................................................................. 3 2.4. Groundwater ............................................................................................................. 3

3. Methods................................................................................................................ 7 3.1. Data storage and analysis ........................................................................................ 7 3.2. Aggregation of data to annual water levels .............................................................. 7 3.3. Removal of outliers ................................................................................................... 7 3.4. Construction of annual water-table surfaces ............................................................ 8 3.5. Calculation of depth to water table ........................................................................... 9 3.6. Processing of data from years 1994–95 to 2005–06................................................ 9 3.7. Processing of data from years 2006–07 and 2007–08............................................. 9

4. Results and Discussion.................................................................................... 12 4.1. Trend of annual depth to water table ...................................................................... 12

4.1.1. Years 2006–07 and 2007–08 ..............................................................................12 4.2. Year of maximum annual water level...................................................................... 14 4.3. Northern Ivanhoe Plain, Martins Location and Cave Spring Gap........................... 16 4.4. Central Ivanhoe Plain.............................................................................................. 17 4.5. Southern Ivanhoe Plain .......................................................................................... 18 4.6. Packsaddle Plain .................................................................................................... 19

5. Conclusions....................................................................................................... 21

6. Recommendations ............................................................................................ 22

References .................................................................................................................. 65

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LIST OF FIGURES Figure 1. Ord River Irrigation Area Stage 1 ........................................................................... viii Figure 2. Annual wet season rainfall at Kununurra Airport ...................................................... 3 Figure 3. Interpreted distributions of (a) deep, basal sand and gravel, (b) intermediate depth, overlying sand and (c) surface clay and silt (Reproduced from: Smith 2008) .............. 5 Figure 4. Water-table cross sections for (a) Section A–A’ Packsaddle Plain, (b) Section B–B’ central and northern Ivanhoe Plain and (c) Section C–C’ northern Ivanhoe Plain and Cave Spring Gap; the cross-section lines are marked on Figure 3 (Reproduced from: Smith 2008) .................................................................................................................... 6 Figure 5. Water-level data from piezometer PN9D; grey symbols are unprocessed water levels; red symbols and red line are the annual water levels; the green line is ground surface elevation at the piezometer location ............................................................... 8 Figure 6. Piezometer point data and control elements used to construct the annual water-table surfaces for the period 1994–95 to 2005–06 ...................................................... 10 Figure 7. LiDAR digital terrain model acquired in September 2006....................................... 11 Figure 8. Trend in annual depth to water table 1995–96 to 2005–06 .................................... 13 Figure 9. Annual depth to water table 2006–07 and 2007–08............................................... 14 Figure 10. Year of maximum water-table elevation 1995–96 to 2005–06 ............................. 15 Figure 11. Water-table cross section A-A’ through northern Ivanhoe Plain and Cave Spring Gap............................................................................................................................. 16 Figure 12. Water-table cross section B-B’ through northern Ivanhoe Plain and Martin’s Location ................................................................................................................................. 17 Figure 13. Water-table cross section C-C’ through northern Ivanhoe Plain and Martin’s Location ................................................................................................................................. 17 Figure 14. Water-table cross section D-D’ through central Ivanhoe Plain ............................. 18 Figure 15. Water-table cross section E-E’ through southern Ivanhoe Plain .......................... 19 Figure 16. Water-table cross section F-F’ through Packsaddle Plain.................................... 20

LIST OF TABLES Table 1. Average wet season rainfall at Kununurra Airport for various time periods............... 3

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ACKNOWLEDGMENTS External funding for this study was provided through the National Action Plan for Salinity and Water Quality (NAP) program. The project was proposed and conducted with overview from the East Kimberley Reference Group, which is a subregional group of Rangelands NRM Western Australia.

We also wish to thank the following people and organisations for their contributions to the project:

Liz Brown – Rangelands NRM Western Australia

Anna Price – Brolga’s Environment

Gae Plunkett – Department of Agriculture and Food Western Australia

Chris Robinson – Department of Agriculture and Food Western Australia

Paul Wilkes – Curtin University

Dr Olga Barron – CSIRO Land and Water

Dr Elise Bekele – CSIRO Land and Water

Western Australia Department of Water

Department of Agriculture and Food Western Australia

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EXECUTIVE SUMMARY The Ord River Irrigation Area (ORIA) is located in the far northeast of Western Australia approximately thirty kilometres from the Northern Territory border and one-hundred kilometres from the northern coast (Figure 1). The irrigation area was established as a cotton monoculture during the early 1960s but cotton has not been grown commercially since the mid-1970s. The ORIA produces a variety of crop types including horticulture and field crops with the most recent introduction being Indian Sandalwood.

