| The research on response of water cycle to changing environment is one of the studying hotspots in water science research in 21st century. With development of eco-hydrology, people have realized that not only the composition of different land use types, but also the spatial distribution of landscape and the slope gradient could effect on hydrological process. The distributed hydrological models, which reflect the basic thought of eco-hydrological simulation, are now being widely used to evaluate and analyze the eco-hydrological response of watershed to land use change.In this study, the SWAT model which has a wide range of applications was studied based on the theories of eco-hydrological, and the Guanhe watershed of Danjiangkou reservoir region was chosen as the case area. The response sensitivity of SWAT (Soil and Water Assessment Tools) model to changes of landscape pattern was investigated, and the corresponding modification of the SWAT model was made to improve sensitivity to changes of land use and runoff simulation. By scenarios analysis, the characteristics of runoff variation in the background of landscape pattern change was discussed. The results obtained would provide more valuable scientific support for the integrated management and sustainable use of water resources in Danjiangkou region.The major contents and conclusions of this study were summarized as follows:1 The application of SWAT model in the case study watershed. The spatial database and attribute database of SWAT model was established by using GIS and RS technology, and the basic framework of SWAT model was then built by dividing the watershed into sub-basins and set down the HRU (Hydrologic Response Units) division rules.2 Analysis of SWAT model sensitivity to changes of landscape pattern. Two land use scenarios with different landscape metrics, which are characterized by landscape composition and configurations, were set and proposed to quantify landscape pattern.By comparing the results of SWAT model using the real land use data in 2000 to that using the simulated land use data, which have exactly same mean slope and areal of each land use type in each sub-basin as the real land use map. It was found that, the SWAT model could display a substantial correlation between the runoff, sediment yields, non-point source pollutants and with slope and area of land use type, but failed to demonstrate the sensitivity of the results of SWAT model to the difference in landscape metrics. Perhaps because the model calculated each HRU separately, then summarized them for the output of the watershed. With farther analysis, it is found that the runoff outputs of SWAT model, by using land-use data with different landscape characteristics, could not only match the measured data, but perfectly match each other, just by adjusting model parameters. Therefore, an outwardly reasonable output of SWAT model could be a perfect simulation of real situations, or come from shrewd calibration of model parameters. Hence, considering the spatial differentiation among land use types and the interaction between HURs, determining the main parameters of SWAT model is a sufficient condition for improving precision of SWAT model results.3 Modification of SWAT model application. In order to improve the sensitivity of SWAT model on changes of land use and runoff simulation, the SWAT model divided each sub-basin of study area into 4 slope gradients and calculated the CN value of each land use type under different slope gradient. Furthermore, the modified SWAT model achieved the forest hydrological adjusting function under each slope gradient through improving the methods to estimate surface runoff.4 The applicability analysis of modified SWAT model. The sensitivity of modified SWAT model to changes of land use was tested and the simulation accuracy between original and modified SWAT model based on different temporal and spatial scales was compared.(1) The research on input data. Compared to the original model, the modified model generated much more HRUs. And, the mean slope difference and mean slope of slope gradient between of HRUs became smaller, thus more details of the topographic feature of Guanhe watershed can be described. By adjusting CN value, The modified model could much more accurately demonstrated the effects of underlying surface condition on runoff process.(2) Watershed runoff atdifferent temporal scales. A notable improvement was achieved by the modificated SWAT Model in monthly and daily hydrological simulation, comparing to the original model simulation for both model calibration and validation periods, especially in rainy season. The regression analysis indicated that the modified model demonstrated a much more accurate non-linear relation between hydrological response (output) and hydrological characteristics (input). It could be used as a more accurate and effective tool for hydrological response caused by land use change.(3) The water yield of sub-basin. The annual water yield of each sub-basin simulated by modified and original SWAT model were compared and the results showed that water yield simulation by the modified model could be more reasonable under landscape spatial distribution. Further study found that the modified model can effectively simulate the change of forest hydrological adjusting function during different rainfall, and more accurately reflected the details of watershed runoff process caused by the change of the underlying surface.(4) Parameter adjustment. It was found the CN values of modified model were generally greater than that of the original model, perhaps because most of the slope ratio of the study area is greater than 5%, and the CN difference between the two models varied with the change of topography and different land use types. Ignoring the "subtraction effect" caused by the interaction between different land use types, the original SWAT model had a fitted runoff curve smoother than the measured runoff fitted curveTherefore, an outwardly reasonable output of SWAT model could be a perfect simulation of real situations, or come from shrewd calibration of model parameters. 5 The land use scenarios analysis by modified and original SWAT model. Two of land use scenarios were set in view of the land-use planning of Henan Province and land-use practical situation of Guanhe watershed, quantitative analysis of the water yield under the each land use scenario was conducted respectively by modified and original model, and the influence of land use and land cover changes on runoff of Guanhe watershed was studied. The results showed that, the modified SWAT model, taking account of the forest hydrological adjusting function, could provide a better trend analysis of runoff generation and convergence characteristics in case of land use change. It was suggested that a riparian vegetation buffer strip with a width about 500 m would be significantly helpful to better effect on soil and water conservation of the watershed.In conclusion, compared to the original model, the modified SWAT model could provide much sensitive responses to land use change under a variety of rainfall conditions, which better reflects the theories of eco-hydrological. The modified SWAT model laid the foundation for enhancing the simulation accuracy of modeling water and soil loss and non-point source pollution, thus making the efforts to the soil and water conservation work in Guanhe watershed much more effectively. |
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