Abstract
Abstract. Billions of people rely on groundwater as being an accessible source of drinking water and for irrigation, especially in times of drought. Its importance will likely increase with a changing climate. It is still unclear, however, how climate change will impact groundwater systems globally and, thus, the availability of this vital resource. Groundwater recharge is an important indicator for groundwater availability, but it is a water flux that is difficult to estimate as uncertainties in the water balance accumulate, leading to possibly large errors in particular in dry regions. This study investigates uncertainties in groundwater recharge projections using a multi-model ensemble of eight global hydrological models (GHMs) that are driven by the bias-adjusted output of four global circulation models (GCMs). Pre-industrial and current groundwater recharge values are compared with recharge for different global warming (GW) levels as a result of three representative concentration pathways (RCPs). Results suggest that projected changes strongly vary among the different GHM–GCM combinations, and statistically significant changes are only computed for a few regions of the world. Statistically significant GWR increases are projected for northern Europe and some parts of the Arctic, East Africa, and India. Statistically significant decreases are simulated in southern Chile, parts of Brazil, central USA, the Mediterranean, and southeastern China. In some regions, reversals of groundwater recharge trends can be observed with global warming. Because most GHMs do not simulate the impact of changing atmospheric CO2 and climate on vegetation and, thus, evapotranspiration, we investigate how estimated changes in GWR are affected by the inclusion of these processes. In some regions, inclusion leads to differences in groundwater recharge changes of up to 100 mm per year. Most GHMs with active vegetation simulate less severe decreases in groundwater recharge than GHMs without active vegetation and, in some regions, even increases instead of decreases are simulated. However, in regions where GCMs predict decreases in precipitation and where groundwater availability is the most important, model agreement among GHMs with active vegetation is the lowest. Overall, large uncertainties in the model outcomes suggest that additional research on simulating groundwater processes in GHMs is necessary.
Highlights
The critical role of groundwater as an accessible source for irrigation and of drinking water, in particular during dry periods, droughts, and floods, will intensify with climate change because increased precipitation variability is expected to decrease the reliability of surface water supply (Taylor et al, 2013; Döll et al, 2018; Kundzewicz and Döll, 2009)
This study investigates uncertainties in groundwater recharge projections using a multi-model ensemble of eight global hydrological models (GHMs) that are driven by the bias-adjusted output of four global circulation models (GCMs)
Because most GHMs do not simulate the impact of changing atmospheric CO2 and climate on vegetation and, evapotranspiration, we investigate how estimated changes in Groundwater recharge (GWR) are affected by the inclusion of these processes
Summary
The critical role of groundwater as an accessible source for irrigation and of drinking water, in particular during dry periods, droughts, and floods, will intensify with climate change because increased precipitation variability is expected to decrease the reliability of surface water supply (Taylor et al, 2013; Döll et al, 2018; Kundzewicz and Döll, 2009). While demand for groundwater is likely to increase in the future, groundwater abstractions have already led to depleted aquifers in many regions around the globe (Thomas and Famiglietti, 2019; Cuthbert et al, 2019a; Wada et al, 2012; Konikow and Kendy, 2005; Döll et al, 2014b) They have resulted in the reduction in groundwater discharge to rivers, with negative impacts on water availability for humans and freshwater biota, in particular during low-flow periods (Herbert and Döll, 2019). An additional factor in estimating groundwater recharge is the simulation of the groundwater table and, capillary rise and focused recharge This has not been achieved yet in GHMs; recently, global hydrological models (GHMs) started integrating gradient-based groundwater models to better estimate the flows between surface water and groundwater and the impact of humans and the changing climate on the groundwater system (de Graaf et al, 2019; Reinecke et al, 2019). Neglecting capillary rise may lead to an overestimation of the decreases and increases in GWR due to a changing climate
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