This report presents maps of annual water-table elevation and annual depth to water table within the ORIA from 1995 to 2008. During the thirty years preceding 1995 the water-table elevation throughout much of the 16,000 hectare irrigation area had already increased by as much as 10 to 15 metres. The thirteen-year period examined in this study encompassed both the maximum historical water-table elevation observed since irrigation commenced and the apparent recent stabilisation of groundwater rise.

A series of thirteen annual water-table surfaces was produced for the period 1994–95 to 2007–08 using a Geographic Information System (GIS). Each annual map was based on the groundwater elevation data collected between 1 July and 30 June during that year to encompass one full wet season per map. The maps are compiled in Appendix C.

A corresponding series of maps depicting annual depth to water table was produced by computing the difference in elevation between the LiDAR (Light Detection and Ranging) digital terrain model acquired in September 2006 and the annual water-table surfaces produced in this study. The resulting maps are compiled in Appendix B.

Representative cross-sections of the ground surface elevation and annual water-table surfaces were produced for Cave Spring Gap and northern Ivanhoe Plain, Martins Location, central and southern Ivanhoe Plain and Packsaddle Plain.

Results and Discussion Between 1994–95 and 2001–02 the annual water-table surface throughout the irrigation area generally rose in response to above-average rainfall. During the following 3–4 years the water table receded by several metres and groundwater elevations returned to similar levels experienced in the mid-1990s. The sustained decline in the groundwater level after 2001–02 was the first time since irrigation had commenced that the water table had consistently fallen on an annual basis. This provided the first evidence in forty years that groundwater discharge from the aquifer had increased and water-table rise had begun to stabilise.

Beneath northern Ivanhoe Plain and Cave Spring Gap the groundwater elevation was controlled principally by recharge from rainfall and groundwater interaction with the irrigation supply-channel and drain networks. Since around 2000, groundwater drainage from Cave Spring Gap toward drain D4 on northern Ivanhoe Plain has been apparent. Prior to that, the direction of subsurface drainage was generally north-east from Ivanhoe Plain through the sand and gravel beds within Cave Spring Gap. An area of shallow depth to water table persists within the south-west part of Cave Spring Gap.

Areas of shallow water table that were less than two metres below the ground surface had developed beneath Martins Location by the late-1990s. In the central part of Martins Location a plantation of Mahogany trees established in 1997 subsequently drew down the water-table by up to eight metres directly beneath the trees. Elsewhere the aquifer is poorly drained and areas of shallow water table persist outside of the plantation. The groundwater elevation is deepest to the east where the aquifer is still filling in response to clearing and irrigation development in 1995.

Water-table fluctuation within the central part of Ivanhoe Plain was 3–4 metres since 1995. Depth to water table below the ground surface was generally greater than four metres and was smallest around 2001— at which time the water-table elevation was at the historical

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maximum value in the area. To the east, the groundwater level was controlled mainly by the Ord River; however, the much steeper water-table slope near to the river indicates a restricted connection to the conductive sand and gravel beds within the aquifer.

Groundwater elevation under the southern part of Ivanhoe Plain was especially steady, with less than one metre variation during the past thirteen years. The water-table elevation was strongly controlled by Lake Kununurra in the south and east, and by the lower Ord River in the west. Depth to water table generally increased in a westerly direction toward the river and was mostly greater than four metres below the ground surface.

The aquifer beneath Packsaddle Plain filled initially by rapid leakage of surface water from Lake Kununurra into the conductive sand and gravel beds underlying the irrigation area. Subsequent irrigation development during the 1970s further raised the groundwater elevation. Between 1994–95 and 2007–08 the depth to water table beneath Packsaddle Plain was generally 2–4 metres below ground surface. Nevertheless, shallow groundwater zones persist around the shore of Lake Kununurra and along the banks of Packsaddle Creek where there is groundwater associated salinity.

Conclusions Careful analysis of groundwater-level records to produce annual values, and the production of annual water-table surfaces and depth-to-water-table maps is a valuable approach for tracking and reporting long-term trends in groundwater level beneath the ORIA. In the past, water-table maps were produced infrequently and without consideration or explanation of seasonal variation.

Determining changes in the groundwater condition throughout the irrigation area on an annual basis is difficult and uncertain without the collection of adequate water-level data and careful analysis of that data. Seasonal variation of groundwater level is typically greater than the change in mean annual level from year to year and therefore the seasonality must be removed to make valid comparisons across different years. This requires multiple measurements per year in both the wet and dry seasons.

To achieve consistency between annual maps also requires a consistent set of groundwater measurement locations, and an explicit and repeatable method for interpolating the point measurements to generate a continuous water-table surface.

