An integrated approach for the determination of regional evapotranspiration using mesoscale modelling, remote sensing and boundary layer measurements

A method is presented for the calculation of regional evapotranspiration by means of mesoscale modelling using the "Lokal-Modell" (LM) of the German Weather service with 2.8 km horizontal resolution. The shortcomings of the model with respect to the quantitative simulation of cloudiness and precipitation are overcome by the assimilation of precipitation and insolation (derived by radar and METEOSAT data, respectively) into the SVAT module of the LM. Three case studies are presented in order to quantify the influence of the assimilation on the simulated evapotranspiration process. The simulations are validated using ground-based measurements of surface layer quantities, turbulent fluxes and boundary layer structures. Different methods for the determination of the turbulent flux of latent heat are intercompared. Simulated surface: energy fluxes are strongly influenced by the assimilation, which highly improves the results for the evapotranspiration field in the case of a strong rain event. The validation studies for cases under fair weather conditions show large uncertainties in the simulated energy fluxes. The sensitivity with respect to the soil moisture content is investigated. An artificially increased soil moisture results in a significantly better representation of the turbulent fluxes in the LM, but the agreement with simultaneously measured boundary layer structures is reduced.