Study on the estimation to the amount of evapotransplration based on changes in soil water

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The amount of evapotranspiration can be evaluated mainly by two processes. One depends on the transpiration by plants which is related to the water absorbed from the soil through their roots, the other is due to direct evaporation from the ground surface. We can evaluate the water balance for a soil layr by adding ; the amount of water which infiltrates from the ground surface during rainfall, gravitational water moving down and passing through the layr, evaporation water through the ground surface and transpiration water by plant leaves. When the observed suction value at the upper and lower portions of the layr is known at each boundary of unsaturated transient flow in the soil layr, the input/output flux at the upper and lower boundary can be calculated using a physical law which is related to the soil-water flow. On the basis of this idea, an unsaturated water flow simulation is carried out to evaluate the amount of evapotranspiration by using soil suctions observed with the soil column obtained at actual forest site (Kuroboku Column) and the man made column (Sand Column) with Japanese standard sand. The analysis results are in agreement with the observed soil water fiux in both column. On the other way, we apply this idea to the actual forest site, measuring the soil suction changes at the upper layr (10cm in depth) and lower layr (40 cm in depth). Changes in the hydraulic gradient are calculated by using observed soil suction changes, and the soil water flux can be estimated with the unsaturated permeability of the site. It is necessary to measure the soil suction changes in the nearest top soil, in order to estimate direct flux from the forest land surface.