WILLOW STAND EVAPOTRANSPIRATION SIMULATED FOR SWEDISH SOILS

A soil model parameterized for an intensively managed willow stand on clay soil was applied to other willow stands on sandy, clay and organi c soils in south-western Sweden. The model was site adapted regarding groundwater conditions and soil physical properties, such as the hydra ulic conductivity and retention curve. To identify differences in evap oration properties between different stands, simulated and measured va lues of soil water tension at different depths were compared. At most sites, the simulated tension levels were higher than measured values. Modified simulations with site adapted parameters regarding root depth , leaf area index and the critical tension for water uptake lowered th e tension levels to an acceptable degree. However, the modifications d id not improve the dynamic behaviour for all sites. The level of agree ment between simulated and measured tensions was largely determined by the assumptions concerning groundwater conditions. At some sites, the root depth and critical tension for water uptake were also important parameters. The main factor limiting evapotranspiration at the unirrig ated sites was the availability of water which in turn was determined by groundwater level, soil properties and root depth. At the irrigated site, where soil water was not limiting, evaporation was only affecte d by a change in leaf area index. Evapotranspiration ranged from 365 m m to 495 mm for the period from May to October. Monthly crop coefficie nts showed similar seasonal patterns of change at the different sites with low values in May and maximum values at the end of the season. Th e increase in crop coefficients was attributed to the increase in leaf area. The high values at the late season were attributed to a low aer odynamic resistance.