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

蒸散量的估算主要通过两个过程进行。一种依赖于植物的蒸腾作用，这与植物通过根系从土壤中吸收的水分有关，另一种依赖于地表的直接蒸发。我们可以通过添加；降雨过程中从地表渗透的水量，向下移动并穿过该层的重力水，通过地表的蒸发水和植物叶片的蒸腾水。当土层内非饱和瞬态流动各边界处的上、下吸力观测值已知时，可以利用与土-水流动有关的物理规律计算出上下边界处的输入/输出通量。在此基础上，利用在实际森林立地获得的土柱（Kuroboku柱）和用日本标准砂制作的土柱（Sand柱）所观测到的土壤吸收量，进行了非饱和水流模拟，对蒸散量进行了评价。分析结果与实测的两柱土壤水通量基本一致。另一方面，我们将这一想法应用于实际的森林场地，测量了上层（10cm深）和下层（40cm深）的土壤吸力变化。利用观测到的土壤吸力变化来计算水力梯度的变化，并利用场地的非饱和渗透率来估算土壤水通量。为了估算森林地表的直接通量，有必要测量最近表层土壤吸力的变化。

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.