Estimation methods of phreatic evaporation for different textures in bare soil area

In order to quantitatively analyze the relationship between the phreatic evaporation and water surface evaporation of different soil textures in bare soil area, soils with different texture and sand samples were taken from the test site in Hefei University of Technology. After pretreatment such as drying and particles sieving, 4 types of homogeneous test materials were selected including coarse sand, fine sand, loam and sand soil. Then, a self-made phreatic evaporation measurement device was made with diameters of 60 and 25 mm and a height of 42 mm. It can easily solve the problem of automatic water replenishment during the evaporation process, so that the groundwater depth can always be 0. Based on the self-designed device, a total of 5 groups of comparative test schemes were constructed, including coarse sand, fine sand, loam, sandy soil and water in bare soil area. Under the same environmental conditions, daily phreatic evaporation of the 4 different soil textures and water surface evaporation were observed for a total of 127 days from December 24, 2018 to April 29, 2019. By analyzing the relationship between phreatic evaporation and water surface evaporation for soils with different textures, linear mathematical equations were established. The results revealed that the change trend of phreatic evaporation and water surface evaporation was basically the same during the experiment. The high air temperature would lead to larger soil evaporation. However, the phreatic evaporation of the 4 different soil textures was not equal to the water surface evaporation, and the difference between them was more significant as the air temperature was increased. The determination coefficients between phreatic evaporation value of different soil textures and the water surface evaporation were all greater than 0.97. Especially for fine sand, the correlation coefficient reached 0.99, which indicated that there was a significant correlation between phreatic evaporation and water surface evaporation (P<0.05). And the conversion coefficients were achieved between them according to the linear mathematical equations. Generally, in soil bare areas with similar climatic conditions in Hefei, the conversion coefficient of coarse sand was 0.94, and the coefficients of fine sand, loam and sand are 1.04, 1.14 and 1.19, respectively. The phreatic evaporation value of coarse sand was less than the evaporation of water surface. The phreatic evaporation values of fine sand, loam and sand were greater than water evaporation. Under bare soil conditions, this conversion coefficient was only related to soil texture. In addition, the loam and sandy soil evaporation in the Wudaogou test site of Anhui Province, China was selected to verify the rationality of the conversion coefficient obtained above. The results showed that when the groundwater depth was 0.4 m, the relative error of the loam evaporation calculated by the substitution method and the phreatic evaporation coefficient method were -10.30% and 2.25%, respectively. Moreover, the sand evaporation calculated by the substitution method was 5.11 mm smaller than the measured value with a relative error of -17.79%, while the sand evaporation calculated using the phreatic evaporation coefficient method was only less than the measured value of 0.56 mm, and the relative error was reduced to -1.94%. Therefore, when calculating the phreatic evaporation at different groundwater depths, directly replacing phreatic evaporation with water surface evaporation would inevitably cause larger relative errors. However, the relative error of the calculation result obtained from the phreatic evaporation coefficient method was smaller, and the calculated value was much closer to the measured value. The coefficient method proposed in this paper would significantly improve the reliability and accuracy of the calculation results of phreatic evaporation.