Spatial interpolation of groundwater-elevation values between the measurement locations is a critical aspect of the analysis. An advantage of implementing the method using a GIS is the ability to automate or semi-automate the analysis and visualisation procedures. Previous maps can then be reproduced or updated if the method is improved by new knowledge or better spatial analysis techniques.

Recommendations 1. That suitable groundwater elevation data be collected each year within the ORIA to

enable the production of a reliable annual water-table surface that can be compared with the results from previous years.

2. That the optimum set of groundwater monitoring locations required to produce a reliable annual water-table surface be identified and agreed to, and that those locations be monitored consistently every year.

3. That groundwater elevation at each monitoring location is measured on a minimum of two occasions per year; preferably once at the end of the dry season and once as soon as possible at the end of the wet season.

ORIA Annual Water Table Mapping Page viii

Figure 1. Ord River Irrigation Area Stage 1

Report Title Page 1

1. INTRODUCTION The Ord River Irrigation Area (ORIA) Groundwater Drainage and Discharge Evaluation project was a collaborative study by the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Western Australian Department of Water (DoW), Department of Agriculture and Food Western Australia (DAFWA), Ord Irrigation Co-operative and the Governments of Western Australia and Australia through the National Action Plan for Salinity and Water Quality (NAP).

The broad objectives of the study were to:

1. Assess the quality of groundwater, including salinity, major ions, nutrients and pesticide concentrations, in relation to groundwater discharge management in the ORIA;

2. Delineate and quantify groundwater discharge zones within the ORIA, including groundwater drainage to existing irrigation channels;

3. Evaluate the feasibility of using subsurface drains to control water-table rise in areas that are susceptible to water logging and salinity development; and

4. Evaluate the potential role of commercial tree plantings in managing aquifer water balance, including measurement and estimation of groundwater uptake by trees and net groundwater recharge rates beneath them.

1.1. This report This report addresses item 2 of the project objectives. It presents annual maps of water-table elevation and water-table depth below ground surface in the ORIA for the period 1995 to 2008. The maps provide a summary of annual groundwater conditions and the year to year change in those conditions allowing for seasonality due to rainfall and irrigation and with data outliers removed.

The results were used elsewhere in this project to evaluate groundwater flow directions in the ORIA, to assess annual water-table trends; to identify potential groundwater discharge areas; to classify soil-salinity risk; and to assist in the selection of field sites for other components of the study.

This report includes relevant contextual information about the ORIA, descriptions of the methods used to produce the maps, and a brief discussion of the results. Conclusions and recommendation from the study also are presented.

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2. CONTEXTUAL INFORMATION FOR THE ORIA The Ord River Irrigation Area (Figure 1) was established during the 1960s and 1970s as part of a three-stage plan:

Stage 1: construction of the Kununurra Diversion Dam to form Lake Kununurra; the associated works required to irrigate 10,000 hectare (ha) of land on Ivanhoe Plain; and construction of the township of Kununurra.

Stage 2: construction of the Ord River Dam to form Lake Argyle upstream of Kununurra; and the associated works and supporting infrastructure required to irrigate an additional 60,000 ha of land.

Stage 3: construction of a hydro electric power station on the Ord River Dam and the associated transmission infrastructure

Stage 1 was virtually complete by 1965. The Ord River Dam was formally opened in June 1972 but only around 2,800 ha of the additional 60,000 ha proposed for Stage 2 were developed on Packsaddle Plain. The hydro electric power station was commissioned in October 1996. Further development of the Ord Stage 2 area is subject to ongoing consideration by the Government of Western Australia and the Australian Government.

2.1. Physiography The Ord Stage 1 area is located on the palaeo-alluvial floodplain of the lower Ord River which is bounded along its flanks by rocky ranges of outcropping sandstone and basalt. The relict floodplain extends up to 10 kilometre (km) in width across the river axis. Land elevation varies by only around 10 m within the irrigation area whereas the surrounding ranges rise 300 m to 400 m above the plain.

Packsaddle irrigation area is located on Packsaddle Plain to the south and west of Lake Kununurra, which is held by the Kununurra Diversion Dam. The southern and eastern sides of Packsaddle Plain are bounded by the Carr Boyd Ranges. Ivanhoe Irrigation area is located on Ivanhoe Plain to the north of Lake Kununurra and between the east bank of the Ord River and the western edge of the Pincombe Range. Ivanhoe Plain extends into Martin’s Location, Green Location and Cave Spring Gap.

The township of Kununurra is located approximately 30 km west of the Northern Territory border and around 100 km from the north coast of Western Australia. It is more than 2,000 km from Perth (>3,000 km by road) and approximately 400 km from Darwin (>800 km by road).

