表层有效土壤水分参数化及冠层下土面蒸发模拟

Parameterization of surface soil available moisture and simulation of soil evaporation beneath canopy

通过观测田间微气象数据、土壤表层水分变化状况及荞麦作物冠层下土面蒸发等资料,引进一个表面体积含水率的函数,构建了基于表层有效土壤水分的土壤蒸发模型。该模型包含了土面蒸发的2个过程:水蒸气从土壤孔隙中扩散到地表面及水蒸气由地表面传输到大气中。模型中表层有效土壤水分参数不仅取决于表层土壤含水状况,而且受风速影响。采用波文比能量平衡法及微型蒸发器观测荞麦地实际蒸腾蒸发量及冠层下土面蒸发的变化规律,并验证模型精度。结果表明,所构建模型可以成功预测冠层下土面蒸发,其平均相对误差为13.5%。该研究对于实现土壤蒸发及作物蒸腾的分离估算,减少无效水分消耗具有重要意义。

Soil evaporation consumes a large part of evapotranspiration during the crop growth season, especially during the seedling or sparse crop growth stage. It has been reported that soil evaporation makes little contribution to crop yield, and thus it has been seen as invalid water consumption. Separate determination of soil evaporation and transpiration is required in many irrigation management programs or yield analysis models. However, it is quite difficult to directly measure soil evaporation and transpiration separately. To achieve this purpose, a soil evaporation model was developed using a new defined soil moisture function based on the actual measurement of meteorological data （air temperature, relative humidity, and wind speed）, soil surface moisture and soil evaporation data. The model combined two processes of water vapor transfer： one is the vapor transport in air while the other is molecular diffusion of vapor in the surface soil pore with the vapor being carried from the interior soil pore to the land surface. For the field observation, air temperature and relative humidity were measured in three different heights above the buckwheat canopy in order to determine the actual evapotranspiration with Bowen ratio energy balance method. Leaf area index and plant height was measured regularly, with the maximum values of 2.25 and 62.7 cm, respectively. The variation of surface soil water content （5 cm） was from 11.2% to 30.9%. An important parameter, surface moisture availability, in the proposed model was decided by surface soil moisture and wind speed. It was shown that surface soil water content was the main factor affecting surface moisture availability, and wind speed had slight influence on it. The modeled surface moisture availability with soil content and constant wind speed was compared to calculated value with varied wind speed. By assuming surface moisture availability to be 1 in the model, another important parameter, bulk transfer coefficient, could be calculated. It has been reported that the bulk transfer coefficient for bare field is mainly influenced by soil texture and atmospheric stability. In this study, average value of bulk transfer coefficient was applied for three different leaf area stages based on the analysis of its actual variation. Actual evapotranspiration and soil evaporation beneath the buckwheat canopy respectively measured by Bowen ratio energy balance method and micro-lysimeter were compared and the soil evaporation measured by micro-lysimeter was applied to validate the accuracy of the model. It was shown that the soil evaporation beneath the buckwheat canopy during seedling stage was quite close to actual evapotranspiration measured by Bowen ratio energy balance. The average hourly soil evaporation measured by Bowen ratio energy balance and micro-lysimeter were 0.16 and 0.17 mm, respectively; while the average relative error between two methods was 12%, root mean square error was 0.077, and correlation coefficient was 0.89. It was also shown that the soil evaporation beneath the buckwheat canopy could be reproduced using the constructed surface moisture availability model with average relative error of 13.5%, root mean square error of 0.249, and correlation coefficient of 0.95. The study is very important in separately estimating soil surface evaporation beneath the canopy and crop transpiration, and in decreasing invalid water consumption through soil surface beneath the canopy.