32.-Simulation of an inland delta aquifer system to evolve pre-development management schemes-A case study in Okavango delta, Botswana

Abstract

Development of groundwater resources to meet the increase demand for drinking, industries, irrigation and other purposes is ever growing in developed and developing countries. The over development of groundwater resources leads to the decline of water level causing socio-economic and environmental degradation. It is, thus, imperative to manage the groundwater resources in an optimal manner. Management schemes can be evolved, only if the groundwater potential is assessed in more realistic manner. Mathematical modelling in conjunction with detailed field investigations have been proved to be a potential tool for this purpose. Evolving pre-development management schemes is still works out to be better choice. One such study was carried out in Kunyere River valley, Okavango Delta, Botswana. Kunyere River valley has three tributaries viz. Marophe, Xudum and Matsibe Rivers. The valley falls in the southeastern fringe of Kalahari Desert (Botswana) which tends along the Kunyere fault in a northeast to southwest direction. The materials in the valley system are saturated below a depth of 7 to 9 m below ground level (bgl), where a fresh potable groundwater reserve is present to a maximum depth of 50 to 70 m (bgl) and below this depth, groundwater is brackish one. The groundwater resource in the valley has been quantified through exploratory drilling, test pumping, and hydro-chemical analysis of groundwater samples by the Department of Water Affairs, Govt. of Botswana.

A model with six layers flow regime was conceptualised by making use of available data. Fourth layer of the model is the main fresh water bearing aquifer and the bottom layer is brackish one. Long duration pumping test carried out in this area indicated the leaky nature of the aquifer system. Mathematical model of the basin was constructed and calibrated for steady state condition by using Visual Modflow computer software. Two prognostic runs were made and an optimal one was identified which will ensure minimum upward leakage from the bottom saline unit to the pumping aquifer. This simulation study indicates that substantial development of groundwater potential is possible in this area.