Focused groundwater recharge in a tropical dryland: Empirical evidence from central, semi-arid Tanzania

Study region: Little Kinyasungwe Catchment within the River Wami Basin of central, semi-arid Tanzania. Study focus: Groundwater and its replenishment via recharge are critical to sustaining livelihoods and poverty alleviation in tropical drylands yet the processes by which groundwater is replenished remain inadequately observed and resolved. Detailed observations are examined from central Tanzania, where the Makutapora Wellfield supplies freshwater to the rapidly growing, capital city of Dodoma. New hydrological insights for the region: The prominence of focused recharge from ephemeral stream discharges is shown from: (1) groundwater recharge correlates more strongly with the seasonal duration of ephemeral stream stage exceeding a threshold than seasonal rainfall; (2) hourly monitoring of groundwater-levels and stream stage shows that sustained groundwater-level rises, indicative of groundwater recharge, correspond better to observed pulses of stream discharge from intensive rainfall observed upstream of the wellfield than rainfall recorded proximate to piezometers; and (3) stable isotope ratios of O and H indicate similar compositions of groundwater and ephemeral streamflow; both have undergone evaporative enrichment and are linked to intensive (90th percentile) daily rainfall. This characterisation of focused groundwater recharge from intensive rainfall in this tropical dryland highlights the potential resilience of groundwater resources to climate change amplifying precipitation extremes and informs strategies to augment replenishment of groundwater supplying the city of Dodoma.