2.2. Climate The Ord River region has a wet-dry tropical climate characterised by well-defined wet and dry seasons. Approximately ninety per cent (90 %) of the annual precipitation falls during the wet season between mid-November and March. The average July to June rainfall at Kununurra Airport from 1960–61 to 2007–08 was 796 millimetre per year (mm/yr), although there was significant variation between years (Figure 2). During the same period, the maximum Jul Jun rainfall was 1476 mm in 1999–00 and the minimum was 366 mm in 1969–70.

Following the 1976–77 wet season the irrigation area experienced a sixteen-year period of below average rainfall, which was 15% below the long-term average (Table 1). During the recent wet period from 1998–99 to 2001–02 the total rainfall was 45% greater than the long-term average.

Air temperatures are typically high-to-very-high throughout the year. July is the coolest month, with mean daily maxima and minima of 31 degree Celsius (°C) and 14 °C,


respectively. November is the hottest month, with mean daily maxima and minima of 39 °C and 25 °C. Average annual pan evaporation for 1960 to 2007 was around 2760 mm/yr; greater than three times rainfall. Average monthly evaporation exceeds average monthly rainfall in all months except February.

Table 1. Average wet season rainfall at Kununurra Airport for various time periods

Wet Seasons Period, yrs Average Annual Wet Season Rainfall, mm/yr

1960–61 to 2007–08 48 796

1976–77 to 1991–92 16 673

1998–99 to 2001–02 4 1154

Figure 2. Annual wet season rainfall at Kununurra Airport

2.3. Irrigation and drainage Around 12,000 ha of irrigable land within the Packsaddle and Ivanhoe irrigation areas is serviced by approximately 134 km of clay-lined supply channels and 155 km of unlined surface drains. Downstream of Lake Kununurra on Ivanhoe Plain, the irrigation supply channel system diverts water from Lake Kununurra, through the irrigation area and back into the lower Ord River at various drainage and relief points below the dam. On Packsaddle Plain, which is directly upstream of the diversion dam, lake water is raised by pumping into the supply channel system and the return water flows into the Dunham River via Packsaddle Creek or directly back into Lake Kununurra via irrigation drains.

2.4. Groundwater Groundwater exists in the alluvium and bedrock throughout the Ord region. It is used for public and private potable water supply in the Stage 1 area and is a minor supply for irrigation in areas outside of the channel system. The groundwater quality is variable and can be unsuitable for both drinking and irrigation (Smith et al. 2007).


The palaeo-Ord valley is filled by a complex, aggraded sequence of fluvial sediments that were generally deposited on the palaeo-topographic surface as a sequence of upward-fining gravel, sand, silt and clay (O’Boy et al. 2001, Lawrie et al. 2006). At the valley-scale, these sediments broadly constitute four unconfined aquifer units consisting of: (1) deep, basal sand and gravel palaeochannel sediments with moderate to very high permeability; (2) intermediate depth, overlying sand with moderate permeability; (3) overlying silt, sandy-silt and silty-sand with low permeability, and (4) surface clay and silt with low permeability. In Figure 3, the upper clay, silt and sandy-silt layers are combined. The water-table cross sections presented in Figure 4 depict the contemporary water-table elevation as well as the earliest known water-level recording for each piezometer—which is generally also the minimum recorded water level—and the maximum known water-level recording.

The water table underlying the irrigation area was relatively deep prior to agricultural development in the early 1960s and it is likely that groundwater was replenished mostly by seasonal surface-water infiltration when the Ord River was flowing and during inundation events. Direct groundwater replenishment from rainfall was probably very small because the water table was deep and the soil profile could absorb and store seasonal rainfall without significant deep drainage.

During the past forty years the water table has risen by 10–20 m beneath most of the irrigation area due to hydrological impacts of agricultural development (Smith 2008). The direction of subsurface drainage is now mainly toward the Ord River, Lake Kununurra, Packsaddle Creek and other surface water drains. More recently, the water table beneath northern Ivanhoe Plain intercepted the deepest irrigation drains, which are helping to prevent further groundwater rise.


Figure 3. Interpreted distributions of (a) deep, basal sand and gravel, (b) intermediate depth, overlying sand and (c) surface clay and silt (Reproduced from: Smith 2008)


Figure 4. Water-table cross sections for (a) Section A–A’ Packsaddle Plain, (b) Section B–B’ central and northern Ivanhoe Plain and (c) Section C–C’ northern Ivanhoe Plain and Cave Spring Gap; the cross-section lines are marked on Figure 3 (Reproduced from: Smith 2008)


3. METHODS

3.1. Data storage and analysis A semi automated approach was developed to manage the task of producing annual water-table maps and to analyse the large amount of available water-level data. All data obtained for this project were imported into a Microsoft Access database. Data aggregation and analysis were performed using 1SQL queries and the results were plotted using 2Python and 3R. The maps were produced using 4ArcGIS.

