MODELING SOIL-WATER DYNAMICS IN A FORESTED ECOSYSTEM .1. A SITE SPECIFIC EVALUATION

Soil water dynamics in a forested lowland catchment are studied in order to support a research on biogeochemical processes and soil acidification in particular. The numerical simulation model SWIF, describing soil water dynamics in forests, is calibrated and validated for a specific site by using three years of daily measurements of pressure head profiles. On the basis of model results, the hydrological behaviour of the forest system is discussed, with special attention to transpiration and to the vertical and temporal dynamics of soil water contents, root water uptake and soil water fluxes. As water uptake by oaks and beeches is restricted to the unsaturated soil zone, high groundwater tables in the rather poorly drained duplex soil appear to have a large impact on the soil water dynamics. Suggestions are made on the implications of the hydrology for soil acidification. This discussion shows that a thorough knowledge of the hydrological behaviour of the system can greatly contribute to the understanding of biogeochemical processes and soil acidification. This paper is the first in a series of three papers. The second discusses the possibilities of extrapolating model results from the calibration site to other locations within the research area, which have varying soil horizon thicknesses (Bouten and Witter, 1992). The third paper gives a full description of the model SWIF with some technical details. It also discusses its numerical behaviour (Tiktak and Bouten, 1992).