Forecasts of plant available and seepage water for agricultural usage during recent extreme hydrometeorological conditions in western Germany using a convection-permitting regional Earth-system model

The aim of the ADAPTER project (www.adapter-projekt.de) of the Helmholtz Association of German Research Centres is to develop products and usable information that help improve agriculture’s resilience to extreme weather conditions and climate change in Germany. One of the main hydrometeorological impacts on agriculture is the soil water budget. Here, we use the Terrestrial Systems Modelling Platform (TSMP) in forecast mode forced by ECMWF forecast data over a domain covering most of North-Rhine Westfalia (NRW, Germany). TSMP is a fully-coupled regional Earth system model with COSMO at 1km spatial resolution as the atmospheric component, with the Community Land Model (CLM) for the land surface interface, and ParFlow for the surface and sub-surface part of the water cycle, both models at 500m resolution. This allows a representation of the closed water budget, including three-dimensional sub-surface and groundwater flow. Here, we demonstrate the usefulness of the fully coupled TSMP for agriculture applications, by focussing on two fundamental parameters of the soil water budget: First, in the context of the droughts that affected Europe, and particularly Germany, over the summers 2018 and 2019, one major parameter for estimating and monitoring the water stress of plants is the fraction of plant available water (fPAW). The pressure head simulated by ParFlow is used to calculate fPAW, on the basis of soil parameters like porosity and the Van Genuchten equation. fPAW is calculated over different soil depths from 0.1m to 3m, to provide information about the water stress of plants with different rooting depths. Our results show that the succession of extremely dry summers in 2018 and 2019, when the meteorological drought evolved into an agricultural and eventually into a hydrological drought, has led to very dry soils showing a fPAW below 30-50% over most of NRW, meaning that it became stressful for plants to extract water from the soil. This did not only affect the upper soil layers, as in 2018, but also deeper layers became very dry, thus no longer only impacting shallow root crops, but also plants with a higher root depth like trees. The wetter 2019 autumn allowed a recovery of the soil water content around the field capacity for the upper layers over a major part of the domain, while the deeper soil remains abnormally dry, especially in the south-western part of the domain. Second, knowing the amount of seepage water over a given period is not only important to monitor the groundwater recharge, which has become a major issue in the context of the past two summers, but also to estimate the leakage of nutrients and pollutants from the upper soil to deeper layers or even the groundwater in the context of certain environmental compliance issues. In accordance with the results obtained for fPAW, TSMP simulated seepage water flux during autumn 2019 only for the upper soil layers; this excessive water is only gradually percolating into deeper soil layers, which still remain clearly below the field capacity over a significant part of the domain.