SPATIAL PATTERNS OF THROUGHFALL AND SOIL-WATER DYNAMICS IN A DOUGLAS-FIR STAND

Measured spatial patterns of water uptake were found to be related to measured throughfall patterns around trees, especially in dry months. To simulate these lateral feedback mechanisms, the one-dimensional soil water model SWIF was modified to a quasi-three-dimensional model allowing preferential water uptake from wet sites. Input-output relations, linking soil physical input parameters to simulation results, showed that soil water contents at field capacity and those after a dry period in summer could be used to parametrize the soil physical characteristics. To assess parameter values for the 2.5-ha research area, soil water contents were mapped using data from vertically installed time domain reflectometry sensors. Model results indicated that the spatial distributions of yearly water uptake and percolation fluxes were strongly affected by throughfall patterns, whereas soil water contents primarily depended on soil physical properties. Results also indicated that refined model parametrization can improve the reliability of model predictions of soil water dynamics at specific sites.