Evidence of the dependence of groundwater resources on extreme rainfall in East Africa

Abstract

Groundwater is the world's largest accessible store of fresh water and supplies 36% of the world's drinking water and ~42% of the water used for irrigation 1 . Groundwater is the only reliable source of fresh water in many semi-arid and arid regions where surface waters are seasonally or perennially absent 9 . The long-term viability of groundwater resources as well as the ecosystems and livelihoods that they sustain, depends on replenishment of groundwater by recharge. Over the past 50 years, groundwater depletion has been estimated and observed in several aquifers throughout the tropics and sub-tropics [10][11][12][13] . Such depletion not only threatens ecosystem function and the livelihoods of groundwater-dependent communities in some of the world's poorest regions but is also estimated to contribute to sea-level rise 12,13 . A conceptual understanding of the relationship between rainfall and recharge is fundamental to the development of robust estimates and projections of not only groundwater recharge and depletion but of all components of the terrestrial water balance under changing climates and increasing freshwater demand.

Impact of climate indicators on continental‐scale potential groundwater recharge in Africa

“…We caution that the continental-scale lumped-parameter approach provides only a crude approximation of the water budget and is possible at the cost of several simplifications: (a) ignoring preferential flow into the soil, fractured rock, saprolite (Neumann, 2005;Beven and Germann, 2013), and karst regions (Hartmann et al, 2014); (b) not considering the role of macroporosity (Owor et al, 2009;Taylor et al, 2013); (c) missing proper soil-hydraulic properties pertaining to datasets related to specific Tropical and Equatorial zones in Africa (Minasny and Hartemink, 2011); (d) missing information on land-use changes (urbanization, deforestation) in Africa (Githui et al, 2009;Baker and Miller, 2013); (e) lacking records of water pumping and the role of human consumption of water; and (f) ignoring focused recharge via leakage from surface waters (ephemeral streams, wetlands, or lakes).…”

Response of a fractured bedrock aquifer to recharge from heavy rainfall events

“…The occurrence of heavy rain events has been associated with natural climate variability through its influence on ocean-atmosphere dynamics (Taylor et al, 2013b;Jones et al, 2004;Spry et al, 2014;Mass et al, 2011 Fleming and Quilty (2006) established that groundwater levels tend to be higher during La Niña years and lower during El Niño years, with the strongest correlation observed in the winter and spring. This follows suit with general conclusions that El Niño is associated with less precipitation and La Niña with more precipitation in southwest BC and the Pacific Northwest (Shabbar et al, 1997;Higgins et al, 2007;Fleming and Whitfield, 2010;Fleming and Quilty, 2006).…”

“…According to numerous climate change projections from global climate models (GCMs), the proportion of infrequent, extreme (heavy) precipitation events is anticipated to increase in the 21 st century (Taylor et al, 2013b;IPCC, 2012;Owor et al, 2009;Pall et al, 2007). While increases in global mean precipitation are constrained by the energy budget (the ability of the troposphere to radiate away latent heat released by precipitation), increases in extreme precipitation are controlled by the availability of atmospheric moisture (Allen and Ingram, 2002).…”

“…Given the role of high intensity rainfall on recharge, Mileham et al (2009) considered projected changes in rainfall intensity in southwestern Uganda, and estimated recharge to increase by 53% from a baseline period of to 2070-2100. Taylor et al (2013b) examined statistically extreme months and seasons of rainfall in Tanzania (semi-arid, tropical climate) and found that an increase in recharge from anomalously intense rainfall can be attributed to high rates of evapotranspiration that need to be surpassed in order for potential recharge to reach the water table in deeply weathered granite. Overall, these studies indicate that extreme rainfall enhances groundwater storage.…”

“…Future climate change projections suggest that groundwater recharge will change in the 21 st century (Taylor et al, 2013b; Intergovernmental Panel on Climate Change (IPCC), 2012; Owor et al, 2009;Pall et al, 2007). Temperate areas already receiving large amounts of precipitation are expected to become slightly wetter and the proportion of heavy rainfall events is anticipated to increase (Taylor et al, 2013a;IPCC, 2012).…”