The land surface–atmosphere interaction is described differently in large-scale surface schemes of regional climate models and small-scale spatially distributed hydrological models. In particular, the hydrological models include the influence of shallow groundwater on evapotranspiration during dry periods where soils are depleted and groundwater is the only water supply. These mechanisms are analysed by combining a distributed hydrological model (MIKE SHE) and a regional climate model (HIRHAM) and comparing simulation results to the FIFE area observation data in Kansas, USA. The numerical experiments include five simulations. First MIKE SHE is forced by observed climate data in two versions (1) with groundwater at a fixed uniform depth, and (2) with a dynamical groundwater component simulating shallow groundwater conditions in river valleys. (3) In a third simulation, MIKE SHE is forced by HIRHAM-simulated precipitation. The last two simulations include (4) a standard HIRHAM simulation, and (5) a fully coupled HIRHAM-MIKE SHE simulation locally replacing the land surface scheme by MIKE SHE for the FIFE area, while HIRHAM in standard configuration is used for the remaining model area. The results show a clear correlation between depth to the groundwater and evapotranspiration with a distinct groundwater depth threshold at 0.5–3 m. During the dry summer period, the two MIKE SHE simulations using distributed groundwater reproduced evapotranspiration better than MIKE SHE with unsaturated flow alone and the HIRHAM simulations. This indicates that including dynamic groundwater in a fully coupled climate-hydrology model may improve evapotranspiration fluxes from areas with shallow groundwater tables.
- Programområde 2: Vandressourcer