Potentials and limitations of modelling vertical distributions of root water uptake of an Austrian pine forest on a sandy soil

Root water uptake patterns are often studied with simulation models of the unsaturated soil water flow, as they are difficult to measure directly. Cal ibration of these models is not straightforward and causes uncertainties in simulated uptake distributions. In this paper we study how uncertainties i n the calibration of the SWIF model affect uncertainty intervals in simulat ed uptake patterns of an Austrian pine stand (Pinus nigra var. nigra) on a sandy soil. After calibrating and validating SWIF with a large data set of more than 125 000 measured soil water contents over a three year period, un certainty ranges in simulated soil water dynamics and root water uptake dis tributions were estimated with a Monte Carlo analysis. In general, uncertainties in root uptake patterns were small (typically <2 10(-4) m(3) m(-3) day(-1)) and were higher for trees with a shallow rooting system (0.8 m) than for trees with a deep rooting system (2.5 m). Uncertai nties arose mainly from uncertainties in simulated soil water fluxes and fr om variations in the reduction of uptake during periods of drought. Uncerta inties in soil water contents were far higher (typically 0.01 m(3) m(-3)) t han uncertainties in uptake, illustrating that uncertainties in uptake para meters and those in the distribution of water uptake hardly affect the mode lling of soil water dynamics. Root water uptake models should therefore be validated against measured uptake distributions, which can be determined on sandy soils during dry periods with a high water use when soil fluxes are negligible to uptake. Copyright (C) 2000 John Wiley & Sons, Ltd.