CAUTIONARY NOTES ON THE USE OF PEDOTRANSFER FUNCTIONS FOR ESTIMATING SOIL HYDRAULIC-PROPERTIES

The performance of published pedotransfer functions was evaluated in t erms of predicted soil water content, pressure heads, and drainage flu xes for a layered profile. The pedotransfer functions developed by Ver eecken et al. (1989),Vereecken et al. (1990) were used to determine pa rameters of the soil hydraulic functions theta(h) and K(h) which were then used as input to SWATRER, a transient one-dimensional finite diff erence soil water model with root uptake capability. The SWATRER model was used to simulate the hydraulic response of a multi-layered soil p rofile under natural climatic boundary conditions for a period of one year. The simulations were repeated by replacing the indirectly estima ted water retention characteristic by (1) local-scale, and (2) field-s cale mean observed theta(h) relationships. Soil moisture contents and pressure heads simulated at different depths in the soil profile were compared to measured values using these three different sets of hydrau lic functions. Drainage fluxes at one meter below ground surface have also been simulated using the same three sets of hydraulic functions. Results show that simulations based on indirectly estimated moisture r etention characteristics (obtained from pedotransfer functions) overpr edict the observed moisture contents throughout the whole soil profile , but predict the pressure heads at shallow depths reasonably good. Th e results also show that the predicted drainage fluxes based on estima ted retention functions are about four times as high compared to the d rainage fluxes simulated using measured retention curves.