Analysis of Water Exchange Processes between Groundwater and Surface Water in the Usangu Plains, Tanzania

The groundwater and surface water interface has been proved evident by the existence of effluent and influent streams. Still, the irrigation sector in sub-Saharan Africa, Tanzania included, is predominantly using surface water and groundwater conjunctively without a clear understanding of the contribution of each of the two water sources. This study was conducted to analyze the water exchange processes between groundwater and surface water in the Usangu Plains. Constrained by data scarcity in the study area, only three hydrograph separation techniques (Sliding interval, Fixed interval, and Local minimum) of the Baseflow Index model third version (BFI+ 3.0) were used. These techniques were applied to estimate baseflow, surface runoff and baseflow indices using river discharge data from six gauging stations across six different rivers. Further, the Mann-Kendall (MK) test was used for trend analysis of the long-term time series baseflow index. Results indicate that the groundwater-surface water interaction exists and the baseflow contributes substantially to the sustainable river flows in the Usangu Plains during both dry and wet seasons. Except for the Great Ruaha River at Msembe, the other five rivers manifested a great reliance on the baseflow with more than 90% of it in the river flows. The MK test revealed that at annual, wet, and dry season scale there are statistically non-significant increasing and decreasing trends in the baseflows. Land and water management strategies such as water allocation measures, sound water usage practices and afforestation may be better approaches to counteract the declines of water flows in rivers of the Usangu Plains, especially in the dry season.