Soil water dynamics and long-term water balances of a Douglas-fir stand in the Netherlands

Seasonal dynamics of soil water, the annual water balances over a period of 30 years (1960-1990) and the frequency of occurrence of extreme water shortage were quantified with the model 'SWIF' for a Douglas fir stand on a sandy soil in the Netherlands. This information was needed for evaluating the impact of air pollution and water shortage on forest growth, and to calculate chemical budgets. Measurements necessary for model parameterization included rainfall, throughfall, transpiration, soil water content and soil water pressure head. These data are available for 3 years. For the period without measurements, throughfall and potential evapotranspiration were calculated from synoptic weather data with an empirical model. The effect of using synoptic weather data instead of detailed on-site measured data was analyzed. The results show that after calibration of the empirical model the simulated annual water balance and seasonal soil water dynamics were well described with synoptic weather data as well as with detailed on-site measured data. The modelled annual water balances for the period 1960-1990 are presented as cumulative frequency distributions. Median values of the major terms of the water balance and the frequency of occurrence of extreme water shortage were derived from these diagrams. Annual median precipitation was 834 mm, calculated median interception loss 317 mm, median transpiration 363 mm, median soil water evaporation 32 mm and median vertical drainage at 150 cm depth 195 mm. The sum of the major losses is not equal to median annual precipitation as the median values of the individual terms may occur in different years. Transpiration shows smaller variations (11%) between the years than throughfall (54%) and vertical drainage (112%). Although the median value of transpiration reduction resulting from water stress is low (4%), some years showed extreme water shortage, the highest transpiration reduction being 31% for 1976. Although total transpiration reduction during median years is low, short periods with considerable drought stress occur during these years.