| Recently,extreme weather events have occurred frequently,correspondingly,hydrological disasters like droughts and flash flood have occurred frequently in some areas,threatening lives and properties.Soil moisture is an important part of the hydrologic cycle,its spatial and temporal distribution and dynamic changes have a profound influence on regional climate,hydrological processes,and agricultural productivity.The spatial distribution and dynamics of soil moisture are complicated due to the heterogeneity of rainfall,the anisotropy of soil properties,and the unsaturated lateral flow,the steep slope in the mountainous region could further complicated it.More in-depth observation and analysis are needed.This research takes a small mountainous catchment in southwest China as study area to conduct the field investigation and in-situ monitoring.The occurrence of the non-sequential response of soil moisture is identified by the change of soil moisture level in adjacent soil layers,the subsurface flow path is then indicated by the interconnected non-sequential response areas.The soil moisture dynamics are studied through data analysis and numerical simulation.The main findings are as follows:(1)Point-scale data analysis revealed the presence of rainfall thresholds in the study area.Two observation sites were selected in the study area for continuous soil moisture monitoring and analysis of spatial and temporal variation of soil moisture at point scale.The measurements showed that there was a threshold in the rainfall-runoff process,which could be attributed to the vertical soil stratification,soil moisture dynamics,and other factors.Once the rainfall amount exceeded this threshold,the runoff began to rise rapidly.The soil properties in the catchment changed significantly in the leaching horizon and illuvial horizon,and the correlation of soil moisture at different depths changed abruptly at their interface.The greater heterogeneity in the vertical distribution of soil moisture resulted in the transit perched water table and a higher frequency of preferential flow.The steep slope further promoted the lateral flow in the upper soil layers.When rainfall amount exceeded 40 mm,the depth at which the non-sequential response occurred increased as the rain intensified.(2)Catchment-scale numerical simulations demonstrated the construction of hydraulic channels on the hillslope.A distributed numerical model based on physical processes was used to simulate the hydrological response at catchment scale.The frequency and position of the occurrence of non-sequential responses were calculated to track the subsurface flow path in the catchment.The results indicated that when rainfall started,the water supply of near-surface soil was sufficient,and the generated preferential flow moved downhill in a nearly saturated wedge shape.At the same time,the excess water infiltrated the soil-bedrock interface,with the expanded riparian zone,and gradually increased runoff.After the cessation of rainfall,the soil moisture in the upper soil either directly flowed into the channel from the area near the riparian zone,or infiltrated into the deeper soil layers,and decreased rapidly due to the lack of recharge.At the same time,water in the middle and deep layers continued to flow laterally.During and after rainfall,soil moisture continued to accumulate at the soil-bedrock interface,and the accumulated moisture may be an important source of subsurface storm flow.In the process of soil moisture propagation from the upslope to the foot,the soil at the foot was gradually moistened,and the hydraulic connectivity of the entire slope was enhanced.As a result,lateral water flows at different depths appeared.The soil on the hillslope gradually connected with the soil in the riparian area,completing the confluence process between slopes and river channels.(3)The temporal distribution of rainfall had an important effect on runoff generation.Especially,short-term heavy rainfall was a critical factor in causing runoff surges and the increase of peak flows.When a short-term rainstorm occurred,the main source of peak flows on gentle slopes was Dunne overland flow,and the main source of recession was lateral subsurface flow;Correspondingly,the main source of peak flows on steep slopes was fast lateral flow near the surface,and the main source of the recession was the lateral flow in the middle and deep soil layers.During the rainfall,the continuous infiltration of water generated different antecedent soil moisture conditions,thus affecting the time to peak and total runoff.In addition,when the rainfall was uniformly distributed in time,the response depth of soil moisture increased with the increase of rainfall depth.However,when the rainfall is unevenly distributed in time,a concentrated rainfall of 200 mm could cause variation in the middle and deep soil moisture which did not fluctuate even when the uniform rainfall of 400 mm occurred.This result indicated that the rainfall intensity played an important role in affecting the soil water content in the middle and deep layers.(4)The spatial distribution of rainfall also had a significant influence on runoff processes.When rainfall depth decreased with elevation,peak flows and total runoff would increase comparing with homogeneous rainfall,so did heavy rainfall that moved towards the catchment outlet.During river confluence,the peak flows increased by 0.71 m~3/s for decreasing rainfall type from the upstream of the 1.7 km main channel to the downstream outlet,which was three times the peak flows of spatially homogeneous rainfall.The difference in the peak flows between the two different spatial distributions of rainfall was 0.24 m~3/s at the upstream,and this difference reached 0.70 m~3/s at the outlet,increased by a factor of about two.When the total rainfall amount was constant,decreasing rainfall was more likely to trigger superimposed runoff from different regions during the river confluence,thus increasing the peak flows.During the rainfall-runoff process,the movement and distribution of subsurface soil moisture affected the water deficits to saturation replenished by rainfall at different hillslopes,which in turn affected the runoff.Based on the analysis of soil moisture dynamics,this study investigates the rainfall-runoff process in mountainous areas using the formula of the non-sequential response.The results could provide scientific support for rainfall forecasting and early flood warning. |
