Water resource evaluation and modelling of Motloutse alluvial water course - a case study of Tobane River reach

Abstract

Surface water is generally not available for irrigation as rain fed agriculture is hindered by the harsh climatic conditions in Botswana. In order to suite their agricultural needs communities in the Motloutse River catchment depend on Motloutse River. However, the river is ephemeral resulting in lack of surface water therefore groundwater potential of the Motloutse alluvial aquifer had to be studied to support irrigation development and improve the livelihoods of the communal farmers in these areas. The government of Botswana has also decided to undertake irrigation along the Motloutse using groundwater resources from the sand river beds and also to incorporate groundwater resources in the NSWCP to augment surface water resources. Alluvial aquifers are unconfined, groundwater systems hosted in sand river channels or flood plain presenting a potentially large resource for agriculture as they can store and supply water throughout the dry season. The study area lies in eastern Botswana along the Motloutse River, downstream of the Letsibogo dam in the Motloutse catchment, a sub catchment of the Limpopo basin near Tobane and Bobonong villages. Groundwater evaluation of Motloutse was carried out through integration of geological, hydrogeological and groundwater modelling tools. Geophysical profiling was carried out to obtain information on the thickness and lithology of river alluvium using GPR to help in aquifer volume estimation. An average depth of 6 metres for the gravelly sand was resolved successfully and laboratory analysis of aquifer material was done to estimate aquifer parameters. The hydrochemistry of the Motloutse alluvial aquifer was assessed and the water deemed suitable for irrigation with a potential pollution from the BCL Mine. It was also found that 3 water types exist in the Motloutse alluvial aquifer dominated by Ca-Na-SO4-Cl water class that have to be monitored on regular basis due to contamination from the mine. Consequently, a simple steady state groundwater model was developed to simulate and plot groundwater flow to improve the understanding of ground-water flow in the Motloutse sand river aquifer. Model yielded calibrated K values of 145 and 11m/day for the riverbed and riverbank respectively, calibrated recharge and evaporation of 172mm/yr and 120mm respectively. A sustainable groundwater yield of 120m3/day with the potential to irrigate an area of 2.4 hectares was determined. The model is associated with a number of uncertainties resulting from the simplification and assumptions made regarding complex field conditions and data quality. Thus limitations of the model should be taken into consideration prior to applying the model for groundwater resource management.