Study on Influences of Salinization on the Evaporation Processes in a Semi-arid Area in Western Australia

为了明确地表蒸发与盐渍化的关系，从2000年开始，在西澳大利亚半干旱区（Yalanbee）的裸土区进行了为期3年的气象水文观测。本研究通过对实测数据的分析和辅助蒸发实验等，探讨了二者之间的关系。(1)野外盐壳是夏季（旱季）和冬季蒸发作用形成的。冬季（雨季）土壤被降雨浸湿。然而，旱季的反照率（太阳辐射在地面上的反射率）比雨季小10%。这种反向关系在亚兰贝土的试验中没有被发现，这仍然是一个未来的问题。(2)旱季野外反照率随太阳高度减小，但最小值不是在中午出现，而是在上午10点左右。更多的是由于蒸发使地面干燥，因为土壤湿度的减少通常会使反照率增加。这种情况与圆顶蒸发实验的结果相对应，蒸发速率从上午10点左右开始突然下降，由于太阳辐射的增加，蒸发速率从日出开始持续增加。(3)在旱季，日变化的大气温湿度和太阳辐射引起土壤中的蒸发凝结。通过与这些实际变化相似的气象条件下的室内雅兰比-土柱试验，阐明了当用扩散方程描述土层中的水汽运动时，其修正后的扩散系数约为原扩散系数的100倍。(4)在地下水位较浅的土柱蒸发实验中，观察到上层土壤表面不均匀地形成了含盐结壳，其发展速度非常快，在实验开始后约5天内结壳面积达到60%左右。蒸发速率与地壳面积成反比呈线性减小，水汽有效可转移截面的减小导致水汽通量和蒸发速率的显著减小。少

In order to clarify the relation between evaporation from the ground surface and salinization, a meteorological and hydrological observation was undertaken in a bare soil area in a semi-arid region (Yalanbee), Western Australia for three years from 2000. In this research, through analysis on that obtained data and a subsidiary evaporation experiment so on, that relation was investigated. The following was disclosed:(1) In the field, salty crust is formed by occurrence of evaporation in summer (dry season) and in. winter (wet season) soil is wetted by rainfall events. However albedo (reflection rate of solar radiation on the ground) in the dry season becomes by 10% smaller than that in the wet season. This reverse relation could not be recognized through an experiment using Yalanbee-soil and still remains as a future problem.(2) Albedo in the field in the dry season decreases with solar altitude, but its minimum was detected to appear not at noon but around 10 a.m. This is considered to … More be due to drying of ground surface by evaporation because reduction of soil wetness usually lets the albedo increase. This situation corresponds to such a result by a dome evaporation experiment that evaporation rate decreases abruptly from around 10 a.m. via its continuous increase from sunrise due to increasing solar radiation.(3) In the dry season, daily changes in atmospheric temperature-humidity and solar radiation cause the evaporation -condensation in the soil. Through a laboratory Yalanbee-soil column experiment under a meteorological condition similar to these actual variations, it was clarified that, when vapor movement in the soil layer is described by a diffusion equation, its modified diffusion coefficient becomes approximately 100 times larger than the original one. This increase is concluded to be caused by occurrences of evaporation and condensation, and structure and behavior of the modified coefficient were also clarified theoretically(4) In an evaporation experiment using the soil column with a shallow water-table, salty crust was observed to be formed non-uniformly on the upper soil surface and its development was very fast as the crust area became about 60% within about five days from the beginning of experiment. Also it was recognized that the evaporation rate decreased linearly with inverse of the crust area, and reduction in effective transferable cross-section for vapor leads to the decrease in vapor flux, evaporation rate, significantly. Less