Enhancing Hydrological Processes in Earth System Models: Implementing Groundwater Dynamics for Improved Climate Representations

In the context of climate change, a global, widespread shift to increased water limitation is expected over approximately 73% of terrestrial ecosystems, with important implications for food and water security, CO2 uptake, and evaporative cooling. Water-limited regions, exposed to climate-change-related increasing droughts and intense anthropogenic water use, are extremely vulnerable to transitions towards drier eco-hydro-climatological regimes. In the longer term, the ongoing drought conditions may intensify the decline of groundwater levels, threatening groundwater-dependent ecosystems and exacerbating the risk of desertification, thereby amplifying a positive feedback on regional climate change. In some Mediterranean climate-type regions, such as SouthWestern Australia, a dry and warm transition has already been observed. Recent findings are a clear warning that also over the Euro-Mediterranean sector groundwater level may have a negative trend resulting from a decrease in precipitation and/or increasing withdrawal. 

Soil water storage  and groundwater dynamics represent important hydrological processes related to these transitions but they are greatly simplified in state-of-the-art Earth System Models (ESMs). Therefore, it is  essential to improve the representation of hydrological processes and their coupling with the atmosphere and the land surface in ESMs. In this respect, the land surface model included in EC-Earth (ECLAND) still lacks a representation of groundwater and instead implements a free drainage condition at the bottom of the unsaturated soil column. 

In this work, we intend to implement a more realistic groundwater representation in EC-Earth by including a global-scale water table to replace the free drainage bottom boundary condition. As a preliminary measure, the impact of groundwater on the shallow, unsaturated zone is evaluated by constraining the vertical water fluxes with a static water table depth (WTD) derived from a global estimate simulation based on observations. We evaluated the effects of this implementation on water and energy fluxes against a network of stations in land-only simulations from 1979 to the present, with boundary forcing taken from ERA5 reanalysis. First findings suggest that including a WTD has an impact on water exchanges between saturated and unsaturated soil in water-limited regions, particularly in semi-arid and transitional climates, which can not be neglected in Earth system models.