Abstract
Study regionLittle Kinyasungwe Catchment within the River Wami Basin of central, semi-arid Tanzania. Study focusGroundwater 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 regionThe 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.
Highlights
African climate is characterised by erratic precipitation (UNEP, 2010), which is markedly variable at intra-annual (Peel et al, 2001), annual (Nicholson, 1998), and decadal to millennial (Nicholson, 2000; Olago et al, 2009) timescales
High-frequency observations of groundwater-levels and stream stage from hydrological years 2015–2016 to 2017–2018, together with daily rainfall and wellfield pumpage, show that sustained groundwater-level rises, indicative of groundwater recharge events, correspond better to the timing and magnitude of rainfall observed in areas upstream of the wellfield and in association with observed pulses of stream discharge
Groundwater recharge estimated from piezometry is more strongly correlated with ephemeral streamflow than rainfall
Summary
African climate is characterised by erratic precipitation (UNEP, 2010), which is markedly variable at intra-annual (Peel et al, 2001), annual (Nicholson, 1998), and decadal to millennial (Nicholson, 2000; Olago et al, 2009) timescales. Groundwater supports livelihoods and poverty alleviation throughout sub-Saharan Africa (Braune and Xu, 2010; Cobbing and Hiller, 2019; Gaye and Tindimugaya, 2019), in semi-arid regions (Xu and Beekman, 2019) where surface water resources are intermittent and non-linearly related to precipitation (De Wit and Stankiewicz, 2006). The sustainable use and optimal management of groundwater require knowledge of long-term groundwater recharge and discharge rates, and potential changes to these rates due to climate change and freshwater withdrawals. Understanding this balance and how it may change are predicated on knowledge of the mechanisms by which meteoric water replenishes aquifers as well as the conditions and pathways which generate groundwater recharge. Knowledge gaps are most acute in drylands (Cuthbert et al, 2019; Somaratne and Smettem, 2014)
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