Comparison of root water uptake modules using either the surface energy balance or potential transpiration

摘要

Numerical models simulating changes in soil water content with time rely on accurate estimation of root water uptake. This paper considers two root water uptake modules that have a compensation mechanism allowing for increased root uptake under conditions of water stress. These modules, proposed by Lai and Katul and Li et al. [Adv. Water Resour. 2.3 (2000) 427 and J. Hydrol. 252 (2001) 189] use potential transpiration weighted, for each soil layer, by a water stress and a compensation function in order to estimate actual transpiration. The first objective of the paper was to assess the accuracy of the proposed root extraction modules against two existing data sets, acquired under dry conditions for a winter wheat and a soybean crop. In order to perform a fair comparison, both modules were included as possible root water extraction modules within the Simple Soil Plant Atmosphere Transfer (SiSPAT) model. In this first set of simulations. actual transpiration was calculated using die solution of the surface energy budget as implemented in the SiSPAT model. Under such conditions, both root extraction modules were able to reproduce accurately the time evolution of soil moisture at various depths, sod water storage and daily evaporation. Results were generally improved when we activated the compensation mechanisms. However. we showed that Lai and Katul [Adv. Water Resour. 23 (2000) 427] module was sensitive to soil hydraulic properties through its water stress function, whereas the Li et al. [J. Hydrol. 252 (2001) 189] module was not very sensitive to the specification of its parameter. The latter module is therefore recommended for inclusion into a larger scale hydrological model, due to its robustness.When water balance models are run at larger scales or on areas with scarce data. actual transpiration is often calculated using. models based on potential transpiration without solving the surface energy balance. The second objective of the paper was to assess the loss of accuracy in such conditions for the Lai and Katul and Li et al. [Adv. Water Resour. 2.3 (2000) 427 and J. Hydrol. 252 (2001) 189] modules. For this purpose we compared results from the SiSPAT model solving the surface, energy balance with those of a degraded version where only potential evapotranspiration was imposed as input data. We found that actual transpiration and evapotranspiration were in general underestimated, especially for the Lai and Katul [Adv. Water Resour. 23 (2000) 427] module. when we used the potential evapotranspiration as calculated from FAO standards. The use of crop coefficients improved the simulation although standard values proposed by the FAO were too small. The definition of the potential evapotranspiration was the major source of error in simulating soil moisture and daily evaporation rather than the choice of the root extraction modules or the inclusion of a compensation mechanism. When used for water management studies, a sensitivity to the definition of potential evapotranspiration used to run the models is therefore advisable. (C) 2004 Elsevier B.V. All rights reserved.