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Abstract. Rooting depths in a forested stand (0.11 ha) were estimated indirectly by inverse modeling maps of soil water contents from time domain reflectometry (TDR) measurements at 150 points. These maps were described with a calibrated onedimensional soil water flow model, with specific values for the rooting depth, van Genuchten's [1980] a parameter, and throughfall fraction at each point. At about one third of the 150 points, modeled rooting depths did not fall within the a priori likely range of rooting depths; these values were discarded when kriging the point estimates to a map of rooting depths. The resulting rooting depths range from 0.8 m to 3.5 m within the stand in good agreement with root observations (R 2 = 0.85). Several factors contributed positively to this good match, such as the fairly uniform soil hydraulic properties and the high sensitivity of soil water dynamics to root water uptake. Overall, the results demonstrate the suitability of soil water content maps based on TDR measurements to quantify spatial variability in soil water dynamics. IntroductionRoot water uptake in ecosystems is often studied with soilvegetation-atmosphere (SVAT) models that integrate the simulation of the different elements of the hydrological cycle. SVAT models offer a tool to estimate plant water uptake and its distribution over the soil as plant uptake fluxes cannot be measured directly. From a hydrological perspective, water fluxes to the atmosphere and groundwater are largely controlled by root systems of trees Schaap et al., 1995]. Rooting depths are therefore key parameters in SVAT models but are difficult and tedious to collect by classical methods such as root excavation and soil coring [Atkinson, 1991]. As a consequence, rooting depths are often only known by estimation; hence results of SVAT models are difficult to interpret.Potentially, root distribution parameters like the rooting depth can be estimated by inverse modeling the soil water dynamics. This indirect estimation is preferable to conventional root observation methods, mainly because root information is not directly related to root activity [Gardner, 1991]. Inverse modeling will succeed only if root water uptake significantly influences the soil water dynamics; its use is therefore restricted to shallow root systems (<4 m) and to dry periods when sensitivity of soil water dynamics to root water uptake is largest. Also, root parameters cannot be estimated indirectly without accurately describing the soil water fluxes. Thus inverse modeling requires a calibrated soil water flow model and consequently a monitoring program to provide the necessary data for calibrating a soil water flow model. In addition, roots never grow uniformly through a soil, and root parameters should therefore be studied on a relatively large scale (>100 The use of indirect methods to estimate root parameters has for long time been frustrated by the inability to measure The aim of this study was to estimate rooting depths in a forested pine stand by inverse modeling the soil ...

Root water uptake patterns are often studied with simulation models of the unsaturated soil water flow, as they are difficult to measure directly. Calibration of these models is not straightforward and causes uncertainties in simulated uptake distributions. In this paper we study how uncertainties in the calibration of the SWIF model affect uncertainty intervals in simulated 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, uncertainty ranges in simulated soil water dynamics and root water uptake distributions were estimated with a Monte Carlo analysis. In general, uncertainties in root uptake patterns were small (typically <2 10−4 m3 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). Uncertainties arose mainly from uncertainties in simulated soil water fluxes and from variations in the reduction of uptake during periods of drought. Uncertainties in soil water contents were far higher (typically 0·01 m3 m−3) than uncertainties in uptake, illustrating that uncertainties in uptake parameters and those in the distribution of water uptake hardly affect the modelling 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 © 2000 John Wiley & Sons, Ltd.

Abstract:Root water uptake patterns are often studied with simulation models of the unsaturated soil water¯ow, as they are dicult to measure directly. Calibration of these models is not straightforward and causes uncertainties in simulated uptake distributions. In this paper we study how uncertainties in the calibration of the SWIF model aect uncertainty intervals in simulated 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, uncertainty ranges in simulated soil water dynamics and root water uptake distributions were estimated with a Monte Carlo analysis.In general, uncertainties in root uptake patterns were small (typically 52 10 À4 m 3 m À3 day À1) and were higher for trees with a shallow rooting system (0 . 8 m) than for trees with a deep rooting system (2 . 5 m). Uncertainties arose mainly from uncertainties in simulated soil water¯uxes and from variations in the reduction of uptake during periods of drought. Uncertainties in soil water contents were far higher (typically 0 . 01 m 3 m À3) than uncertainties in uptake, illustrating that uncertainties in uptake parameters and those in the distribution of water uptake hardly aect the modelling 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¯uxes are negligible to uptake.

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