Watershed delineation based on digital elevation models (DEMs) is the prerequisite to set up SWAT model. With topography and hydrologic processes different from the mountainous area, the watershed delineation in plain polders faces three problems: (1) the subbasins and reaches delineated from the DEM do not agree well with the realistic ones; (2) the braided streams cannot be identified by SWAT model; (3) routing processes are affected by the water-control projects seriously. Based on ArcGIS and ArcSWAT, improved DEM-based method and pre-defined method were applied in watershed delineation. In the first method, “Burn-in” and drawing reach and subbasin boundary manually were applied. First, the digital channel network (DCN) was imported using the “Burn-in” function, and the streams and subbasins were delineated based on DEM and DCN. Then the “watershed” and “reach” layers were edited in ArcGIS, where the location, range, and hydrologic connection could be adjusted. In the second method, streams and subbasins could be drawn manually or re-edited. In conclusion, the watershed delineation results using both methods agreed well with the realistic hydrologic processes in polder area and could meet the demands of the following model setup

Application of the Soil and Water Assessment Tool (SWAT) model is limited in plain polders, where hydrological and nutrient processes are confined by water conservancy facilities such as dikes and pumping stations. Watershed delineation techniques are proposed to enable SWAT to simulate these processes in a plain polder in Jiangxiang Town (Nanchang County, Jiangxi Province, China). Drainage unit division and multiple-outlet modelling approaches are introduced, and the main river network and land features are incorporated into a digital elevation model (DEM), so that drainage unit delineation can agree well with real flow direction and concentration. The impoundments of pumping stations are regarded as functioning like reservoirs, which are set up in the SWAT model to simulate confined hydrological processes. The results show that confined hydrological and nutrient processes in plain polders are simulated well; the retention rates of total nitrogen and total phosphorus are estimated as, respectively, 39% and 29% in both streams and impoundments.

Infiltration is one of the major procedures in water balance research and pollution load estimation in paddy fields. In this study, a new method for indirectly estimating infiltration of paddy fields in situ was proposed and implemented in Taihu Lake basin. Since when there were no rainfall, irrigation and artificial drainage, the water depth variation process of a paddy field is only influenced by evapotranspiration and infiltration (E + F). Firstly, (E + F) was estimated by deciding the steady decreasing rate of water depth; then the evapotranspiration (ET) of the paddy field was calculated by using the crop coefficient method with the recommended FAO-56 Penman-Monteith equation; finally, the infiltration of the paddy field was obtained by subtracting ET from (E + F). Results show that the mean infiltration of the studied paddy field during rice jointing-booting period was 7.41 mm day⁻¹, and the mean vertical infiltration and lateral seepage of the paddy field were 5.46 and 1.95 mm day⁻¹ respectively.