Distributed hydrological simulation in the polder areas of the southern plain considering irrigation-drainage operation characteristics

Abstract: Polder refers to the field where embankments are built around low-lying land to prevent the free flow of external water. The basic geographical unit can also be formed in the river network area of the southern plain in south China. The hydrological process is very complicated, because the interior of the polder area is generally low and flat, while the exterior is built with several sluice gates, pumping stations, and irrigation-drainage facilities. Therefore, it is very necessary to find a hydrological model and calibration, as well as the internal water circulation characteristics in the plain polder area of the southern plain of China. Taking the Northern Polder of Lujiang County, Anhui Province of China as the study area, a spatial division was proposed with the improved SWAT model using the "burn-in" algorithm and artificial correcting of sub-basins and river networks. A multi-water source irrigation system was also constructed to add the virtual reservoirs in the polder area, considering the irrigation characteristics. A multi-objective hydrological rate determination was then established using monthly time scales for the polder areas. The different sizes were finally classified to consider the drainage characteristic in the polder areas. The results show that the relative error of the monthly runoff at the outlet of most polder areas was within ±15%, the coefficient of determination was greater than 0.7, the Nash-Sutcliffe efficiency coefficient was greater than 0.6, and the simulation performance was at a medium or good level, indicating the improved SWAT model suitable for hydrological simulation in the study area. The simulated runoff process of a few polder outlets was deviated greatly from the measured one, possibly because the outflow of some pumping stations was artificially affected, resulting in the runoff process out of synchrony with the precipitation. The water balance analysis showed that the precipitation and water diversion from the outer rivers were converted mainly into runoff and evapotranspiration, whereas, the water diversion of outer rivers was approximately equal to the pumping capacity of pumping stations. Besides, a small part was discharged out of the basin through deep leakage. A large amount of evapotranspiration was due mainly to the high intensity of evapotranspiration with rice as the main crop. The coefficient of runoff was smaller than the average coefficient of runoff in the Yangtze River Basin. The reason was that the amount of water was emitted by the evapotranspiration and deep leakage, due to the damming effect of ponds, ditches, inner rivers, and other water bodies in the polder and the drainage of artificially regulated pumping stations. There was a 15% increase in the water diversion from the outer river and the drainage from the pumping station under the single-source irrigation scenario, compared with the current scenario. By contrast, there were a 30% reduction in the pumping drainage and a 60% reduction in the deep leakage without irrigation. The irrigation water consumption showed that the outer river was the main irrigation source, where the proportion of the water supply was more than 80%. The amount of water used for irrigation in the rice season was much greater than that in the wheat season. Among them, the main irrigation water sources for the rice season were the outer river and the polder inner river, whereas, the wheat season was from the outer river and the polder channel. The improved SWAT model can provide an effective way of water management in the polder area of the southern plain of China.