Spatiotemporal trend analysis of groundwater level changes, rainfall, and runoff generated over the Notwane Catchment in Botswana between 2009 and 2019.

Abstract

The semi-arid south-eastern part of Botswana has recently been experiencing severe water shortages, and the demand currently surpasses the supply in the greater Gaborone area. Within the context of increased stormwater runoff generated over the area and the potential for groundwater recharge, this study aims to investigate the relationships between groundwater depths and rainfall amounts and identify their patterns and significance or lack thereof over Botswana’s largest water demand centre that falls within the data-scarce Notwane catchment area (NCA). Trend analysis of monthly rainfall and groundwater levels between 2012 and 2019 and their homogeneity were undertaken using the Mann-Kendal test, followed by the application of the water balance method to estimate runoff over the catchment between 2009 and 2019. Runoff and precipitation between the two periods were compared using paired t-tests. Investigations revealed that rainfall increased insignificantly, whereas groundwater depth generally increased significantly. The homogeneity test revealed a general insignificant increase in rainfall over NCA. No catchment-wide conclusions were obtained regarding groundwater depth increases. Water-balance computed runoff in 2019 was an increase of 13.7% from that computed in 2009, despite the conservative 3% increase in rainfall between the two periods. Increase in runoff could even be higher if land use changes were incorporated. This study revealed that there is groundwater recharge over the catchment, particularly after heavy rainfall events. The results of this study offer insights for identifying groundwater recharge potential zones, which could inform decision making with regard to strategies for induced groundwater recharge to replenish groundwater resources that can conjunctively be used with surface water resources.