ATHYS model and its application on simulation of rainfall-runoff on Fengle catchment of Lake Chao

The rainfall-runoff on the Fengle catchment in the Lake Chao basin was simulated using ATHYS model to provide a scientific basis for the control of erosion, flooding, non-point source pollution, and water utilization in the Fengle catchment area. The model is based on Digital Elevation Model(DEM), land use, soil and climate data of the catchment and the daily discharge data from the outlet Taoxi station. ATHYS model is a distributed hydrological model. The catchment was divided into the same size(90 ×90 m) meshes 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 the daily rainfall measurement from eight rain gauge stations. Fengle basin is one of the biggest tributaries of Lake Chao. The basin area is 1500 km2, The mainstream of the river is about 50 km long and its elevation is from 6 to 463 m. Fengle catchment belongs to the humid subtropical monsoon climate with an average of annual rainfall about 1000 meters. ATHYS model was developed based on 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, ω, ds, the primary parameters in production model, represent the maximum volume of the reservoir(mm), coefficient of infiltration(mm·h-1), coefficient of subsurface runoff(no dimension), coefficient of soil infiltration(1·d-1), respectively. There were two parameters in the transfer function: V0 and K0. V0 is the speed of propagation(m·s-1), 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 various types of landuse in the catchment, such as rice, agricultural, nature, urban and so on. Two landuse types, rice and other, were used in this model. The model was run in a continuous way at the daily timescale. The discharge at the outlet was available for ten years duration in which five years' s discharge data were randomly selected for the calibration purpose. The comparison observed and simulated discharge was based on Nash and Sutcliffe index. Data from other five years were used for validation. The parameter was calibrated using trial and error method. The model demonstrated a good calibration and validation result with the total Nash coefficients being 0.86 and 0.80, respectively. The maximum Nash coefficient in calibration was 0.92, while the minimum value was 0.68. In validation, the maximum and minimum of Nash coefficient were 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 periods. 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 had a great variation by year, while a corresponding relationship was observed between them. The eigenvalues of rainfall and runoff volume had a great inter-annual variation and the average value of annual rainfall was 1238 m and the variation coefficient was 20.2%. The average value of annual runoff depth was 0.52 and the variation coefficient of annual runoff depth reached to 47.0%. There is no big difference in the average of annual runoff volume and daily peak runoff volume between observation and simulation values, but the observed and simulated runoff volume both showed a great inter-annual variation in total runoff volume and daily peak runoff volume. ATHYS model has proven to be able to simulate runoff from Fengle catchment in a continuous way.