Managing the impact of irrigation on the Tosca-Molopo groundwater resource
Van Dyk, Gabriel Stephanus Du Toit
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From 1990 to 2000 rapid development of irrigation from groundwater resources in dolomite aquifers took place in the Tosca Molopo area. This abstraction lead to water levels declining 10 to 20m regionally and up to 60m proximate to intensive irrigation. The purpose of this study was to investigate the impact of irrigation on the resource and initiate actions to manage the resource. This thesis reports on the qualification and quantification of the impact, determination of water use and regulating use to ensure sustainable future use. The Tosca Molopo area is located in South Africa proximate to the Botswana border. The area of interest is characterized by a flat topography. From the watershed in the west at 1210 m the elevation gradually decline to 1070 m in the east over a distance of 60 km. A number of non-perennial rivers drain the area, and although insignificant as surface water resources they play a major role in groundwater recharge. A low annual rainfall, varying from 399 mm in the east to 385 mm in the west, characterizes the study area. Evaporation in the area is high at between 2050 – 2250 mm/a (WRC, 1994) with only a small percentage of rainwater available to recharge groundwater. Two distinctive aquifers namely a primary aquifer formed by fine-grained sediments of the Kalahari Group and fractured/ carstified dolomites of the Ghaap Plato formation contribute to the system. The general flow is from the SW to the NE with the Molopo River the base of drainage. From the observed water level reaction the sediments contribute largely towards the storage of the aquifer system with the fractures of the dolomite contributing to high yielding flow. The MODFLOW PMWIN 5.1.7 (Chiang 2000) software was used to construct a 2-layer finite difference flow model. The model covering 80 km east west and 50 km north south or 4000 km2 was divided into cells of 0.5 X 0.5 km generating 100 rows and 160 columns. Based on the conceptual model provision was made for 2 layers namely the unconsolidated primary aquifer and the underlying fractured dolomite with its aquifer characteristics. The first layer ranges from an elevation of 1160 mamsl at a depth of 10 m in the southwest. To the northeast it range from an elevation of 1080 mamsl to a depth 960 mamsl or a thickness exceeding 120 m. The base of the sediments is the top of the fractured dolomite aquifer with its base at 900 mamsl. Of the number of dolerite dykes intruded into the dolomite the Grassbank and Quarreefontein dykes (both 15 m thick) are the most influential on the groundwater flow. Both these dykes act as no-flow boundaries of the Neumann (impervious) type impeding flow from the south and west of the area. Towards the east the Quarreefontein dyke does not seem to be a no-flow boundary as the water level information indicate connection with the dolomite to the south. The surface and groundwater shed formed by the Banded Iron Formation of the Waterberge forms the boundary to the west. The combination of both a geological contact and watershed is a leaking boundary. The Molopo River forms the eastern boundary. Recharge to the aquifer was determined with the chloride mass balance method with groundwater sample analysis and the Cl rain content 0.8mg/l. Recharge zones as determined from this chloride analysis were used for the model. Recharge in each zone was based on seasonal recharge for the winter (ranging from 0.5% or 0.4 mm to 3% or 1.5 mm) and summer (ranging from 0.5% or 1.6 mm to 3% or 8.3 mm) depending on the precipitation. Groundwater is the sole source of water for both agricultural and domestic requirements. As irrigation use is responsible for 99.5 % of the total use no domestic and stock watering abstraction was considered. Irrigation abstraction was calculated from the registration areas, field observations and reports from users. The volume was then averaged over a six-month period (182.5 days) according to crop cultivated to obtain the daily abstraction from the aquifer. The calibrated model was used to test the following 10-year future scenarios of abstraction and recharge in order to assist in decisions regarding management of abstraction from the aquifer system. Scenario 1 was with average precipitation and recharge at the current high abstraction rate of 16.1 Mm3/a. This scenario was not acceptable due to the regional water level declines of 20 to 30m and 60 to 110 m water level declines proximate to irrigation. Scenario 2 was with was with average precipitation and recharge at the restricted abstraction at 11.1 Mm3/a. This scenario would result in regional water level declines of 10 to 20m and 30 to 60m proximate to irrigation. With strong abstraction control this scenario with controllable water level declines was acceptable. Scenario 3 was similar to scenario 2, but with 20 % less than normal precipitation. The water level declines that will result with this scenario were similar to scenario 1, but it was expected that below normal precipitation would be the exception. The 4th scenario tested was if normal precipitation prevailed and all irrigation abstraction was stopped. The regional water level would recover fully after 10 years with only 10 m still to recover proximate to heavily irrigated areas. The model demonstrated that rates as specified by scenario 2 can be sustainable abstracted from the system at average recharge and that these abstractions would still be sustainable at 20 % less than average recharge as in scenario 3. Management of abstraction of the aquifer was consequently structured to ensure that abstraction would not exceed the sustainable yield of 11.1 M m3/a. Based on the evaluation and modeling of the resource the regulating and management of abstraction was addressed within the legal framework provided by the National Water Act (NWA) to obtain sustainable, equitable and fare dispensation of water use. Only water use exercised before Oct 1998 is recognized as existing water use. Potentially unauthorized users were identified with the use of satellite images. These water users were given the opportunity to proof that they are authorized users through a communication process and to submit supporting evidence. Users who could not submit satisfactory evidence were directed to scale their use down to authorized use by a specific time (summer 2003). These water users appealed to the water tribunal against the ruling of the water use authority, but the tribunal ruled in favor of the water use authority. In line with equitable access, application from new users were still processed with only 60 m3/ha of property owned authorized in accordance with General Authorization as prescribed by regulations of the NWA. With these actions the resource was still over allocated with water use still not within the accepted sustainable abstraction. Therefore it was decided that regulations would be implemented to enforce users to restrict their water use to 60 % of authorized water rights. The NWA makes provision for local management structures to be established to manage their local water use. Such a Water User Association (WUA) was established in the Tosca area and would on the long term enhance the capabilities for water use management. The resource is currently over allocated. It is recommended that the irrigation water use be restricted with 40% of authorized water rights. The water rights are not fairly allocated. Although the above actions are aimed at normalizing the critical damage to the resource and eminent conflict in the area compulsory licensing would be the long-term solution in this area. Compulsory licensing is aimed at sustainable and equitable allocation of water rights. The WUA should ensure that all users comply to abstraction control measures and water level monitoring the boreholes in the monitoring network would indicate if the resource would stabilize and recover to within sustainable use.