Simulation Of Water Movement Based On COUP Model At Forestland On Soils Of Purple Sandstone In Three Gorges Reservoir Area

Research on the law of soil water movement has important significance on revealing the runoff formation mechanism of forest watershed and correcting the flood forecasting model. We chose forest of purple sandstone regions as study object, field observation, laboratory test and model simulation methods (COUP model) were adopted to illuminate the influencing factors of moisture movement of forest soils and the effect of plant roots on preferencial paths in purple sandstone regions. The research could promote the research of soil moisture movement law in the three gorges reservoir area.Through the research of simulation on soil moisture dynamic of the study plots and the corrected model based on preferencial paths, a model was set up, which was appropriate for soil moisture movement simulation in forest land of purple sandstone regions in the three gorges reservoir area. It also has important practical significance on further understanding of woodland moisture redistribution, water cycle mechanism of the basin and the flood development rule in the three gorges reservoir area. At the same time, It has profound theoretical significance on riching the content of forest hydrology, soil physics, watershed hydrology and soil and water conservation. The major results in this research were as follows:(1) The accuracy of the model simulation is affected by soil structure, vegetation types and human disturbance. The results of correlation analysis showed that vegetation types and human disturbance had effect on the accuracy of the soil water characteristic curve model simulation. This resulted in the soil water characteristic curve simulation was different by the same model in the same forest land of purple sandstone, and the accuracy of simulation was higher in the surface soil than the bottom soil. The partial correlation analysis results showed that the soil water characteristic curve was significantly positive correlate to soil clay content, highly significant positive correlate to capillary porosity, significant negative correlate to sand content, negative correlate to soil density. And the soil water characteristic curve under the middle suction value was also negative correlate to noncapillary porosity.(2) Point pattern analysis indicated that the spatial position of preferential flow paths and plant roots had a significant positive correlation in the0-20cm depth soil surface layers. This may be related to the growth of shallow root system. Soil moisture conduction properties was affected by the quality and diameter class of preferrencial paths.(3) The response of simulation value on rainfall of the original COUP model was faster than the actual response in the surface soil, and slower than the actual response in the bottom soil. The fluctuation of the simulation value in the bottom soil was smaller. After considering the preferrencial paths, the response of simulation value of soil moisture movement in the bottom soil on precipitation was more sensitive, which was similar with the measured value. This indicated that the response speed of soil was affected by the existence of preferrencial paths, which resulted in the occurrence time of soil moisture peak in the deep soil was ahead of time. The cumulative flow of preferential flow in mixed forest plots was higher than the coniferous forest plots in the same time. This indicated that the Preferrencial paths in mixed forest plots were with better development. This may be due to the broad leaved tree species grow faster and need more water than the conifer tree species. There are more root systems of the broad leaved tree species, so the roots act frequently with the soil in the process of absorbing the moisture, which result in the collapsing and restructuring of soil particles and producing more paths that water could easily flow through.(4) The response of forestland soil moisture in the purple sandstone area on rainfall had certain hysteresis, and showed multimodal phenomenon. For the mixed forest, soil water loss was mainly affected by plant transpiration, but for the coniferous forest, it was mainly affected by soil transpiration. There existed preferrential flow phenomenon in different soil depth of different plant plots under the heavy rain process, which became more obvious with the increase of precipitation.The simulation results of soil water dynamic in forestland of purple sandstone area during different types of rainfall process by the corrected model were obviously better than the orginal model. The simulation captured the results that the surface soil moisture peak was smaller, but the response of soil moisture in the bottom soil on the precipitation was severe.