3.2. Aggregation of data to annual water levels Seasonality and irregular sampling can have unwanted consequences for calculating annual values based on simple arithmetic means. Sampling bias was common in the water-level data and occurred at various time scales; for example, when many water-level measurements were taken during a particular week, month or season but fewer measurements were taken during intervening periods.

To overcome this, water-level data from each groundwater-monitoring location were sequentially aggregated (i.e., summed and averaged) at daily, then monthly, then half-yearly and, finally, yearly time intervals. By that procedure, all water-level values within each day were averaged to obtain a single mean-daily water level for all days with data; those daily means were then averaged to obtain a mean-monthly water level for all months with data—and so on. The final values were considered to provide reasonable estimates of the annual water level for each year within the period of interest.

An example result is presented in Figure 5, which depicts the unprocessed and processed water-level data for piezometer PN9D. The unprocessed measurements (grey symbols) were collected irregularly and exhibit seasonally. The annual water levels (red symbols and red line) pass smoothly through that data and provide a useful impression of annual water-table change during the ten-year period of interest. Similar graphs for all of the piezometers used to produce the water-table maps are presented in Appendix A.

3.3. Removal of outliers Outliers can be identified using statistical tests; however, a simple visual method was used in this study. After completing an initial calculation of the annual water level for each year, the graphs of the unprocessed and processed data (Appendix A) were compared visually. If it was apparent that an annual water-table estimate was unreasonably influenced by incongruous data then those unprocessed data were tagged as outliers and the calculations were re-done ignoring those values. An example is evident in Figure 5; the anomalous water-level value in 1995 was considered to be an outlier and was ignored in the calculation of the annual water level for that year.

1 Structured Query Language 2 Python is an interpreted, interactive, object-oriented programming language and is the scripting language incorporated with ArcGIS (web: http://www.python.org/) 3 R is a free software environment for statistical computing and graphics (web: http://www.r-project.org/index.html) 4 ArcGIS is an integrated collection of GIS software produced by Environmental Systems Research Institute (ESRI) (web: http://www.esri.com/)


Figure 5. Water-level data from piezometer PN9D; grey symbols are unprocessed water levels; red symbols and red line are the annual water levels; the green line is ground surface elevation at the piezometer location

3.4. Construction of annual water-table surfaces Annual water-table maps were produced for each year using the processed annual water-level values and spatial analysis tools available in ArcGIS. Figure 6 depicts the piezometer locations, the hull area for producing the water-table maps and the other control elements that were used. A control element was aligned parallel to the bank of the Ord River in the central part of Ivanhoe Plain, where the water-table elevation data near to the river were sparse, to ensure relatively uniform water-tables slopes along the river bank.

The following procedure was applied for each map:

1. At each measurement location the water level was set equal to the annual value determined by the above sequential-averaging method

2. Points around the border of the map and along a control element added to central Ivanhoe Plain were assigned annual water levels by extrapolation of the above piezometer point data using inverse distance weighting

3. Points along the shoreline of Lake Kununurra were assigned fixed annual water levels equal to the average water level in the lake

4. Points along the bank of the Ord River, below the Kununurra Diversion Dam, were assigned annual water levels based on field measurements taken in October 2002

5. The annual water-table surface was constructed as a Triangular Irregular Network (TIN) using all of the above point-data locations and values

6. Finally, the water-table surface was slightly smoothed to improve the curvature of the water-table contours without significantly changing the match between the observed point-data values and the contoured-data values at those locations.


3.5. Calculation of depth to water table Annual depth to water table for each year was calculated as the difference in elevation between the LiDAR (Light Detection And Ranging) digital terrain model depicted in Figure 7 and the annual water-table surfaces determined by the above procedure.

3.6. Processing of data from years 1994–95 to 2005–06 The first series of depth-to-water-table maps for the years 1994–95 to 2005–06 were produced in 2006 using a digital elevation model (DEM) that was developed by CSIRO for groundwater modelling of Ivanhoe and Packsaddle Plains (Smith et al. 2006). Those maps were redone in 2008 using the newly-acquired and more-accurate LiDAR DEM. Two additional maps also were produced for the interceding years 2006–07 and 2007–08.

When the initial series of maps was created in 2006 the pre-existing groundwater monitoring data was analysed to determine a set of groundwater bores with reliable water-level data for every year within that period. This approach enabled each annual water-level map to be produced using the same set of measurement locations and prevented apparent year-to-year changes that can result from year-to-year differences in the spatial interpolation. The set of measurement locations used for that period are depicted in Figure 6 and on the maps for years 1994–95 to 2005–06 in Appendix B and Appendix C.

