Hydrological Response To Land Use And Climate Changes In Fengle Catchment

The hydrographs at the outlet of the Fengle catchment of Lake Chao basin were simulated with ATHYS model, in order to provide a scientific basis for the studies of erosion and flood control, water use and non-point source pollution process of Fengle catchment. The model is based on Digital Elevation Model (DEM), landuse, soil and climate data of the catchment and daily discharge data from the outlet Taoxi station. ATHYS model is a distributed hydrological model. The catchment was divided into regular meshes of 90m><90m that describe DEM, landuse and soil properties. The rainfall at the location of the mesh was interpolated using the Thiessen method and the rainfall data was based on daily rainfall data from 8 rain gauge stations. Fengle basin is one of the biggest tributary of Lake Chao. the area of the basin is 1500 square kilometers, the mainstream of the river is about 50 kilometers long and the elevation is from 6 to 463 meters, Fengle catchment belongs to humid subtropical monsoon climate, the average of the annual rainfall in this catchment is about 1000 meters. ATHYS model comes with a variety of production and transfer computing models. The runoff from each mesh was calculated using a reservoir model, and the runoff volume was routed to the outlet using a lag and route method to get the hydrograph and to obtain the discharge volume. STO, INF, co, ds are the primary parameters in production model, they represent the maximum volume of the reservoir (mm), coefficient of infiltration (mm/h), coefficient of subsurface runoff (no dimension), coefficient of soil infiltration (1/d) respectively. There were 2 parameters in the transfer function:Vo and K0. V0 is the speed of propagation (m/s), and K0 is a diffusion coefficient without dimension. V0 and K0 are assumed here to be identical for each mesh, and must be calibrated from rainfall and discharge data. There are many different kinds of landuse in the catchment, such as rice, agricultural, nature, urban and so on. At first, two kinds of landuse types were used in this model:rice and other. And then five kinds of landuse types were used:rice, agriculture, nature, water and urban. Here, a foundation of study on land use change was built. The model could run in a continuous way and independent way at the daily timescale. The discharge at the outlet was available for ten years duration; 5 years were randomly selected for calibration purpose. The comparison observed and simulated discharge was based on Nash and Sutcliffe index. The 5 years left were used for validation. The parameter calibrated by trial and error method.The model demonstrated good calibration and validation results. When simulating in continuous way with simple land use types, the total Nash coefficients being 0.86 and 0.80 respectively. The maximum of the Nash coefficient in calibration is 0.92 while the minimum value is 0.68. In validation, the maximum and minimum of Nash coefficient are 0.92 and 0.68. Except for one value, the correlation coefficient between observed and simulated discharge was over 0.9 for both calibration and validation period. The model can generally well reproduce the hydrograph except for some peaks and some baseflow periods. The relative error of the annul runoff volumes between observed and simulated runoff was about 23.1% in calibration period and 34.6% in validation period. The rainfall and runoff both have a big variation by year while they have a corresponding relationship with each other. May to August is the wet period. The eigenvalues of rainfall and runoff volume are both have a big interannual variation, the average value of annul rainfall is 1238 meters and the variation coefficient is 20.2%. The average value of annul runoff depth is 0.52 and the variation coefficient of Annual runoff depth reach 47.0%. There is no big difference in the average of annual runoff volume and daily peak runoff volume in observation and simulation. But the observed and simulated runoff volume both have a big interannual variation in total runoff volume and daily peak runoff volume. ATHYS model was proved to be able to simulate runoff from Fengle catchment in both ways:continue and independent.In the hydrological response of land use change on Fengle watershed, two methods of land use scenarios assumptions were established:extreme scenario assumptions and future land use scenario assumptions. The extreme assumption divided into extreme rice, extreme nature and extreme urban. Based on historical land use data of Fengle basin, considering the impact of land use change factors and the land use planning of "land Use Planning of Hefei (2006-2020) ", four kinds of future land use scenarios were constructed. The results show that land use change produced certain effect on runoff and runoff procession:in these three kinds of extreme hypothetical scenarios of land use types, the overall trend in annual runoff volume and runoff coefficient are:extreme rice> extreme nature> extreme urban, the peak volumes of the runoff were in the order:extreme urban> extreme nature> extreme rice. The three runoff hydrographs fit each other very well except some peak value. In four kinds of future land use scenarios, the total annual runoff, depth of annual runoff, runoff coefficient and peak volume of runoff have the same trends, as follows:scenario 1< scenario 2<scenario 3< scenario 4. In simulation of runoff hydrograph, four kinds of future land use scenario have a consistency. Except some peak value, there were a big similarity on trend of hydrograph, the time of the peaks were stable.In the study of hydrological response to climate change, seven climate change scenarios were assumed:-30%,-20%,-10%,0,+10%,+20%,+30%. The simulation results show that:the climate change have a big influence on runoff, when rainfall decreasing, the variation of runoff would be reduced with the increase of the decreased magnitude; when rainfall increasing, the variation of runoff would be improved, the more increase on rainfall, the more increase on runoff; and comparing the impact on runoff changes of the reduced rainfall and the increased rainfall, it could be found that with the same magnitude of rainfall changes, the change of runoff would be bigger if the rainfall increased.