裸地における土壌水分と蒸発量の関係についての検討

Abstract

土壌水分量の減少にともなって裸地面からの蒸発が抑制される過程について, 蒸発環境が一定の恒温恒湿室におけるポット実験で検討を加えた。標準砂と田上山のマサ土を用いたところ, 土層厚25cm, 水分が十分存在するときの蒸発量約2mm/dayの条件下で, 土壌の平均体積含水準が標準砂12. 1％, マサ土17. 3％以下で蒸発量の低下が見られた。蒸発量の低下が見られたのは実験開始後, 標準砂は26日目, マサ土は38日目であった。蒸発量の低下が生じた後の減少傾向は, はじめが急で次第に緩慢となるが, 標準砂の方がマサ土よりも変化が急であった。一次元不飽和浸透モデルにより土中の水分移動をシミュレートしたところ, 臨界含水率以下では含水率の減少に比例して蒸発が抑制されるという蒸発抑制条件を組み込むことにより, 土壌内の水分移動と蒸発量の変化傾向を実験結果とほぼ適合させることができた。このシミコレーションから得られた蒸発量の低下がはじまる土壌表層の含水率は, 標準砂5. 4％, マサ土17. 0％となり, PF表示では標準砂PF2. 23, マサ土PF2. 47と推定きれた。

The relation between the amount of evaporation from bare ground and its soil moisture content has been investigated experimentally. The materials used were the Toyoura standard and (Soil-A) and the weathered granitic soil (Soil-B). The soil colums had 21cm in diamenter, 27cm in height and the materials were packed 25cm in depth uniformly. The colums were settled in air conditionning room having a constant temperature and relative humidity. The evaporation rate under nonstressed condition in this room was about 2mm/day all the time. The evaporation rate from colums and the mean soil moisture content were measured by weighing and the soil moisture suction was measured by tensiometer attached to the colum. The decrease of the evaporation began on the 26th day using Soil-A and on the 38th day using Soil-B and the mean volumetric moisture contents at the beginning time of evaporation decreasing were 12.1% and 17.3%, respectively. The water movement in the soil colums has been simulated by the one dimensional model of the saturated-unsaturated flow solved by a finite difference method under the condition that the evaporation from surface has been expressed as a function of the volumetric moisture content in the surface layer. The calculated trends of the soil moisture suction and the evaporation rate using the soil water characteristics of both materials have agreed with the measured results adequately. In this simuration, the soil moisture content in the surface layer at the beginning time of evaporation constriction has been estimated 5.4% (PF2.23) for Soil-A and 17.0% (PF2.47) for Soil-B.