3.7. Processing of data from years 2006–07 and 2007–08 The same set of monitoring bores used for 1994–95 to 2005–06 could not be used in 2006–07 and 2007–08 due to a change in the groundwater monitoring program after 2005–06. Some of the previous measurement locations were common during those years; however, others were missing and a few were additional. Thus, the 2006–07 and 2007–08 maps were produced using the available groundwater monitoring data for those years; the locations are depicted in Appendix B and Appendix C. Several inconsistencies between the pre- and post-2005–06 maps are apparent, which are discussed in the results and discussion section following.


Figure 6. Piezometer point data and control elements used to construct the annual water-table surfaces for the period 1994–95 to 2005–06


Figure 7. LiDAR digital terrain model acquired in September 2006


4. RESULTS AND DISCUSSION This section of the report presents selected maps and cross-sections produced from the results. The full series of maps depicting the annual depth to water table and annual water-table elevation are compiled in Appendix B and Appendix C, respectively.

4.1. Trend of annual depth to water table In Figure 8 and Figure 9 the maps of annual depth to water table are arranged chronologically to provide a general impression of water-table change throughout the irrigation area during the past thirteen years. From 1994–95 to 2001–02 the water-table elevation generally increased on an annual basis and then began to recede from around 2002–03. The sustained decline in the groundwater level after 2002–03 was particularly significant because it was the first time that the water table had consistently fallen throughout the irrigation since irrigated agriculture was established during the 1960s (Smith et al. 2006).

It now appears that during the mid-1990s the water-table elevation had begun to stabilised relative to the mean long-term rainfall; however, four consecutive years of above-average rainfall from 1998–99 to 2001–02 (Table 1) continued to raise the groundwater level during that period. Subsequent recession of the water table coincided with a return to average rainfall conditions from around 2002–03. Although some parts of the aquifer are still filling, it seems likely that the groundwater level throughout much of the irrigation area will remain at around its current elevation, with 1–2 metre interannnual variation in response to rainfall variation and current land use.

4.1.1. Years 2006–07 and 2007–08 The methods section explains that the maps for years 2006–07 and 2007–08 were produced using a different set of measurement locations due to a change in the groundwater monitoring program within the irrigation area. In particular, the number of measurement locations within Martins Location (Figure 1) and near to the Ord River on northern Ivanhoe Plain was insufficient in those years to produce reliable water-table surfaces for those areas. The locations are indicated by “no data” labels on Figure 9 and the maps for these should be disregarded in those regions.

This change in the quality of result emphasises the importance of collecting reliable water-table information each year from a consistent set of groundwater monitoring locations.


Figure 8. Trend in annual depth to water table 1995–96 to 2005–06


Figure 9. Annual depth to water table 2006–07 and 2007–08

4.2. Year of maximum annual water level The four-year period of above-average rainfall from 1998–99 to 2001–02 resulted in an historical maximum water-table elevation throughout most of the irrigation area. This pattern is evident in Figure 10, which depicts the year of maximum water table elevation across the irrigation area. The areas rendered in yellow and green shades experienced a maximum historical groundwater elevation prior to or during 2001–02. Smaller areas rendered in red are currently at their maximum historical elevation values, indicating that the aquifer is still filling at those locations. Notably, the water table is still rising in the eastern part of Martins Location which was not developed until 1995. At that time the water-table depth below the ground surface was around 15 m but the water table began to rise almost immediately after the area was cleared (Smith et al. 2006).

Areas around the margin of Lake Kununurra are rendered in blue tones indicating that the water-table elevation was highest in 1995–96. This reflects a slightly higher lake level at that time and coupling between water-table elevation and lake level along the shoreline. The currently lower groundwater level is beneficial for groundwater management and was caused by lowering of the lake level for operational improvements (Chafer, T. CEO Ord Irrigation Cooperative, 2007, pers. comm. 25 June).

The maximum historical water-table elevation in the central part of Martins Location (Figure 1) also occurred earlier than elsewhere. This was caused by the establishment of a Sandalwood–Mahogany plantation in around 1997 (Carter et al. 2009). From around 2001, the plantation was unirrigated and significant growth and uptake of groundwater by the Mahogany trees lowered the water table beneath the plantation by up to eight metres during the following five years.


Figure 10. Year of maximum water-table elevation 1995–96 to 2005–06


4.3. Northern Ivanhoe Plain, Martins Location and Cave Spring Gap Figure 11 to Figure 13 present cross-sections through the northern part of Ivanhoe Plain, Martins Location and Cave Spring Gap. They depict the LiDAR ground-surface elevation and selected annual water-table profiles along the cross-section lines indicated on Figure 10. Both the pattern of general water-table rise from 1995–96 to around 2001–02 and the more recent stabilisation of the water table can be observed.

