Study On Storm Event Hydrological Processes In The Three Gorges Reservoir Forest Watershed Based On Modular Modeling System

In which extent that forest vegetation affect flooding is a scientific problem still need to be solved. The study on storm event hydrological processes in the Three Gorges Reservoir watershed based on MMS (Modular Modeling System) sponsored by national natural science foundation program-The forest vegetation impacts on flooding in the watershed of the Three Gorges Reservoir- is been done from the hydrological processes such as the effect of forest vegetation on precipitation, runoff and infiltration etc.. A storm event distributed hydrological model (PRMS_ Storm) with physical mechanism is built and the model is calibrated and validated by the data of the Xiangshuixi forest watershed of Simian Mountain. According to the results of case study, the values of main parameters of the Three Gorges Reservoir Region are work out in order to do further study for the different scale. And the storm event process in the different vegetation cover scenarios is simulated and could be useful for discovering forest watershed eco-hydrological process in the Three Gorges Reservoir Region.First of all, the storm event distributed hydrological model (PRMS_Storm) with physical mechanism is built based on MMS (Modular Modeling System) with ArcGIS tools.Main model parameters of Xiangshuixi forest watershed in Simian Mountain are acquired by field investigation, plot observation and experiments. In which, the maximum value of soil non-capillary moisture capacity is in the mixed forest of Cephalotaxus fortunei Hook, with Pinus massoniana and broad-leaved trees and the minimum one is in the wild land; the amount of hydraulic conductivity varies from 0.6 to 6.3mm/min and the maximum is Phyllostachys pubescens, the minimum is in the mixed forest of Pinus massoniana and broad-leaved trees; the maximum of water holding capacity of litters is Cryptpomeria fortunei Hooibrenk forest, the minimum is in the Castanea mollissima forest stand; the surface roughness coefficient varies from 0.0182 to 0.2575 and results also show that there is typical power function relationship between interception and rainfall. 16 channel networks and 30 hydrologic response units in Xiang shuixi watershed are obtained by using the GIS tools.The model-fit efficiency is 0.738 and over 80% of result is qualified in the first watershed. And in the second watershed, the model-fit efficiency is 0.697 and over 83.3% of result is qualified. Accordingly, the simulation precisions of two nested watershed are high.Main model parameters in the Three Gorges Reservoir Areas are gained in terms of soil map, topographical map and land use map. In which, the maximum of soil non-capillary moisture capacity is in the yellow brown soil and the minimum is in the paddy soil. And in 5 main forest types of the Three Gorges Reservoir Area, the amounts of soil infiltration parameters of kpar, psp and rgf have great difference. The mean of kpar from high to low is as follows: yellow brown soil, purple soil, lime stone soil, paddy soil and yellow soil, but the maximum of psp and rgf are all in the yellow soil. Furthermore, the correlation coefficients of canopy interception equation of 4 main forests are between 0.60 and 0.81; the maximum of water holding capacity of litters is coniferous and broad-leaved mixed forest, the minimum is in the bamboo forest stand. At last, the amount of correlation degree affecting roughness by using the grey correlation analysis method is calculated, that is: soil density in 0～20cm (0.68), soil total porosity (0.66), soil non-capillary porosity (0.64), the gravel content greater than 2mm (0.61), slope (0.48) and litter (0.44).The storm event processes under the different forest coverage and forest community scenarios are simulated, the results show that: when forest coverage is 0, 20 percent, 40 percent and 60 percent, the corresponding average peak flow under 12 storm events are 0.294 mm/min, 0.248 mm/min, 0.214 mm/min and 0.167 mm/min, but under a 100-year storm recurrence scenario, the corresponding average peaks flood are 1.16 mm/min, 1.002 mm/min, 0.849mm/min and 0.659mm/min. The result also suggested that peak flow reduction in the different forest distribution is different even in the same forest coverage, and it means that effect on reducing peak flow is more obvious when forest vegetation is distributed in low elevation of the watershed. The amount of average runoff of 12 measuring storms under no vegetation condition is 1.13 times, 1.26 times, 1.45 times and 2.47 times of those forest coverage is 0, 20 percent, 40 percent and 60 percent and current condition, and runoff in 100-year recurrence storm event is 1.09 times, 1.17 times, 1.29 times and 1.61 times.The peak flow and runoff of three forest community spatial pattern scenarios which are mixed conifer-broadleaf forest type, broadleaf forest type, and synthetic arrange type (mixed conifer-broadleaf forest, broadleaf forest and shrub forest) in the Three Gorges Reservoir Areas are simulated, the results indicated that the reduction effect on peakflow of mixed conifer-broadleaf forest type and general arrange type are better than broadleaf forest type. At the same time, mixed conifer-broadleaf forest type is the best for reducing peakflow in the study region and the synthetic arrange type is selected considering forestation actual situation.