GROUNDWATER RESOURCE ASSESSMENT USING MODELLING AND REMOTE SENSING: A CASE STUDY OF SESHEKE, ZAMBIA

Abstract

Groundwater is the major source of domestic water supply in rural Zambia. Sustainable utilization of these resources requires accurate knowledge of its quantity and quality. One the tools most used in assessing groundwater resources is the use of numerical groundwater models. The reliability of groundwater modelling is largely constrained by the quality of input data. Earth Observation data and groundwater modelling can help in quantifying the groundwater fluxes in an area where data is scarce. This paper discusses the assessment of groundwater resources in Sesheke using Geographical Information System (GIS), Remote Sensing (RS) and Groundwater modelling. The aquifer system was modelled using software Processing Modflow for Windows (PMWIN) as a pre and post processor for MODFLOW assuming steady state conditions. Only the upper aquifer was modelled under unconfined conditions represented by three numerical layers of varying thickness 130m to 364m. The grid cell size of the model was taken as 250 X 250m and 11,863 active cells were used to represent the entire study area which is 11,501Km2. The model area and the elevation of the top layer were delineated using Shuttle Radar Topographical Mission (SRTM) Digital Elevation Model (DEM) and topographic maps. A combination of trial and error method and automatic method were used to calibrate the model using the observed hydraulic heads until the Root Mean Square Error (RMSE) reached 17m. From the sensitivity analysis hydraulic conductivity proved to be the most sensitive parameter of the model. From the pump testing analysis the average transmisivity was found to be 3.614m2/day and the hydraulic conductivity was in the range of 0.742 – 2.614m/day for the sandstone and 0.09-24m/day for the loose top Kalahari sands. From the Surface Energy Balance system (SEBS) the annual average actual Evapotranspiration (ET) was calculated as 745mm against the annual precipitation of 765mm. The estimated annual recharge from the groundwater model is 54.7mm in the north and 28mm in the southern part of the model. The model results suggest that the main recharge area is in the northern part of Sesheke which agrees well with hydraulic head survey conducted during the field work where high hydraulic heads in the north where comparable to the southern part of the District.