SIMULATING EVAPORATION FROM SHORT-ROTATION FOREST - VARIATIONS WITHIN AND BETWEEN SEASONS

A physically based soil water model was applied to a fertilized and irrigated 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. The Penman-Monteith combination equation is used to calculate potential daily transpiration, soil evaporation and potential interception evaporation. Daily meteorological data are used as driving variables. Evaporation, estimated by the energy balance/Bowen ratio method, and soil-water tension measurements made over several years, were used to verify the model. Measured evaporation of water intercepted by the vegetation over 1 year was also compared with simulated monthly values. Good agreement was found between simulated evaporation and evaporation determined from Bowen ratio measurements. The cumulative seasonal evaporation 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 through October. On a mean seasonal basis, transpiration was 66%, soil evaporation 23% and interception evaporation 11% of total evaporation. The monthly interception evaporation comprised 5-23% of precipitation. The study period showed interannual variations attributable to variation in climate (including irrigation) as well as to stand age and development. This indicates that the model is quite general in many respects; it is tentatively suggested that it be used for accurate simulation of water balance components of short-rotation forest on a clay soil in this type of climate.