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

Abstract

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.

On the identification of macroscopic root water uptake parameters from soil water content observations

“…These results suggest therefore that even during a very dry period for which sensitivity for root water uptake is intuitively assumed to be at a maximum, soil water content dynamics is still much controlled by some soil parameters. Such a finding, yet not quantified by a classical sensitivity analysis, is similar to that presented by Gardner [1991] suggesting that the dominant factor in the uptake calculations is the nonlinear water retention curve, and by Musters et al [2000] showing that the uncertainties in plant uptake parameters and those in root distribution hardly affect simulated soil water content dynamics. Other soil parameters (α, K sat , θ r , λ) have a similar or smaller influence on the soil water content than root water uptake parameters.…”

“…This method consists in determining the root water extraction at different depths on the basis of a simplified water balance neglecting the vertical soil water fluxes, considering therefore that all soil water depletion is due to water uptake by plant roots [see, e.g., Rasiah et al , 1992; Musters et al , 2000]. This approach was adopted for our field data set for the period between 6 July and 2 August as this period matches exactly all the conditions previously specified maximizing the success of this method.…”

“…Notwithstanding these successful applications of the inverse method, some recent studies also illustrated the shortcomings of this approach. Musters et al [2000] and Musters and Bouten [2000] presented results showing with the same data set that the soil water content dynamics was hardly affected by root water uptake parameters and that identifiability problems might arise for those parameters.…”

Micro-variability of hydrological processes at the maize row scale: implications for soil water content measurements and evapotranspiration estimates

“…This approach considers that the whole soil water depletion is due to the root water uptake. The methodology is particularly well adapted for dry periods during which root water uptake is larger than soil water fluxes (Musters et al, 2000;. Such conditions are met in our study and the root water uptake is quantified as follows…”

Assessment of water balance simulations for large-scale weighing lysimeters

“…A sensitivity study of Hupet et al (2002) shows for soils with different textures that soil water content is quite insensitive to crop parameters, in particular to root water uptake parameters, at least as compared to soil hydraulic parameters. Moreover, results of Musters et al (2000) illustrate that uncertainties in measured soil water contents were far higher than uncertainties in root water uptake parameters and that uncertainties in uptake parameters hardly affect simulated soil water dynamics. We observed similar results in the measured water retention curves.…”