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

蒸散量主要由两个过程估算。一种是植物通过根系从土壤中吸收水分的蒸腾作用，另一种是地表直接蒸发。我们可以通过增加降雨时从地表入渗的水量、向下移动并通过土层的重力水、通过地表的蒸发水和植物叶片的蒸腾水来评价土层的水分平衡。在土层非饱和瞬态流的各边界处，当土层上、下部分的吸力观测值已知时，可利用与土-水流动有关的物理定律计算上、下边界处的输入/输出通量。在此基础上，利用实测林地土柱（黑北土柱）和日本标准砂人工土柱（砂柱）观测的土壤吸力，进行了非饱和水流模拟，以估算蒸散量。分析结果与实测的土壤水流量基本一致。另一方面，我们将这一思想应用于实际的森林立地，测量了上层（10 cm深度）和下层（40 cm深度）土壤吸力的变化。通过观测到的土壤吸力变化计算水力梯度的变化，并可以估计土壤水通量与非饱和渗透率的网站。为了估算林地地表的直接通量，有必要测量最近表层土壤的吸力变化。

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.