Why is it important to understand the impacts of climate change on groundwater?
Groundwater is a vital resource in sustaining multiple human activities, including irrigation and the provision of drinking water, and essential for multiple freshwater ecosystems. It is a crucial resource in dry regions and during dry periods in many parts of the world when surface water is less available. Demands for groundwater have intensified in the last decades, depleting many groundwater storages (called aquifers) worldwide. This pressure is expected to increase in the future as overall demands for freshwater are projected to grow globally. At the same time, climate change is likely to decrease the reliability of surface water availability, which may increase the pressure on groundwater.
An important indicator in understanding future groundwater availability and sustainable groundwater use is “groundwater recharge.” Simplified, it describes how much water from precipitation will end up in an aquifer. Knowing that is key because only when the volume of water pumped out of the aquifer is less or equal to the amount going in (recharge) can we ensure sustainable use.
However, we have yet limited information about groundwater recharge and even less about how climate change and different global warming levels are likely to affect future groundwater recharge, and thus, availability in the various parts of the world. The following outlines research that we have carried out in collaboration with other recognized global modelling experts, advancing the knowledge frontier on groundwater by building on the collective effort developed under the ISIMIP framework.
Groundwater recharge is challenging to investigate
Groundwater recharge is challenging to estimate. It depends on several variables, including topography, soils, land cover, land use, atmospheric CO2 levels, and the local geology. Those regions receiving a steady high rainfall, with relatively flat topography, with porous soils and geology, and cover of natural vegetation, are likely to have higher groundwater recharge capacity than highly urbanized and steep regions.
There is yet limited understanding of the spatial and temporal factors that influence groundwater recharge globally. Groundwater recharge accumulates all the uncertainties from multiple other components in the water cycle. That means groundwater recharge is influenced by how much it will rain in the future, with what frequency, what fraction will be evaporated, what fraction will be converted into runoff, and how surface and groundwater flows are connected. An additional challenge is that field measurements of groundwater recharge are rare. In particular, there is a lack of long observation time series, which prevents scientists from contrasting to what extent simulations match actual observations. These uncertainties and shortcomings are particularly acute in arid and semi-arid regions, where groundwater is also the most strategic water resource.
How did we investigate the impacts of climate change on groundwater recharge?
To model the impacts of climate change on future groundwater availability, we compared current (1°C warmer than pre-industrial) and past (pre-industrial) groundwater recharge rates against the recharge under three different global warming scenarios: +1.5°, +2° and +3°C.
We investigated the impacts of climate change on future groundwater from two perspectives. One, by exploring how future greenhouse gas emissions are likely to affect climate variables such as future rainfall and temperature and how such physical changes could modify groundwater recharge. Two, by analyzing how future emissions will influence plant physiology and the consequences of such changes on groundwater recharge. The hypothesis here is that with increasing CO2 emissions, terrestrial plants work more efficiently, which reduces their transpiration rates; thus, less water is absorbed by plants through their roots and more water for recharging aquifers. Whether this CO2 effect on vegetation is likely to have a significant impact on groundwater recharge is something this study aimed to address.
We used eight state-of-the-art global hydrological models to investigate groundwater recharge, namely: WaterGAP2, CLM4.5, H08, JULES-W1, LPJmL, PCR-GLOBWB, CWatM, and MATSIRO. These models represent groundwater recharge in different ways and with varying levels of complexity.
To estimate the impacts of global warming under three global scenarios (+1.5°, +2° and +3°C), each of the eight models was forced with four climate models: GFDL-ESM2M, HadGEM2-ES, IPSL-CM5A-LR, and MIROC5. We applied a statistical test (for a detailed explanation, see section 2.5 in Reinecke et al. (2021)) to determine if the changes in groundwater recharge between the current (and past) climate and future global warming scenarios are significant. With this test, we are able to highlight regions where the agreement among the hydrological models is highest and exclude areas where the agreement among models is low.
What did we find?
Compared to today’s warming of already 1 °C, the mean of the different combinations at 3 °C global warming shows decreases of recharge of over 100 mm per year in various regions across South America and in the Mississippi Basin, and declines of up to 50 mm per year in the Mediterranean, East China, and West Africa. Other regions, such as Indonesia and East Africa, show mean increases of up to 100 mm per year However, as displayed in Figure 1, individual models sometimes compute much larger changes for these regions.
