Model For Soil Evaporation And Experimental Verification

The quantitative calculation method of soil evaporation is widely applied in the water conservancy, hydrology, agriculture, ecology, environment, geography and meteorology etc. Penman derived the soil potential evaporation formula from energy balance equation and mass transport equation, but the formula cannot apply under the condition of soil moisture deficit, so the application of soil potential evaporation formula is limited and it is necessary to derive a formula suitable for calculating soil evaporation under any conditions.This paper takes quantitative calculation method of soil evaporation as research object, based on the soil-atmosphere continuum and the theory of energy balance and mass transport, taking the water surface evaporation as the reference evaporation surface, using mathematical modeling approach for predicting soil evaporation. The mainly research achievements of this paper are as flowers:1) Modified the concept of Reference Evaporation Surface: Reference Evaporation Sur-face is the evaporating fountain which has steady evaporating state. This paper takes the air-dry soil surface, saturated soil surface and water surface as the reference evaporation sur-face. Air-dry soil surface evaporation represents the lower limit of soil evaporation under the same weather, for the quantity of evaporation is zero under the any weather. Saturated soil surface evaporation represents the upper limit of soil evaporation under the same weather, for the quantity of evaporation is the soil potential evaporation under the any weather. Water sur-face evaporation represents the upper limit of moisture evaporation under the same weather, for the quantity of evaporation is the moisture potential evaporation under the any weather. Reference Evaporation Surface not only provides the scope of evaporation, but can simplify the energy balance equation and mass transport equation by mathematic methods.2) By validating the model of diurnal soil temperature variation, conclusions are as fol-lows:I,There is negligible error to estimate the diurnal average temperature of the characteris-tic surface by the formula ?T=0.5(Tmax+Tmin), and to estimate the amplitude of the changes of the characteristic surface diurnal temperature variation by the formulaδT =0.5(Tmax-Tmin).II,The characteristic surface diurnal temperature is equal to the average temperature at 8:30am to 10:00am. 3) Three mathematic models of soil evaporation were established to precisely estimate the soil evaporation. The first model: Model for estimating soil evaporation based on water sur-face evaporation, the second model: Model for estimating soil evaporation based on dry soil and saturated soil surface evaporation, and the third model: Model for estimating soil evapo-ration based on saturated soil surface evaporation. The first model is based on water surface, and the results show the parameters such as the diurnal maximum and average temperature in the water surface and drying soil surface, diurnal water surface evaporation, and wind speed are involved in the model. The second model is based on dry soil and saturated soil surface, and the results show the parameters such as the diurnal maximum temperature in the dry soil surface, drying soil surface, and saturated soil surface, and diurnal saturated soil evaporation are involved in the model. The third model is based on saturated soil surface, and the results show the parameters such as the diurnal maximum and average temperature in the saturated surface and drying soil surface, diurnal saturated soil evaporation, and wind speed are in-volved in the model.4) The verified results of the models indicate that the theoretical values fit well with measured ones, and three models'correlation coefficient are 0.88, 0.95 and 0.94. The second model has the highest correlation. Conclusions are as follows:I,The complex physical process of soil evaporation is avoided in the model for the in-troduction of reference evaporation surface, which makes the soil evaporation be easy to be calculated.II,The results show only a few parameters are required for those models, and other em-pirical parameters are not necessary.III,With the development of remote sensing temperature technique, there will be supe-riority in calculating soil evaporation on large area.IV,Since the models are simple and they make calculation easy when having too many regions, so they can be used to study the spatial variation of soil evaporation.