An optimality-based model of the coupled soil moisture and root dynamics

S J Schymanski,J Beringer,M Sivapalan,L B Hutley,M L Roderick

doi:10.5194/hess-12-913-2008

Abstract

Abstract. The main processes determining soil moisture dynamics are infiltration, percolation, evaporation and root water uptake. Modelling soil moisture dynamics therefore requires an interdisciplinary approach that links hydrological, atmospheric and biological processes. Previous approaches treat either root water uptake rates or root distributions and transpiration rates as given, and calculate the soil moisture dynamics based on the theory of flow in unsaturated media. The present study introduces a different approach to linking soil water and vegetation dynamics, based on vegetation optimality. Assuming that plants have evolved mechanisms that minimise costs related to the maintenance of the root system while meeting their demand for water, we develop a model that dynamically adjusts the vertical root distribution in the soil profile to meet this objective. The model was used to compute the soil moisture dynamics, root water uptake and fine root respiration in a tropical savanna over 12 months, and the results were compared with observations at the site and with a model based on a fixed root distribution. The optimality-based model reproduced the main features of the observations such as a shift of roots from the shallow soil in the wet season to the deeper soil in the dry season and substantial root water uptake during the dry season. At the same time, simulated fine root respiration rates never exceeded the upper envelope determined by the observed soil respiration. The model based on a fixed root distribution, in contrast, failed to explain the magnitude of water use during parts of the dry season and largely over-estimated root respiration rates. The observed surface soil moisture dynamics were also better reproduced by the optimality-based model than the model based on a prescribed root distribution. The optimality-based approach has the potential to reduce the number of unknowns in a model (e.g. the vertical root distribution), which makes it a valuable alternative to more empirically-based approaches, especially for simulating possible responses to environmental change.

Highlights

The weakest component of soil-vegetation-atmosphere transfer (SVAT) models is their link with the soil environment (Feddes et al, 2001)
The model was used to compute the soil moisture dynamics, root water uptake and fine root respiration in a tropical savanna over 12 months, and the results were compared with observations at the site and with a model based on a fixed root distribution
Improvements in our understanding and parameterisation of root water uptake are necessary to increase our confidence in the outputs of global circluation models (GCM) that depend on accurate estimates of vegetation water use

Summary

Introduction

The weakest component of soil-vegetation-atmosphere transfer (SVAT) models is their link with the soil environment (Feddes et al, 2001). SVAT models within numerical weather prediction and climate models do a poor job at simulating soil moisture dynamics, which in turn means that the fluxes of water and heat to the atmosphere are poorly represented. Improving this is a priority to advance weather forecasting (Giard and Bazile, 2000). Schymanski et al.: Optimality and soil water-vegetation dynamics water uptake by adding a sink term to Richards’ equation This sink term typically depends on the description of an “effective root-density distribution” of varying complexity and other variables (Varado et al, 2006). A number of such observations have been compiled by Jackson et al (1997)

Objectives

Methods

Findings

Conclusion