Attribution of trends in modeled and observed surface water fluxes

Evaporation and runoff are the major surface water fluxes. Together they determine water availability on land, which ecosystems and humans rely on. The mechanisms driving trends in evaporation and runoff in the context of global change are not fully understood, which is also reflected in a significant uncertainty of projections of these water fluxes across Earth system models. To better understand trends in surface water fluxes and model discrepancies, we examine whether i) trends in total precipitation or ii) changes in the partitioning of precipitation to evaporation and runoff primarily control those changes. While changes in the partitioning are partly related to trends in total precipitation, additional factors such as land cover change and altered precipitation variability act as important contributors.
We analyze the trends in surface water fluxes across the 21st century utilizing data from several Earth system models. We compute differences in the mean daily partitioning of precipitation as well as mean daily precipitation between the first and last 25 years of the century. We find that changes in runoff are primarily driven by changes in precipitation partitioning, while changes in evaporation are dominated by changes in precipitation amount. However, for both surface water fluxes the spatial pattern of the relative importance of total precipitation versus precipitation partitioning varies among models. Our results highlight the importance of considering both changes in precipitation amount as well as changes in partitioning when investigating long-term trends in surface water fluxes.
Additionally, we will compare Earth system model simulations with observation-based data to test the robustness of our conclusions. Understanding the drivers of trends in surface water fluxes can inform related process-based modelling. This enables more accurate projections of the terrestrial water cycle as a basis for more targeted long-term regional water management.