Study on Soil Moisture Transport Characteristics of Thick Layered Cohesive Soil in Jianghan Plain under Persistent Drought Conditions

Affected by continuous high temperature and low rainfall, the Yangtze River basin had experienced the most severe meteorological drought in 2022 since the establishment of complete meteorological observation records. Based on meteorological monitoring data from field test sites, this study selected August-December of 2022 as the research period with significant differences in rainfall and temperature compared with previous years. The response of cohesive soil moisture to rainfall, evaporation and groundwater level under drought conditions was analyzed by using soil moisture content and matrix potential monitoring data at different depths. The results showed that the moisture content and water transport of shallow soil (0～1.4 m) were affected by both soil evaporation and precipitation infiltration. During drought, the water movement was dominated by evaporation, with the depth about 1.4m and the effective infiltration was only formed by long-term stable rainfall . The water content of the middle-layer soil (1.4～3.5 m) was relatively high and stable, which was basically not affected by evaporation and rainfall during the monitoring period. Soil water migrated downward in the way of "piston flow", and the zero flux surface near the low permeability layer was affected by the heterogeneity of the soil layer during the infiltration process. Water content and water transport in deep soil (3.5～6.0 m) were closely related to groundwater level, and the correlation became weaker and the lag time became longer as the soil depth decreased. Compared with the middle soil, the changes of soil moisture content and water potential gradient in deep soil were more intense due to the dual effects of low permeability layer prevention and the decrease of groundwater level.