Evaporation and its influencing factors in farmland soil in the arid region of Northwest China

Micro-lysimeter, as a simple and effective method, has been widely used to measure soil evaporation. However, there are still some controversies in its manufacture and application. The objective of this paper is to explore the applicability of micro-lysimeter in measuring soil evaporation under the conditions of bare soil and film mulching in the arid region of Northwest China, and to further analyze the main influencing factors of soil evaporation. The evaporations of bare soil and with plastic film mulch were measured by several micro-lysimeter methods, and sap flow combined with soil water balance method (E(F-B)) was conducted to compare and verify the results. Partial Least Squares (PLS) method was used to evaluate the main factors affecting soil evaporation, and the contribution rates of different factors to soil evaporation were obtained. Results showed it is feasible to measure evaporation of bare soil by micro-lysimeter. The measured results of micro-lysimeters with back cover (sealed) and with gauze cover (unsealed) were similar. The linear relationship between them was E(unsealed)=1.008E(sealed) (R2=0.876 1, P<0.05), The linear relationship between measured results of micro-lysimeters with and without soil replacement was E(no soil replacement) = 0.919E(soil replacement) (R2=0.890 1, P<0.05). Those are suggested that micro-lysimeter with gauze cover (unsealed) and without soil replacement should be adopted in arid areas of Northwest China, since its simplicity, economy and little damage to farmland compared with other micro-lysimeter methods. Evaporation measured by traditional method of 'evaporation determined by bare soil micro-lysimeter multiplied by hole rate' (E (LB×HR) was significantly lower than that in the field with plastic film mulch, and the daily evaporation approached 0. There were no significant difference in evaporations measured by micro-lysimeter between with plastic film hole (E(LM)) and bared soil (E(LB)) method, and their average values were 0.96 and 1.24 mm/d, respectively, during the maize whole growth period. It was found that there was a big deviation between E (F-B) and E (LM) method, indicating that the soil evaporation is still large although the farmland is covered with plastic film, which further proves that it is wrong in traditional concepts that the evaporation can be neglected under the condition of plastic film mulching. Although measured evaporation in bare soil using micro-lysimeters was feasible, there would be some deviation using the method in the field with plastic film mulch. Thus evaporation through little mulch hole (0.5%-5.0%) was relative high (1.02 mm/d) and it should not be ignored. Partial Least Squares (PLS) method has been applied in many disciplines and is an effective method to solve the collinearity of independent variables. In this study, contribution rates of different influencing factors to soil evaporation were obtained using PLS method. The contribution values of main influencing factors with Variable Importance in the Projection (VIP) values above 0.8 from high to low in order were: soil water content, solar radiation, leaf area index, hole rate, temperature, and the contribution rates were 23.9%, 18.3%, 17.0%, 14.5% and 13.9%, respectively.