Spatio-temporal variation of water resource and fluxes in the Okavango basin

As part of its development planning process, Angola is seeking opportunities to intensify agricultural production and maximize efficient use of the abundant water resources of the Upper Okavango Basin while sustaining flows to the Okavango Delta, which provides critical ecosystem services for sustaining wildlife populations and a tourism dependent local economy. A SWAT-MODFLOW model was developed and implemented to simulate major hydrologic processes: surface runoff, ET, percolation, lateral flow, stream water seepage, groundwater return flow, groundwater level, and water balance in the Cubango-Okavango River Basin. Due to the limited rainfall data availability, we generated a rainfall dataset using a gridded station-based GPCC and satellite-based CHIRPS dataset to force the model. The model was tested against monthly average stream discharge at two stream gauging stations at Dirico and at Mohembo (inflow to the Delta) during the 20032010 and 2003-2019 period. Because of the lack of groundwater level monitoring data, we used a baseflow separation technique to calibrate the geological parameters of the study area. Using expert knowledge, field observation, and available streamflow measurements from the 1960s to 70s at Longa, Quiriri, and Cuito, we assumed that baseflow rates in these areas would be similar to the present because the landscape, while changing, has remained largely intact around these areas since that time. The monthly Nash-Sutcliffe coefficient (NSE) values of streamflow discharge for calibration and validation were calculated to be 0.50 and 0.51 for Dirico, and 0.76 and 0.70 for Mohembo, respectively. Results show that the model captures the dominant baseflow patterns throughout the watershed (Longa, Quiriri, Cuito > 0.9) and the critical patterns of groundwater/surface water interactions. In addition, we observed substantial spatio-temporal variability of groundwater head, recharge to the water table, and water balance components. Findings gained during this study can support stakeholders and researchers in better understanding the effects of different land management practices on water resources, identify critical variables in protecting freshwater ecosystems and biodiversity, and provide a baseline for future conservation strategies.