SIMULATING EVAPORATION FROM SHORT-ROTATION FOREST - VARIATIONS WITHINAND BETWEEN SEASONS

A physically based soil water model was applied to a fertilized and ir rigated short-rotation willow stand on a clay soil. The model is based on an extension of Richards' equation, and the water retention curve and saturated conductivity are determined by analyses of soil cores. T he Penman-Monteith combination equation is used to calculate potential daily transpiration, soil evaporation and potential interception evap oration. Daily meteorological data are used as driving variables. Evap oration, estimated by the energy balance/Bowen ratio method, and soil- water tension measurements made over several years, were used to verif y the model. Measured evaporation of water intercepted by the vegetati on over 1 year was also compared with simulated monthly values. Good a greement was found between simulated evaporation and evaporation deter mined from Bowen ratio measurements. The cumulative seasonal evaporati on exceeded the Penman open water evaporation by up to 31% in 3 years out of 4; it ranged between 416 and 584 mm for the period from May thr ough October. On a mean seasonal basis, transpiration was 66%, soil ev aporation 23% and interception evaporation 11% of total evaporation. T he monthly interception evaporation comprised 5-23% of precipitation. The study period showed interannual variations attributable to variati on in climate (including irrigation) as well as to stand age and devel opment. This indicates that the model is quite general in many respect s; it is tentatively suggested that it be used for accurate simulation of water balance components of short-rotation forest on a clay soil i n this type of climate.