Response Mechanism Of Shallow Soil Moisture To Precipitation In Loess Plateau ——A Case Study Of Dongzhi Tableland

Soil moisture is the link between atmospheric precipitation and vegetation growth,maintaining the stable development of terrestrial ecosystems.In the Loess Plateau,0-30cm shallow soil is the main root layer of grassland and important agricultural farming layer.precipitation is the main source of shallow soil moisture.Under the background of global climate change,the variation trend of annual precipitation in the Loess Plateau is uncertain,but the intensity and frequency of extreme precipitation increase,and the instability of precipitation increases.In addition to the total annual precipitation,the annual precipitation distribution,precipitation intensity and precipitation persistence can significantly affect the process of canopy interception,soil evaporation,soil water infiltration and redistribution,thus affecting the shallow soil moisture.Under the current rapid climate change,it is urgent to clarify the response mechanism of shallow soil moisture to precipitation in the Loess Plateau,which is conducive to scientific guidance of agricultural production,ecological restoration and ecological construction,and better play to its ecological potential.In this paper,the process-based IBIS model was used to simulate and verify the shallow soil moisture of 0-30 cm in Dongzhiyuan study area from 1981 to 2020.R~2was 0.89 in the parameter adjustment period,and RMSE was 0.63;validation period R~2was 0.85,RMSE was 1.18,model simulation accuracy was high,suitable for this study.Based on the simulation results,the variation characteristics and trends of atmospheric precipitation and 0-30cm soil moisture in the study area in recent 40 years were analyzed by linear fitting,Mann-Kendall mutation detection,skewness and kurtosis coefficients.The response of shallow soil moisture to precipitation and rainfall intensity in growing season was studied by Pearson correlation analysis and function fitting,and the conversion efficiency of precipitation to shallow soil moisture under different rainfall intensities was obtained.Finally,the principal component analysis method was used to analyze the key process affecting the change of shallow soil moisture,and the dynamic response of shallow soil moisture under different precipitation conditions was explained from the mechanism.These results deepened the understanding of the dynamic change of ecological hydrological process in the Loess Plateau.The main conclusions are as follows:（1）On the interannual scale,the shallow soil moisture content and annual precipitation in the study area from 1981 to 2020 showed a non-indigenous downward trend in general,and there was a significant positive correlation between them.From the monthly scale,the monthly shallow soil water content and precipitation were significantly positive correlation.the shallow soil moisture content was low from April to May;the shallow soil moisture increased slightly from June to August,which was the consumption period of soil moisture;the soil moisture content is high from September to October,which was the supplementary period of soil moisture.From the daily scale,the relationship between single continuous precipitation and shallow soil moisture in the study area was well fitted by power function.Single precipitation in the range of 0-20mm,rainfall intensity and shallow soil moisture increment was significantly positively correlated;single precipitation in the 20-50mm range,rainfall intensity and shallow soil moisture increment correlation was not obvious;single rainfall>50mm,rainfall intensity and shallow soil moisture increment was significantly negatively correlated.（2）Different levels of rainfall intensity affect the efficiency of precipitation into shallow soil moisture:light rain（≤5mm/day）has no effect on shallow soil moisture;the precipitation conversion efficiency of moderate rainfall（5.1-25mm/day）was the highest（0.43mm/mm）;Precipitation conversion efficiency of heavy rain（25.1-40mm/day）was 0.28（mm/mm）;the lowest precipitation conversion efficiency of rainstorm（>40mm/day）was 0.17（mm/mm）.（3）The vegetation canopy interception evaporation process,vegetation transpiration process and surface evaporation process mainly affect the dynamic change of soil moisture.At the same time,the saturated and super-osmotic flow processes also feedback and act on the soil moisture change process.The correlation of these key soil hydrological processes explains the response mechanism of shallow soil moisture to precipitation under different rainfall intensities and rainfall persistence.