In particular, it has become evident that drain D4, which is typically 3–4 metres deep, has been receiving a significant quantity of groundwater discharge since the late-1990s. From 1999–00 onward there has been a significant component of groundwater drainage from Cave Spring Gap toward northern Ivanhoe Plain and drain D4. Prior to that, the direction of subsurface drainage was generally north-east from northern Ivanhoe Plain through the sand and gravel beds within Cave Spring Gap. An area of shallow depth to water table persists within the south-west part of Cave Spring Gap.

Sections B–B’ and C–C’ both depict the water-table drawdown beneath the Sandalwood-Mahogany plantation in Martins Location. The water-table depression under the trees is not evident in the water-table profile for 2007–08 because insufficient groundwater levels were collected that year. Section B–B’ also features the area of deeper water table beneath the north-east part of Martins Location where the aquifer is still filling.

A steeper water-table gradient adjacent to the Ord River at the south-west end of section B–B’ signifies that there is reduced connectivity between the river and conductive sand and gravel beds within the aquifer. This restriction of the subsurface flow prevents effective drainage of the aquifer to the river from northern Ivanhoe Plain.

Figure 11. Water-table cross section A-A’ through northern Ivanhoe Plain and Cave Spring Gap


Figure 12. Water-table cross section B-B’ through northern Ivanhoe Plain and Martin’s Location

Figure 13. Water-table cross section C-C’ through northern Ivanhoe Plain and Martin’s Location

4.4. Central Ivanhoe Plain Figure 14 depicts ground surface elevation and annual water-table profiles through the central part of Ivanhoe Plain along section D–D’.

To the east, the watertable is controlled mainly by the Ord River. A steeper water-table gradient near to the river indicates restricted subsurface drainage—as is the case further north. Elsewhere in this area, the variation in the water-table elevation during the past


thirteen years was around 3–4 metres. The annual depth to water table below the ground surface was generally greater than 4 metres throughout, including in 2001–02 when the water-table elevation was at its historical maximum value in the area.

Figure 14. Water-table cross section D-D’ through central Ivanhoe Plain

4.5. Southern Ivanhoe Plain Figure 15 depicts the ground-surface elevation and the annual water-table profiles along section E–E’ through southern Ivanhoe Plain.

Throughout the area the water-table elevation is strongly controlled by the Ord River to the west and Lilly Creek Lagoon to the south and east. The lagoon is an inlet of Lake Kununurra and has the same surface water level as the lake. Lake Kununurra and the lower Ord River are apparently well-connected by conductive sediment beds, which constantly transmit lake water through the aquifer to the river. Fluctuation of the annual groundwater level in response to rainfall and irrigation was less than one metre during the past thirteen years.


Figure 15. Water-table cross section E-E’ through southern Ivanhoe Plain

4.6. Packsaddle Plain Figure 16 depicts the ground-surface elevation and the annual water-table profiles along section F–F’ through Packsaddle Plain, where the elevated groundwater level is controlled principally by the water level in Lake Kununurra and groundwater drainage into Packsaddle Creek. The aquifer beneath Packsaddle Plain formed initially as a subsurface extension of the lake but subsequent irrigation development in the 1970s raised the water table above the lake in some parts.

Prior to construction of the Ord River Dam and formation of Lake Argyle in 1972, Lake Kununurra filled seasonally and was below full capacity for around eight months each year. Since then, there has been sufficient up-stream storage in Lake Argyle to maintain Lake Kununurra at full capacity, with only minor level adjustments for operational requirements.

From 1994–95 the annual depth to water table below the ground surface was generally 2–4 metres throughout but was shallower in the lowest parts of the landscape, including around the shore of Lake Kununurra and along the banks of Packsaddle Creek where salinisation has occurred due to prolonged groundwater evaporation (Smith and Price 2009). The contemporary water-table elevation is similar to the water-table elevation in the mid-1990s and was around 1–2 metre higher in around 2002.


Figure 16. Water-table cross section F-F’ through Packsaddle Plain


5. CONCLUSIONS Careful analysis of groundwater-level records to produce annual values, and the production of annual water-table surfaces and depth-to-water-table maps is a valuable approach for tracking and reporting long-term trends in groundwater level beneath the ORIA. In the past, water-table maps were produced infrequently and without consideration or explanation of seasonal variation.

Determining changes in the groundwater condition throughout the irrigation area on an annual basis is difficult and uncertain without the collection of adequate water-level data and careful analysis of that data. Seasonal variation of groundwater level is typically greater than the change in mean annual level from year to year and therefore the seasonality must be removed to make valid comparisons across different years. This requires multiple measurements per year in both the wet and dry seasons.

