| Precipitation and vegetation changes are two key factors that have significant impacts on hydrological characteristics. However, the impacts of forest vegetation on hydrology are the debating focus for a long time. It is known to most of scholars that, for discrete rainstorms, hydrological benefits of forest vegetation are significant, such as decreasing flood, reducing peak flow and prolonging the time of runoff confluence, while for continuous rainstorms, they will be greatly reduced. But under the circumstances of discrete rainstorms and continuous rainstorms, how much are the contribution rates of precipitation and vegetation changes to watershed hydrological variations respectively? The research achievements of quantitative evaluation are rare.Therefore, the study was carried out to quantitatively evaluate the contribution rates of impacts of precipitation and vegetation changes on hydrological characteristics under the circumstances of discrete rainstorms and continuous rainstorms respectively in Dakeng small watershed of Northwestern Jiangxi Province. In the thesis, theoretical basis will be provided for construction and hydrological benefits'evaluation of forestry ecological engeerings, such as Natural Forest Conservation and Returning Farmland to Forestry or Pasture. What's more, the research achievements will contribute to correctly recognize the vegetation functions and extent of flood drop and hazard reduction.In the thesis, four approaches were adopted to analyze the data, including time-serial eigenvalue, statistical regession, watershed contrast by itself and hydrological model simulation. Based on the previous achievements, by means of the data of precipitation and hydrology from 1966 to 1993, aviation remote sensing images and landuse current situation map during two various periods, additional data of rainstorm-runoff, vegetation and soil from field investigation in 2007, firstly, the changes of precipitation, hydrological characteristics and vegetation have been analyzed. At the meantime, the reponse of hydrological characteristics changes to precipitation and vegetation changes and the reponse of runoff characteristics to rainstorms characteristics under the circumstances of rainstorms have also been studied. Secondly, according to digital elevation model of 1:10000, the properties of topography and soil, vegetation types, Dakeng small watershed was divided into 16 channels and 31 hydrological reponse units. The storm event physically-distributed hydrological model PRMS_Storm was built based on Module Modeling System in the period of 1981~1994. Furthermore, key parameters of the Model were calibrated and validated. Finally, by means of hydrological simulation of discrete rainstorms and continuous rainstorms in the period of 1995~2007 (on behalf of the data in 2007) respectively, the contribution rates of hydrological changes (the depth of runoff and peak flow) resulting from precipitation and vegetation changes were quantitatively evaluated under the circumstances of discrete rainstorms and continuous rainstorms respectively.The results have indicated that, mean yearly total precipitation, wet days and the depth of runoff was 1559.4 mm, 154.8 d and 791.6 mm, which monthly variation presented a single-peak pattern during a year and which yearly variation was great from 1966 to 1993 in Dakeng small watershed. What's more, yearly precipitation presented increasing tendency totally, while yearly wet days presented very small decreasing tendency and yearly depth of runoff presented decreasing tendency totally. Under the circumstances of discrete rainstorms and continuous rainstorms, compared with the period of 1981~1994, mean runoff coefficient in the period of 1995~2007 was decreased 55.6% and 55.6% respectively, while mean peak flow was decreased 67.5% and 72.5% respectively and mean time of peak flow lagged maximum rainfall intensity was increased 72.5% and 71.4% respectively. And the results have demonstrated that changes of landscape pattern, vertical structure and quality of vegetation contribute to the functions of conservating water, decreasing peak flow and prolonging the time of runoff confluence to realize in Dakeng small watershed during two various periods.The results of quantitative evaluation have suggested that, with regard to single rainstorm, the contribution rates of changes of precipitation and vegetation to hydrological variations are obviously different. Taking the six selected discrete rainstorms as examples, the contribution rates of precipitation changes to variations of depth of runoff are 37.3% and 83.6% respectively for 70# and 90#, while ones of vegetation changes are 62.7% and 16.4% respectively. The contribution rates of precipitation changes to variations of peak flow are 57.1% and 92.0% respectively for 73# and 67#, while ones of vegetation changes are 42.9% and 8.0% respectively. With regard to the six selected continuous rainstorms, the contribution rates of precipitation changes to variations of depth of runoff are 79.7% and 92.5% respectively for 79# and 91#, while ones of vegetation changes are 20.3% and 7.5% respectively. The contribution rates of precipitation changes to variations of peak flow are 73.3% and 97.5% respectively for 74# and 58#, while ones of vegetation changes are 26.7% and 2.5% respectively.The results of quantitative evaluation have also showed that, under the circumstances of discrete rainstorms, the contribution rates of changes of precipitation and vegetation to variation of depth of runoff are 67.1% and 32.9% respectively and the former is 2.04 times of the latter, while ones of changes of precipitation and vegetation to variation of peak flow are 76.8% and 23.2% respectively and the former is 3.31 times of the latter. Under the circumstances of continuous rainstorms, the contribution rates of changes of precipitation and vegetation to variation of depth of runoff are 86.9% and 13.1% respectively and the former is 6.63 times of the latter, while ones of changes of precipitation and vegetation to variation of peak flow are 88.0% and 12.0% respectively and the former is 7.33 times of the latter. As is known, variations of depth of runoff and peak flow are mainly resulted from precipitation changes for single rainstorm. Because of different rainstorm types, the contribution rates of changes of precipitation and vegetation to hydrological variation are also different. What's more, the contribution rate of precipitation changes under the circumstances of continuous rainstorms is greater than that under the circumstances of discrete rainstorms, while the result is on the contrast for vegetation changes. |
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