Daily Runoff Simulation by an Integrated Catchment Model in the Middle and Lower Regions of the Changjiang Basin, China

To simulate daily runoff in the middle and lower regions of the Changjiang (Yangtze River) basin, we used an integrated, spatially distributed model consisting of the Hydrological Simulation Program—FORTRAN (HSPF) and two newly developed submodels, the paddy runoff model (PRM), simulating the runoff from paddy fields, and the lake discharge model (LDM), simulating the hydraulic effect of the Changjiang mainstream on the discharge from neighboring lakes. In the PRM, the ratios of the simulated monthly evapotranspiration relative to the observed potential evaporation were close to or exceeded 1.0 in both of the test subcatchments in the full ponding period with plentiful irrigation water, corresponding to many previous observations of evapotranspiraion rates from paddy fields. Whereas the original HSPF could not consider the backwater effect of the water level in the mainstream, the daily discharge from the lakes simulated by the LDM was dependent on the water level in the mainstream and corresponded to the observed data at the hydrologic stations on the channels connecting each lake with the mainstream, as shown by the daily $R^2$ (Nash–Sutcliffe coefficient) values for Dongting Lake (0.89 in the calibration period, 0.90 in the verification period) and for Poyang Lake (0.84 in the calibration period). For the runoff response from the whole basin simulated by the integrated model, the deviations of runoff volume and values of $R^2$ were $-4.2\%$ and 0.95 in the calibration and verification periods and 1.6% and 0.93, respectively, at Datong hydrologic station, which shows the good applicability of this model as a tool for management of water resources in the Changjiang basin.