To achieve consistency between annual maps also requires a consistent set of groundwater measurement locations, and an explicit and repeatable method for interpolating the point measurements to generate a continuous water-table surface.

Spatial interpolation of groundwater-elevation values between the measurement locations is a critical aspect of the analysis. An advantage of implementing the method using a GIS is the ability to automate or semi-automate the analysis and visualisation procedures. Previous maps can then be reproduced or updated if the method is improved by new knowledge or better spatial analysis techniques.


6. RECOMMENDATIONS The following recommendations are made based on the results of this study:

1. That suitable groundwater elevation data be collected each year within the ORIA to enable the production of a reliable annual water-table surface that can be compared with the results from previous years.

2. That the optimum set of groundwater monitoring locations required to produce a reliable annual water-table surface be identified and agreed to, and that those locations be monitored consistently every year.

3. That groundwater elevation at each monitoring location is measured on a minimum of two occasions per year; preferably once at the end of the dry season and once as soon as possible at the end of the wet season.


APPENDIX A Measured groundwater elevations and estimated annual values 1994–95 to 2005–06

Description:

1. Grey symbols – water-table elevation measurement.

2. Red symbol and red line – annual water-table elevation calculated after the removal of data outliers.

3. Green line – ground-surface elevation at the monitoring bore location.













APPENDIX B Annual depth to water table 1994–95 to 2007–08

Description:

1. Each depth to water table map was calculated as the difference between the LiDAR digital-elevation model and the annual water-table surfaces (Appendix C).

2. A common set of water-level measurement locations (i.e., monitoring bores) was used for the map from 1994–95 to 2005–06; the bore locations are depicted on those maps.

3. Due to a change in the groundwater monitoring program after 2005–06, different sets of monitoring bores were used to produce the maps for 2006–07 and 2007–08; the locations are depicted on each map.

4. The maps for 2006–07 and 2007–08 are unreliable in Martins Location and near the Ord River on northern Ivanhoe Plain; groundwater levels were not collected in those areas during that period. The areas are marked on the maps by “no data” labels, where the interpolated depth to water table should be disregarded.
















APPENDIX C Annual water-table elevation 1994–95 to 2007–08

Description:

1. The annual water-table surfaces were constructed from the annual groundwater levels depicted in Appendix A; the method is presented in the main report.

2. A common set of water-level measurement locations (i.e., monitoring bores) was used for the map from 1994–95 to 2005–06; the bore locations are depicted on those maps.

3. Due to a change in the groundwater monitoring program after 2005–06, different sets of monitoring bores were used to produce the maps for 2006–07 and 2007–08; the locations are depicted on each map.

4. The maps for 2006–07 and 2007–08 are unreliable in Martins Location and near the Ord River on northern Ivanhoe Plain; groundwater levels were not collected in those areas during that period. The areas are marked on the maps by “no data” labels, where the interpolated water-table elevation should be disregarded.
















REFERENCES Carter J, Smith A, Ali M, Silberstein R, Doody T, Byrne J, Smart N, Robinson C, Palmer C, Slaven T, Smith P and Palmer, D (2009) Interaction between trees and groundwater in the Ord River Irrigation Area. CSIRO: Water for a Healthy Country National Research Flagship.

Lawrie K, Clarke J, Hatch M, Wilkes P, Apps H (2006) Improving hydrogeological models of aquifer systems for salinity and water management in the Ord irrigation area: A pilot study into the use of geophysics and other geoscience methods. CRC LEME Restricted Report 232R. Cooperative Research Centre for Landscape Environments and Mineral Exploration.

O'Boy CA, Tickell SJ, Yestertener C, Commander DP, Jolly P, Laws AT (2001) Hydrogeology of the Ord Irrigation Area. Hydrogeological Record Series Report HG 7. Western Australian Water and Rivers Commission.

Smith AJ, Pollock DW and Palmer D (2006) Groundwater management options to control rising groundwater level and salinity in the Ord Stage 1 Irrigation Area, Western Australia. Science Report 70/06. CSIRO Land and Water.

Smith AJ, Pollock D, Palmer D and Price A (2007) Ord River Irrigation Area (ORIA) groundwater drainage and discharge evaluation: Survey of groundwater quality 2006. Science Report 44/07. CSIRO Land and Water.

Smith AJ (2008) Rainfall and irrigation controls on groundwater rise and salinity risk in the Ord River Irrigation Area, northern Australia. Hydrogeology Journal, 16, 1159–1175.

Smith A and Price A. 2009. Review and assessment of soil salinity in the Ord River Irrigation Area. CSIRO: Water for a Healthy Country National Research Flagship.

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