ELSEVIER Ecological Modelling 83 (1995) 245-253 E(OLOGI(liL mODELLInG Water balance of the Solling spruce stand as simulated with various forest-soil-atmosphere models Willem Bouten a, *, Per-Erik Jansson b a Landscape and Environmental Research Group, University of Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam, Netherlands b Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, S-750 07 Uppsala, Sweden Accepted 11 January 1995 Abstract In connection with the workshop 'Comparison of Forest-Soil-Atmosphere Models,' a number of models were applied to the Soiling spruce site in Germany. A common set of driving variables and validation data was available for a period of 14 years. Four models, all using the Richards' equation for water transport in the soil profile, were compared and evaluated with respect to differences in various water balance components. In addition, simulations of annual water budgets were compared for five other models with simplified representation of hydrological processes. Major differences were found in the simulated results of the partitioning of evapotranspiration into interception losses, transpiration and forest floor evaporation with respect to both temporal dynamics and total amounts. These differences were found to be mainly caused by different parameterizations and calibration procedures using either tensiometer measurements or chloride concentrations. Generally it was difficult to identify differences caused by the various model concepts because of the more important differences in parameterization procedures. The large variation in results reflects uncertainties in assigning the water balance terms of the Solling site on the basis of the available measurements rather than major differences in the understanding of hydrological processes. Keywords: Forest ecosystems; Hydrology; Model comparison; Soil I. Introduction Almost all models which were presented at the workshop on 'Comparison of Forest-Soil-Atmo- sphere Models' (van Grinsven et al., 1995) con- tain a module to describe hydrological processes. Models aim at quantifying (i) vertical soil water fluxes, as water is the main transporting agent for chemical constituents, (ii) water uptake by the * Corresponding author. forest, as transpiration is a key process in the functioning of plants, or (iii) the soil moisture condition, as it regulates a number of biological and chemical processes in the soil. The models used do not cover the state of the art with respect to detailed understanding of all hydrological pro- cesses in a forest ecosystem. Detailed models describing micro-meteorological exchange of wa- ter and energy between forest and atmosphere are not represented. The time resolution of the available data was not suitable to run these mod- els. 0304-3800/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-3800(95)00102-6