On transpiration and soil moisture content sensitivity to soil hydrophysical data

Sensitivity of evapotranspiration E and root zone soil moisture content h to the parameterization of soil water retention Psi(theta) and soil water conductivity K(Psi), as well as to the definition of field capacity soil moisture content, is investigated by comparing Psi1-PMSURF and Theta-PMSURF models. The core of PMSURF (Penman-Monteith Surface Fluxes) consists of a 3-layer soil moisture prediction module based on Richard's equation in combination with the Penman-Monteith concept for estimating turbulent heat fluxes. Psi1-PMSURF and Theta-PMSURF differ only in the parameterization of the moisture availability function F(ma). In Psi1, F(ma) is parameterized by using Psi(theta) and K(Psi) hydrophysical functions; in Theta, Fma is parameterized by using hydrophysical parameters: the field capacity theta(f) and wilting point theta(w) soil moisture contents. Both Psi1 and Theta are based on using soil hydrophysical data, that is, there is no conceptual difference between them in the parameterization of E even if in Psi1 Fma depends on 12 parameters, while in Theta only on two soil/vegetation parameters. Sensitivity tests are performed using the Cabauw dataset. Three soil datasets are used: the vG (van Genuchten), CH/vG (Clapp and Hornberger/van Genuchten) and CH/PILPS (Clapp and Hornberger/ Project for Intercomparison of Land-surface Parameterization Schemes) datasets. The vG dataset is used in van Genuchten's parameterization, while in Clapp and Hornberger's the CH/vG and CH/PILPS datasets are used. It is found that the consistency of soil hydrophysical data in the simulation of transpiration is quite important. The annual sum of E obtained by Psi1, E Psi1, differs from the annual sum of E obtained by Theta, E Theta, because of the inconsistency between the fitting parameters of Psi(theta) and K(Psi) and the theta(f), and not because of the differencies in the parameterization of F(ma). Further, theta(f) can be estimated not only on the basis of using soil hydrophysical functions (the theta(f) so obtained is theta(Soil)(f)) but also on the basis of analysing the transpiration process (the theta(f) so obtained is theta(tr)(f)). theta(tr)(f) values estimated from the condition E(Theta) approximate to E(Psi1) are in acceptable accordance with the theta(Soil)(f) values proposed by Wosten and co-workers. The results are useful in optimizing the parameterization of transpiration in land-surface schemes.