Study On Constitutive Relationship Of Runoff Generation At The Hillslope Scale

Hillslope is the basic unit for hydrological responses. The rainfall-runoffrelationship at hillslope scale is the fundamental mechanism for hydrological simulation.Based on hillslope scale, establishing macroscopic constitutive relationships forhydrological model is an important measure in bridging the gap between grid scaleequations, such as soil infiltration equation, and macro-scale model, such as river basinmodel. Based on hillslope hydrological response mechanism, a numerical model forhillslope runoff generation has been built in this thesis, which has also been calibratedand verified by field observations and experimental data. Through sensitivity analysison hillslope threshold-linear effect, hydrological constitutive relationship at hillslopescale is obtained. In addition, its upscale applicability has been discussed, which set thefoundation for scales coordination in watershed hydrological modeling.In this study, the THRM model is developed. The THRM model couples with SaintVenant equation for surface runoff and Richards equation for variably saturated soilwater movement (including subsurface flow and groundwater flow). A finite differencescheme with improved boundary conditions is adopted in this research. By introducingselective algorithm and parallel computing technique, the THRM model reveals a highcomputational efficiency and stability in simulating subsurface flow of the experimentalhillslope, which is valuable in assessing the hillslope runoff generation mechanism. Amodel based sensitivity analysis is also carried out. The impact of boundary condition,grid size and initial soil moisture on simulation result and model stability has beenassessed, which provides insightful information in facilitate the understanding of themechanism in subsurface flow.By conducting extensive investigation, twelve characteristic factors have beensystematically selected, including landform, soil features, bedrock features, weather andinitial condition factors, to represent the hillslope hydrological responses. Therainfall-runoff relationships have been explored by carrying out over40,000numericalexperiments with THRM. A “threshold-linear” two-phase rule has been observed, whichsuggests that if the rain fall intensity is lower than the threshold value, no runoff can beobserved; on the other hand, if the rainfall intensity exceeds the threshold value, the a linear relation has been observed between rain fall intensity and runoff. The thresholdeffect of rainfall-runoff has thus further analyzed, which has been regarded as the basinconstitutional relationship. The hydrological constitutive relationship at hillslope scalecan be expressed in the “threshold-linear” form, which is mainly controlled by the soilthickness, slope and bedrock leakage factor. The relationship is also supported by fielddata. The qualitative research of landform factors such as soil distribution, longitudinalprofile and bedrock roughness have also been revealed in this study.Basing on the hydrological constitutive relationship, an experimental watershed ata typical rocky mountain area of North China has been built. Data from this semiaridand semi-humid area provides another validation for the constitutive relationship. Therunoff components at the small basin outlet and response time have been analyzed,which reveals that the hillslope runoff generation response time is large enough toignore the time for river routing of the small basin. Therefore, the stormflow part ofsmall basin outlet can be obtained by directly superimposing the runoff calculated fromhillslopes.