Study On Eco-hydrological Process To Land Use/Forest Cover Change Of Small Typical Watersheds In Beijing Mountain Area

With the influence of climate change and human activities, Beijing city is facing the eco-environmental problems of less water resource and serious soil erosion, and so on. Beijing mountain area as an important ecological barrier of the city, plays an important role on conserving water resource and maintaining soil and water. But unreasonable configuration and structure of water conservation forest cannot play its role of ecological function. In order to understand the role of forest vegetation on water resources further, the study combined with "Study on effects of typical forest on formation mechanism of water resources" which was supported by "Twelfth Five-Year" National Science and Technology, and took Beijing mountain typical watershed-Banchengzi and Hongmenchuan as study area, analyzed the changing trends of Land use/cover change, precipitation and runoff in two watersheds, and explained the effects of land use change and precipitation on runoff. Afterwards, WetSpa extension model was used to simulate the transport of runoff under different land use/forest vegetation, and offered some supports on vegetation establishment and water resources management. The main conclusions were as follows:(1)In Banchengzi watershed, the transfer characters of land use/forest vegetation in the study period is that broad-leaved forest land increased, coniferous forest, shrubbery and water decreased, mixed forest first increased and then decreased, and farmland and other land use were irregular. In the basin farmland and water are located in flatland and gentle slope at the altitude of250-350; the forest are located in slope gradient (>15°) at every different altitude and aspect. Fragstats software results show that the Landscape and patch connectivity become better and fragmentation of them become lower. In Hongmenchuan watershed, the transfer characters of land use/forest vegetation in the study period is that broad-leaved forest, coniferous forest, shrubbery and farmland increased, mixed forest and water decreased. In the basin farmland and water are located in flatland and gentle slope lower than550m; the forest are located in slope gradient (>6°) at every different altitude and aspect. The Landscape and patch connectivity become lower, shape become complex and fragmentation of them increase. Different land uses in two basins were at the state of the balance of the two-way high-speed conversion.(2)The mean annual precipitation and runoff in two watersheds presented decrease from1990to2006, but the decrease rate of runoff was much larger than the decrease rate of precipitation. Based on the double mass curve the study periods of two watersheds were divided into two stages-base period (1990-1997) and change period (1998-2006). Compared with base period, runoff of change period in Banchengzi watershed decreased132.6mm. It was estimated that land use/forest vegetation change and precipitation variables accounted for27.9%and72.1%of the reduction in mean annual runoff respectively. Compared with base period, runoff of change period in Hongmenchuan watershed decreased132.7mm. It was estimated that land use/forest vegetation change and precipitation variables accounted for44.4%and55.6%of the reduction in mean annual runoff respectively. Land use in later period played a role in the runoff reduction.(3)After the WetSpa extension model calibration and validation, the results showed that three hydrological evaluating indexes in two watersheds basically satisfied the simulating needs and reached B level. The relative error in two watersheds was below15%, and Nash-Sutcliffe model efficiency coefficient and coefficient of determination were more than0.6for daily flow, monthly flow and annual flow. On the basis of it, the WetSpa extension modeling baseflow processes were validated by baseflow separation of digital filtering method, and baseflow in two watersheds was at about50%respectively.(4)WetSpa extension model was used to simulate the impact of land use/forest vegetation change on the flow, and results showed that the simulated runoff of land use in2005was lower than land use in1990in wet year, but there were no difference between different land use in normal and dry years in Banchengzi watershed. The runoff followed the order of shrubbery> coniferous forest> mixed forest> broad-leaved forest under the extreme scenarios of a single type. Land use in2000in Hongmenchuan watershed played a very important role in water conservation in wet and normal year. The capacity of different forest vegetation types in regulation and storage runoff followed the order of shrubbery<coniferous forest<mixed forest<broad-leaved forest under the extreme scenarios of a single type in Hongmenchuan watershed. Runoff components of broadleaf forest in two basins showed that interflow and groudflow had the largest proportion, and broadleaf forest had the largest regulation and storage runoff capacity. With the increase of forest cover rate, the runoff of two watersheds decreased. The reduction of runoff in two watersheds became smaller significantly when forest cover rate increased to over60%, but the proportion of interflow and groudflow continued increasing.