Modelling the spatial and temporal variations of the water balance for theWeser catchment 1965-1994

This study describes the application of the physically based SVAT model PRO MET (PRocess Oriented Model for EvapoTranspiration) in the mesoscale catchm ent of the Weser (Northern Germany, approx. 37,500 km(2)) utilizing a 30 ye ars time series of meteorological input data. To enable a representative an alysis of the spatial and temporal variations of the water balance componen ts, the modelization is performed continuously without re-initialization of the state variables or specific calibration. Therefore, PROMET is expanded with the one-layer snow model ESCIMO (Energy balance Snow Cover Integrated Model) to provide an integrated model structure for continuous simulations of the water cycle. All necessary input data fields are integrated in a fo ur-dimensional GIS data structure with a raster grid spacing of 1 km: a DEM , soil texture information derived from digitized maps, landuse distributio n computed by unmixing a time series of NOAA/AVHRR satellite images and met eorological input data fields which are spatially and temporally interpolat ed using data provided by the standard measurement network of the German We ather Service (DWD). Spatially non-distributed physical soil and plant para meters are either derived from measurements or taken from literature. The s tudy presents the structure of ESCIMO and its validation at the point and t he catchment scale. Then, the modelled mean annual evapotranspiration, aET, as obtained by application of the linked models PROMET/ESCIMO is compared with the corresponding term ET calculated by inserting the measured precipi tation and gauged runoff into the water balance equation. It is started fro m the assumption that for the 30 years period, the overall underground stor age change DeltaS is negligible. The mean annual deviation aET-ET over the 30 years period is 10.9 mm, indicating that the results represent a valid l ong-term description of the water balance. The patterns of the simulated wa ter balance components are discussed with respect to the physical-geographi cal properties of the environment and their interaction with the predominan t meteorological conditions. The study then addresses smaller spatial and t emporal scales: for the subcatchment scale, the variation of aET-ET and thu s the model error is considerably larger; for shorter periods, aET-ET is us ed for the estimation of DeltaS, The simulation accuracy as depending on th e length of the aggregation period and the mean monthly evolution of the st orage are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.