A SUB-BASIN WATER RESOURCE QUANTIFICATION AND AQUIFER PRODUCTIVITY ASSESSMENT FOR THE NORTHWEST BOREHOLE SCHEME NEAR OPUWO, NAMIBIA

Namibia is the most arid country in southern Africa, and is classified as experiencing medium-water stress from 2010-2014 by the Water Resources Institute (WRI) (World Bank, 2015) (Gassert, 2013). Increased water-resources management responsibility at the municipal level, population growth and urbanization trends necessitate community-scale quantification of water resources. An annual water balance for the contributing sub-basin to Opuwo, Namibia was performed. The Behnke and Maxey method was used to estimate PET, and the methodology outlined by Allen, et al was used to determine soil moisture response to individual rainfall events for one hypothetical year. Water balance results indicate that of the 379 mm of annual rainfall, approximately 84% is lost to evapotranspiration, 15% contributes to surface runoff, and only 1% recharges groundwater resources. Results of the water balance are reconciled with a groundwater flow model (using GMS 9.1 formulating the input for MODFLOW-2005) of the aquifer serving the wellfield in Opuwo. The aquifer model was based upon abstraction and well data for 2013. The modeled extent covers a land area of 45 km2 and consists of 1 layer used to represent the alluvial water-bearing structure. Hydraulic conductivity was estimated using the Theis method for unconfined aquifers in AQTESOLV. Through parameter estimation and calibration the MODFLOW model is able to duplicate pumping water levels in the wells within 1 m of observed values. The calibrated model was further used to explore hypothetical scenarios based upon increased water demand associated with population growth of 50% and 100%. Results indicate that sufficient groundwater flow exists to meet increased demand. However, predicted drawdown levels eliminate utilization of some screened intervals in well casings, negatively impacting production capacity of the boreholes. Rehabilitation of current wells, construction of additional wells, and implementation of check dams to increase recharge in the watershed may buffer against future drawdown as a result of increased pumping rates