Modeling resistance of soil evaporation and soil evaporation under straw mulching

Abstract: Evapotranspiration is the main way of soil water depletion in field, and soil evaporation is an important part of evapotranspiration, and it may account for 32%-50% of the total evapotranspiration during the whole crop growth period. Soil evaporation does little effect on the formation of crop biomass and yield, so reducing soil evaporation is important for improving water use efficiency and agricultural water saving management. In recent years, straw mulching has been used widely due to its capability of improving soil structure, increasing soil organic matter, reducing soil evaporation and maintaining soil water content. Mulching affects exchange of energy and heat between soil surface and free air, so the mechanism for soil evaporation under mulching condition may be different from that without mulching. Considering the fast development of straw mulching, therefore modeling soil evaporation under straw mulching is of important for making agricultural water management. In this paper, the factors (soil water content and mulching amount) that affect soil evaporation had been analyzed, the values of mulching resistance were calculated, the equations for calculating resistance of mulching and the resistances of soil evaporation and soil evaporation under straw mulching had been established, the parameters for the equation had been determined. The experiment was carried out from late July to middle August in 2010 and from middle July to middle August in 2011, respectively. The undisturbed soil with winter wheat plants was prepared and put into plastic buckets when winter wheat was maturity, two main treatments (stubble and stubble+mulch treatments) were set up, there were five sub-treatments for each of the two main treatments. The stubble heights for stubble treatments were 0, 5, 15, 25, 35cm. Stubble heights were 0, 5, 15, 25, 35 cm with mulching amounts of 0.47, 0.43, 0.348, 0.267, 0.187 kg/m2 in stubble+mulch treatment, respectively. The total biomass (stubble biomass) for each stubble treatment was 0, 0.04, 0.122, 0.203, 0.283 kg/m2, respectively; the total biomass (stubble biomass and mulching amount) in each stubble+mulch treatment was 0.47 kg/m2. During the prepared period, water was applied to the soil until all the soil in the plastic bucket was saturated, and then the excessive water was discharged out off plastic bucket through small hole at the bottom of plastic bucket to make sure soil water content was about field capacity at the beginning of the experiment. The soil evaporation and soil temperature were measured daily and soil water content in 0-2 cm depth was measured every 2 ~ 5 days according to the soil evaporation. Meteorological data, including relative humidity, hourly mean air temperature, and wind speed were measured with an automatic weather station. Results showed that the relative soil evaporation (the ratio of soil evaporation to water evaporation from 20 cm diameter pan) kept relatively stable when soil water content was higher than 0.23 cm3/cm3 (60% field capacity), the ratio declined linear with the decreasing of soil water content when soil water content was higher than 0.05 cm3/cm3 (wilting point) and less than 0.23 cm3/cm3. Mulching affected soil evaporation, reducing stubble height and increasing mulching amount could reduce soil evaporation when the total biomass was the same. The total amount of soil evaporation during experiment period reduced by 8%-51% under stubble+mulch treatment with mulching amount of 0.348-0.47 kg/m2 compared with stubble treatment when stubble height was 0-15 cm. Mulching resistance was 158-600 s/m in this study and it was exponentially increasing with the mulching amount, and the resistance of soil layer decreased linearly with the soil water content increasing when soil water was less than 0.23 cm3/cm3 and higher than 0.05 cm3/cm3. The validation processes showed that the developed method and related parameters (mulching resistance and the resistance of soil layer) performed well for modeling soil evaporation under mulching condition.