1　Introduction

It is essential to develop proper rainfall-runoff model for the prediction of discharge rate of the lower plain areas with multi-level drainage system,characterized with the high density drainage canals and pump stations.Due to extremely high percent of farming area,high density of crisscrossed drainage canals and ditches with large number of sluices and pump stations,the hydrological process of the lower plain area is strongly affected by the human activities,such as the different operating rules of sluice and pump station,and the layout and cropping structure of agriculture.Therefore,the rainfallrunoff process of the low plain areas is a non-natural process under multi-level artificial control,resulting in that the direction of flow,the velocity of flow,the rate of flow and water level is most controlled by the operating rules of sluice and pump station.

The past two decades have witnessed the development of a number of rainfall-runoff models for the lower plain areas of different complexity,ranging from conceptual model to physically based mode(Wu,et al.,2001;Kim,et al.,2005;Jia,et al.,2005).However,the current rainfall-runoff simulation model focusing on the description of the natural water cycle process(Chahinian,et al.,2005)is on the basin scale,and the agricultural hydrological model,focusing on the description of the natural-agricultural compound water cycle process is on the soil column scale and field scale.The agricultural hydrological model on the basin scale is lacking in the description of the mechanism of natural-agricultural compound hydrological circulation system(Moussa,et al.,2002).In a word,research on rainfall-runoff model to the scale of drainage area is still in the stage of initiation and exploration(Chen,et al.,2003).It is an effective method to simulate the rainfall-runoff process of the lower plain areas by coupling the distributed hydrological model of the drainage area,the hydrodynamic model of hydraulic structure including sluice or pump at each level,and the optimization model for the operation of the pump station.However,on the scale of the drainage area,this method requires a lot of high-precision data with performance computing equipment,so in fact this method does not get good results for engineering applications in lower plain areas.How to build a simple and reliable rainfall-runoff simulation method is critical in the face of increasingly urgent flood disaster control in lower plain area.The Diaocha lake area,located in the middle reach of the Yangtze River,is selected as the study area,a combination of the hydrodynamic model,the hydrologic model,the sluices and pump model is adopted to simulate the rainfall-runoff process in the Diaocha lake area.