Soil moisture and root water uptake in climate models. Research Programme Climate Changes Spatial Planning

More accurate simulation of the energy and water balance near the Earth surface is important to improve the performance of regional climate models. We used a detailed ecohydrological model to rank the importance of vegetation and soil factors with respect to evapotranspiration modeling. The results show that type of lower boundary condition, root zone depth, and temporal course of leaf area index have the strongest effect on yearly and monthly evapotranspiration. Soil texture data from the WISE database in combination with HYPRESS pedotransferfunctions can be used to derive more accurate Mualem-van Genuchten type soil moisture retention and hydraulic conductivity functions. We added recent literature data on root densities of agricultural crops to the root data base of Schenk and Jackson. We tested the HTESSEL land routine of the regional climate model RACMO for Western Hungary, which shows systematically too low evapotranspiration and too high air temperatures in many numerical regional climate studies. Satellite remote sensing data, in combination with the SEBAL algorithm, were used to derive evapotranspiration fluxes at a 1x1 km grid for the year 2005. Compared to satellite data, HTESSEL somewhat underestimated evapotranspiration fluxes. This underestimation occurred mainly in regions with irrigation and shallow groundwater, factors which are not included in HTESSEL. Tests with other reduction functions for root water uptake, more realistic soil depth, and a concept for groundwater influence did not yield more accurate spatially distributed evapotranspiration fluxes for Western Hungary.