Warming above pre-industrial temperatures: °C
This figure shows the averaged increase/decrease in groundwater recharge under different global warming scenarios and with respect to pre-industrial times. When activating the button for the statistical significance of the grid cells, only those regions where a large majority of model projections are in agreement with the sign of the trend (either increase or decrease) are displayed. The default view compared the change between present-day warming of 1 °C and a global warming scenarios of + 3°C
If only statistically significant changes are considered for all global warming levels compared to the present day, consistent patterns of decreasing groundwater recharge emerge for southern Chile, Brazil, central continental USA, the Mediterranean, and East China. On the contrary, consistent and statistically significant increases can be observed for northern Europe and, in general, northern latitudes and East Africa. Statistically significant changes could only be derived for a small percentage of the total land area. Only about 15% of the continental area, on average for all global warming levels, show statistically significant increases or decreases. However, the patterns of non-significant mean changes are consistent with the significant changes and show, e.g., for the Amazon, larger more extensive areas of increases and decreases around the statistically significant changes.
Limitations and knowledge gaps
Field measurements of groundwater recharge are challenging and scarce
Local measurements of groundwater recharge are limited because those are difficult to obtain. Commonly, several indirect means need to be combined to obtain an estimate; for example, changes in groundwater level combined with precipitation measurements and evapotranspiration estimates can be used to infer groundwater recharge rates. Thus, uncertainty in these measurements is also high. The limited number of measurements and its associated uncertainty make it difficult to validate model simulations.
A related problem is that recharge measurements are local. Comparing these local measurements to global, spatially coarse model outputs is challenging. Our study results have been compared to a global dataset that was extrapolated from existing local measurements. The agreement with the model outputs varies largely between models showing that further research is necessary. This research needs to (1) improve the available observations on groundwater recharge, which then (2) can be used to better evaluate and improve the existing models.
Groundwater recharge is a highly uncertain process
Estimation of groundwater recharge is complex as uncertainties from the water balance calculation accumulate. As explained before, changes in, for example, precipitation, runoff, or evapotranspiration all may influence groundwater recharge and have uncertainty attached. In particular, in dry regions with low precipitation, this possibly leads to significant errors as minimal changes in precipitation may already lead to large changes in recharge. This limits the reliability of climate impact studies on groundwater recharge.
Because groundwater recharge is a complex process involving different inflows and outflows highly variable in space and time, most hydrological models, especially global ones, cannot capture all inherent complexity and thus incorporate simplified processes in their routines.
Two processes have been excluded in this study because only some models implement it or simplify it so much that no reliable estimate is to be expected:
Focused recharge, which is a water flow flowing from surface water bodies (rivers, lakes, wetlands) to the groundwater.
Capillary rise is a flow opposite to groundwater recharge occurring in regions with very shallow groundwater (less than 3 m). Similar to a drinking straw in your favourite beverage where water rises inside the straw several millimetres above the level of the drink itself (the involved forces are called adhesion, cohesion and surface tension), groundwater rises through the soil. It can be then accessed by plants through their roots.
Limited understanding of the impact of rising CO2 levels on groundwater recharge
Climate change can affect groundwater recharge in different ways, either through changes in climate variables such as temperature rise and rainfall amount and distribution, but also through plant physiological changes induced by rising CO2 emissions. Scientists have shown that certain plants adapt to higher concentrations of atmospheric CO2 by closing their stoma as a means to regulate the entry of CO2 and the exiting of water vapour back to the atmosphere. Some plants’ physiological adaptation results in less water being transpired, which means that plants could become more efficient consuming less water (less transpiration), thus making more water available for groundwater recharge. The extent to which this adaptation has a real impact on groundwater recharge is not yet clear. Among the eight models used in this study, only four are able to account for such an effect (CLM 4.5, JULES-W1, LPJmL, and MATSIRO). Our study shows that differences between the models that consider that effect and models that do not are substantial.
This article was written in collaboration with the ISIpedia Editorial Team.
Please contact the ISIpedia Editorial Team (email@example.com) for more information or questions about this report.
Cover image: Ilka Franz
1 International Centre for Water Resources and Global Change (UNESCO), 56002 Koblenz, Germany
2 Institute of Physical Geography, Goethe University Frankfurt, 60438 Frankfurt, Germany
3 Senckenberg Leibniz Biodiversity and Climate Research Centre (SBiK-F) Frankfurt, 60325 Frankfurt, Germany