Validation of TERRA-ML with discharge measurements

We evaluate the runoff-simulation performance of a water transport model (routing scheme) coupled to the Land Surface Parameterization module TERRA-ML of the operational COSMO (Consortium for Small-Scale Modelling) weather forecast model. In addition to the successful implemention of the routing scheme, we also included an alternative vertical soil water transport parameterisation in TERRA-ML in order to estimate the uncertainty caused by the component of the LSP central to runoff generation. A combination of two data sets, both operational products by DWD, is used for precipitation input. These are the hourly precipitation data set RADOLAN RW, which is based on radar data and is calibrated by rain gauges, as well as the daily REGNIE data set, which is only based on gauge data. The mesoscale Sieg river catchment located in Western Germany is used as the evaluation testbed. The extended TERRA-ML was run in standalone mode (decoupled from the atmospheric part of the COSMO model) with I x I km spatial resolution from April to September 2005 based on and provided with spatially more detailed descriptions of topography, land use and soil texture. The model was driven by operational COSMO analysis data and two different sources of observed precipitation (gauge and radar measurements). The results are compared to discharge measurements. They indicate a good representation of the observed discharge by the extended TERRA-ML system. The additionally implemented linear vertical soil water parameterization overestimates total discharge less (6 %) than the default exponential parameterization (20 %) when compared to a gauging station located at the lower reaches of the river Sieg. Suggestions are given on how to further enhance the modelled discharge by improvements in the LSP scheme.