考虑风速影响的土表蒸发计算方法

土表蒸发是土体与大气相互作用的主要方式之一。由于土表蒸发与环境因素和土体性质密切相关,目前准确计算土表蒸发量仍存在困难,而其中的难点便是确定蒸发过程转折点的影响因素和表征方法。针对风速对蒸发过程的影响规律,新研发了一套控制气候的非饱和土蒸渗试验装置,可综合控制空气温度、相对湿度和风速等气象参数。蒸发试验选取了3种不同的土样,分别设置12种不同的气候条件进行。试验结果表明风速越大,即空气动力阻滞系数越小,蒸发过程出现转折点时的临界含水率越大。理论推导和试验分析表明单纯用含水率单变量不能准确预测土表蒸发量,还应考虑不同土体性质和风速等额外变量。结合理论推导,提出一种计算土表蒸发量的新方法,它将表层1cm的土体含水率作为变量,同时利用田间持水率和空气动力阻滞系数来表征土性和风速的影响。

New method for calculating soil surface evaporation considering effect of wind speed

Soil surface evaporation is one of the main processes in the soil-atmosphere interaction. Since it is highly related to meteorological factors and soil properties, the determination of evaporation from soil surface remains a challenge, and the key point is to determine the critial water content where the evaporation changes from constant rate stage into falling rate stage. To investigate the effect of wind speed on soil evaporation, a climate control apparatus is newly developed, with a feature of completely controlling air temperature, relative humidity and wind speed. 12 climatic conditions are applied to 3 kinds of soil specimens to carry out the evaporation tests. The results show that a lower aerodynamic resistance always leads to a higher critical water content, only the water content cannot allow an accurate estimation, and the additional variables accounting for soil texture and wind speed must be included as well. Moreover, a simple approach to parameterizing the evaporation is presented by using the water content of top 1-cm layer as a variable and considering the effect of soil texture and wind speed.