| Over the past40years, water yield, from the mountains areas of North China, showd a sharp reduction, causing a serious water shortage. Reductions in water yield from the mountains have put tremendous pressure on the ecological environment. It has severely limited the social and economic development for the middle and lower reaches of Hai River. Climate variation and landuse change may be responsible for the reduction of water yield. A quantitative assessment of the hydrological effects of forestation, especially on basin water yield, is critical for the sustainable forestry development within this dry region. Therefore, Banchengzi and Hongmenchuan watersheds located in Miyun reservoir catchment were selected as the research objects. The meteorological and hydrological data in the past twenty years were collected, and the characteristics and the dynamic evolution of landscape patterns during1990to2010were analyzed. Moreover, water balance model and separation evaluation method were applied to quantify the respective contribution of climatic change and land use change to annual runoff. WETSPA model was applied to simulate the runoff under different scenarios. The main results were as follows:(1) The mean annual precipitation of Banchengzi watershed showed a fluctuant decreasing but not remarkable trend from1989to2011, while the runoff showed a significant decreasing trend(p<0.01). Combined with the results of cumulative runoff rate curve and independent sample t-test, the research time sequence of runoff was divided into:1989-1997(natural period) and1998-2011(compared period). The mean annual precipitation of Hongmenchuan watershed in from1989to2009also showed a fluctuant decreasing trend and the runoff showed a significant decreasing trend(p<0.01); the mean annual precipitation which caused runoff yield was decreasing but the trend was not significant(p>0.05); In the same way, time sequence of runoff in Hongmenchuan watershed was also divided into1989-1997(natural period) and1998-2009(compared period).(2) Forest cover rate of Banchengzi watershed generally fluctuated but increased in the whole research period. Landscape heterogeneity was high and landscape fragmentation degree is increasing; Forest land was also the landscape matrix in Hongmenchuan watershed. In addition to forest land, construction land and farmland area increased obviously. Its landscape fragmentation level was high, but the degree of fragmentation was in a decreasing trend. The mean annual biomass of Banchengzi watershed was21.42×104t and the increasing of that from1989to2011reached an extremely significant level. The annual biomass variation of Hongmenchuan was consistent with that of Banchengzi, but the increasing trend was more obvious. The increasing trend of biomass per unit area in Banchengzi and Hongmenchuan reached a significant level at the99%confidence interval.(3) The influence of climate change on runoff in Banchengzi watershed contributed for80.24%; as the main representative of human activities, land use change contributed for19.76%. The contribution of climate change to runoff reduction in Hongmenchuan watershed accounted for43%; the contribution rate of land use change as the consequence of human activity to runoff reduction accounted for57%. As precipitation of Banchengzi and Hongmenchuan increased by1%, runoff increased by an average of0.29%and0.09%, respectively; and the increase of every1%in potential evaporation led to the decrease of0.21%and0.05%, respectively.(4) The contribution of forest land change in Banchengzi to runoff accounted for81.29%of the contribution rate of land use change to runoff and the contribution of grassland and farmland to runoff was only10.68%and8.03%of that. Other land use types accounted for small proportion of the watershed area, so we ignored the influence of them. In study period, the contribution rate of forest land change in Hongmenchuan to runoff accounted for74.37%of the contribution rate of land use change to runoff and the contribution rate of grassland and farmland change to runoff was only17.08%and8.55%, respectively.(5) Interspersion juxtaposition index (IJI) and Mean patch size (MPS) of tree layer in Banchengzi watershed are the main factors affecting surface runoff and base flow. Patch density (PD), MPS and IJI of the shrub layer had the greatest influence on surface runoff and base flow. IJI of tree layer in Hongmenchuan watershed has the closest relationship with surface runoff coefficient and base flow coefficient. So does the shrub layer in Hongmenchuan.(6) Based on WETSPA model, it shows the runoff relative error between the simulation and the measured value was within15%, determination coefficient was better than0.60, Nash-Sutcliffe coefficient was better than0.5. Results of verification and calibration completely meet the requirements, so this model could be applied to the two study watershed well.(7) Runoff of the two study watershed differs in different forest types:that of the broad-leaved forest was the minimum and that of shrubbery was the maximum. After all the forest types of Banchengzi watershed converted into the mixed forest, forest cover rate was98.33%, the biomass per unit area was35.81t/hm2, mean annual runoff was the minimum54.63mm. After all the forest types converted into shrub land, forest cover rate was the same98.33%, biomass per unit area decreased to5.14t/hm2and runoff depth increased by18.63%. Under this circumstance, forest cover rate of Hongmenchuan watershed was91.12%, biomass per unit area reduced from32.25t/hm2to4.85t/hm2, annual runoff depth increased by16.45%. After all the forest types of Banchengzi watershed and Hongmenchuan watershed converted into the grassland, the mean runoff depth increased by38.09%... |
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