Understanding the Importance of Convective Rain Events and Tracing Their Impact on the Catchment with Isotopes

Climate change is often associated by the public with a reduction in precipitation, but in fact for most of the world’s regions scientists predict a change in rainfall pattern. This change in rainfall pattern includes prolonged drought phases interrupted by heavy rainfall events, where a significant portion of the yearly rainfall can occur within a couple of hours. Following the Clausius-Clapeyron superscaling hypothesis, rising temperatures will be accompanied by more frequent and intense convective precipitation. Convective rain events impact the water cycle in a different manner than other precipitation mechanisms, with marked spikes in rain rates. These can cause excess overland flow, soil erosion, and urban flooding, especially when occurring over dry and/or sealed land. At the same time, these intense rain events contribute less to the recharge of aquifers and due to flooding and soil erosion impair water quality of drinking water reservoirs. Considering that the occurrence of convective rain events will increase with climate change, they pose a real threat to achieving the Sustainable Development Goals (SDG). Integrated water resource management (IWRM) requires a better understanding of the impact of such events on surface and groundwaters. Water isotopes have proven throughout to be ideal tracers of water movement from the rain drop to the spring, well, river or reservoir. However, convective rain events might challenge our data collection and interpretation techniques: we need to fill gaps in systematic data collection (temporarily and spatially), consider short-lived radioisotopes and additional tracers that can provide us information on the timescales and fate of the water delivered by a convective rain event. The CRP puts forward an innovative approach of high-frequency sampling of rain events, sampling of the saturated and unsaturated zone, a suite of isotopes (incl. sulfur-35, sodium-22 and radon-222), in combination with high-frequency sensors at the outlet of the catchment. The aim is to revisit the application of isotopes in catchment hydrology on short time scales provoked by convective rain events and provide recommendations for (1) high-frequency and event-based sampling of water isotopes, (2) application of short-lived radioisotopes, and a (3) combination with concentration-discharge relationships. The CRP will particularly contribute to SDG 6.4: Ensure freshwater supplies by understanding and predicting the impact of climate change on freshwater resources, as well as feed into SDG 2.4, SDG 13.1 and 3, and SDG 15.1.