Identification of water scarcity hotspots for the 21st century – the case study of irrigation demands in Germany

In many temperate regions globally, sufficient precipitation and moderate temperatures have meant that green water has sufficed for agriculture. However, the effects of climate change demonstrate that additional crop water is now more frequently required in many of these areas, particularly for dry summer years, with irrigation demands expected to continue increasing. In Germany, this effect has become noticeable over previous years, exemplified by the reduced crop yields in the recent summer droughts of 2018 and 2020.

Our study, performed within the scope of the WADKlim project, identifies critical hotspots for water stress through high-resolution hydrological modelling and statistical analyses to determine groundwater recharge and theoretical irrigation requirements from now until 2100. We set up and calibrated the mGROWA hydrological model over a historical period (1961-2020) at a high spatial (100 m) and temporal (daily) resolution. The calibrated model was then run until 2100 for three climate scenarios (1 x RCP2.6; 2 x RCP8.5), which were selected as a stress test for the system. As model outputs, we derived the spatio-temporal patterns of groundwater recharge as well as crop water requirements, through the application of irrigation rules typical for Germany and accounting for the spatial distribution of different crop types. We converted the theoretical crop water requirements into requirements only for areas that are equipped for irrigation, incorporating multiple scenarios for the rate at which irrigation infrastructure could expand in Germany.

Our results demonstrate the large spatial and interannual variations in irrigation demands throughout Germany. We quantify how the multiplicative effect of warmer and drier summers in combination with increased areas equipped for irrigation is expected to strain water resources in the future. For example, we estimate that mean annual irrigation demands in Germany could increase by as much as 700% by 2075-2100, considering the “worst-case” scenario of climate projection and increase in irrigated areas. Regarding groundwater availability, owing to the expected increase in winter precipitation in Germany, our modelling results show pronounced regional variations in whether or not annual groundwater recharge is expected to increase in the future. Finally, we included estimates of other water requirements and aggregated the results to determine overall water demands at the district level and calculate ratios of water use to groundwater recharge per district. Our results highlight the hotspots in Germany where water stress is expected to increase the most throughout the 21^st century, which could likely lead to conflict between different water users (agricultural, industry, public supply).

Determining the spatio-temporal characteristics of how water stress will change requires comprehensive assessments of water availability, crop water requirements, areas equipped for irrigation infrastructure, and other water uses. In addition, the large variability in climate projections means that results from such assessments provide large ranges of expected water stress conditions. We have developed and tested a comprehensive methodology for identifying and mapping water hotspots, which we implemented for Germany using three climate projections. Our methodology is transferable to similar (data-rich) regions, and can also be applied for a complete ensemble of climate projections.