Development of hydro-pedotransfer functions to predict capillary rise and actual evapotranspiration for grassland sites

New hydro-pedotransfer functions (HPTF) for flat grasslands are presented to estimate both annual capillary rise from the groundwater into the root zone and actual evapotranspiration on a regional scale. Based on easily available site information, soil water components such as percolation rate and therefore groundwater recharge can also be evaluated. This information may be obtained without detailed knowledge of soil hydraulic properties and daily weather data. Rather, the only data needed is soil texture class, groundwater depth, summer rainfall and potential evapotranspiration (ET0) according to the FAO guideline. The basic idea is to evaluate the increase of actual evapotranspiration (=gain, G) caused by capillary rise from groundwater compared to identical site conditions, but without groundwater influence. This gain (G) represents an effective parameter to express both the soil and climate dependent effective capillary rise for a given grassland site. To develop hydro-pedotransfer functions expressing gain, we first used the numerical simulation program SWAP in order to calculate water balances for a broad spectrum of soils, groundwater depths, and climate conditions. Secondly we analyzed this data statistically in order to obtain simple equations for predicting G without using a numerical model. The new hydro-pedotransfer functions were developed and tested for different climate regions in Germany.