Effects of Rainfall Variability and Groundwater Abstraction on Mtoni and Bububu Springs Discharge in Masingini Catchment, Zanzibar, Tanzania
I, Abdalla, Yussuf
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Spring discharge from the Masingini catchment plays a crucial role in the Zanzibar Municipality water supply scheme. However, there has been remarkable attenuation in spring discharge over the years due to a change in rainfall patterns and groundwater over-exploitation. Rainfall is the only source of freshwater recharge on Zanzibar, and groundwater is the primary source of water supply in Zanzibar. The study's main objective was to assess the effects of rainfall variability and groundwater abstraction on Mtoni and Bububu springs discharge in the Masingini catchment, Zanzibar. Specifically, the study evaluated rainfall variability on the Masingini catchment between 1992 and 2018 to assess spring discharge at least ten years before and after the water policy; determined groundwater levels in the Masingini catchment; evaluated spring discharge trends in the Masingini catchment and assessed the response of the Mtoni and Bububu springs discharge with regard to rainfall variability and groundwater abstraction in the Masingini Catchment. The monthly rainfall data was collected from the Tanzania Meteorological Agency in the Zanzibar office. Monthly groundwater levels data of fifteen (15) boreholes for wet and dry seasons were obtained from field measurements and supplemented by Zanzibar Water Authority's groundwater abstraction data collected. The spring discharge data were collected from the Zanzibar Water Authority. The data analysis was carried out using the Mann-Kendall test, Surfer 18, ArcGIS 10.8, and multiple regression. The results revealed a statistically non-significant downward trend in all three rainfall seasons for both rainfall stations due to the decline of seasonal rainfall over the years. Annual rainfall showed a significant decreasing trend for Kizimbani Agromet station (p < 0.05). On the other hand, the groundwater level dropped by an average of 5±2.6m between the long rainy and dry seasons. The results showed that water level contours are ranging from 2 to 46m, meanwhile, groundwater is flowing from high level to low ones. Besides, the contour maps revealed that the aquifer system drains toward West-north and West-south of the catchment. The drawdown range from -30.35 to -2m indicates that the groundwater table and head levels are declining with pumping age, which has resulted in diversion on the groundwater flow system and affects the natural groundwater discharge system. Besides, the maximum and minimum spring discharges corresponded to wet and dry seasons, respectively. There was a significant decreasing trend in annual spring discharge at p = 0.05 and p = 0.01 for Mtoni and Bububu spring discharge, respectively. The M-K revealed a significantly positive correlation of annual spring discharge and rainfall, τ = 0.333 and τ = 0.453 for Mtoni and Bububu springs, respectively, implying that discharge increases as rainfall increases. The average annual discharge of Mtoni and Bububu springs declined by 23.9% and 18.9%, respectively, before and after water policy. However, groundwater abstraction showed a significant increasing trend over the years (2013-2016), τ = 0.929 (p = 0.01). The multiple regression revealed that rainfall and groundwater abstraction accounted for 78.0% and 71.2% of the variation in average daily spring discharge for Mtoni and Bububu springs, respectively. Hence, the study concluded that rainfall variability and groundwater abstraction were the primary factors for declining spring discharge on the catchment. The study urged ZAWA to engage in rainwater harvesting technologies and improve water services to meet water demands for sustainable groundwater management.