A distributed water balance model to estimate direct groundwater recharge: A case study from the nhoma and khaudum catchments, Namibia

Abstract

The most important water resource in drylands such as Namibia is groundwater and itsrecharge including temporal and spatial variations need to be assessed reliably for sustainable water management. In this paper, a grid-based conceptual water balance model witha simplification at the catchment scale level is used to assess groundwater recharge. Thereliability of the model predictions are verified by the independent chloride mass balancemethod. The distributed, process-oriented, physically based water balance model MOD-BIL used in this study considers the major water balance components and calculates aspatially differentiated water balance by simulating water fluxes and storages at temporaland spatial resolutions based on meteorological, topographic, soil physical, land coverand geological input parameters. In this study it is set up for the upper Khaudum andNhoma catchments in the Kalahari of north-eastern Namibia and northwestern Botswanaat a spatial resolution of 500 x 500 m, calculated daily for a period of 22 years. A meanannual area groundwater recharge of 11.5 mm a⁻¹ is calculated for the catchments, butspatial variations between 0 and 17.5 mm a⁻¹occur depending on the variability of vegetation, soil and geomorphology. Groundwater recharge only occurred on a few daysduring the simulation period.