Inverse modeling and uncertainty analysis of potential groundwater recharge to the confined semi-fossil Ohangwena II Aquifer, Namibia

Wallner, Markus; Houben, Georg; Lohe, Christoph; Quinger, Martin; Himmelsbach, Thomas

Inverse modeling and uncertainty analysis of potential groundwater recharge to the confined semi-fossil Ohangwena II Aquifer, Namibia | Modélisation inverse et analyse d’incertitude de la recharge potentielle d’eaux souterraines de l‘aquifère captif semi-fossile d’Ohangwena II en Namibie Modelado inverso y análisis de incertidumbre de la recarga potencial de agua subterránea en el acuífero confinado semi-fósil de Ohangwena II, Namibia 纳米比亚承压的半-化石Ohangwena II含水层潜在地下水补给的反演模拟及不确定性分析 Modelagem inversa e análise de incertezas da recarga potencial das águas subterrâneas para o aquífero confinado semifóssil Ohangwena II, Namíbia

2017

Wallner, Markus | Houben, Georg | Lohe, Christoph | Quinger, Martin | Himmelsbach, Thomas

The identification of potential recharge areas and estimation of recharge rates to the confined semi-fossil Ohangwena II Aquifer (KOH-2) is crucial for its future sustainable use. The KOH-2 is located within the endorheic transboundary Cuvelai-Etosha-Basin (CEB), shared by Angola and Namibia. The main objective was the development of a strategy to tackle the problem of data scarcity, which is a well-known problem in semi-arid regions. In a first step, conceptual geological cross sections were created to illustrate the possible geological setting of the system. Furthermore, groundwater travel times were estimated by simple hydraulic calculations. A two-dimensional numerical groundwater model was set up to analyze flow patterns and potential recharge zones. The model was optimized against local observations of hydraulic heads and groundwater age. The sensitivity of the model against different boundary conditions and internal structures was tested. Parameter uncertainty and recharge rates were estimated. Results indicate that groundwater recharge to the KOH-2 mainly occurs from the Angolan Highlands in the northeastern part of the CEB. The sensitivity of the groundwater model to different internal structures is relatively small in comparison to changing boundary conditions in the form of influent or effluent streams. Uncertainty analysis underlined previous results, indicating groundwater recharge originating from the Angolan Highlands. The estimated recharge rates are less than 1% of mean yearly precipitation, which are reasonable for semi-arid regions.

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