Introduction

Three ports of Jingjiang containing(Songzi port),Songzi River Estuary(Hudu Taipingkou),Ouchi River(Ouchikou),is the main channel of the main stream of the Yangtze River dividing the sediment into the Dongting Lake.Three outlets along Jingjiang river reach,namely Songzi outlet,Taiping outlet,and Ouchi outlet,is the main passage for Yangtze River to divide flow and sediment into Dongting Lake.After impoundment of the Three Gorges Reservoir,the middle and lower reaches of Yangtze River will have a long distance erosion in a long time,and the river bed will be cut down.In the same discharge,the water level along the course will decline to different degree.This water level decline,especially the water level at the three outlet gates,will make the split flow of three outfalls to reduce[1-2].Undoubtedly,the reduce of split flow has certain influence on irrigation and water supply in this area(Hereinafter referred to as〝Jing Nan river network area〞).The calculation of water resources supply and demand balance is the foundation of analyzing the influence of the Three Gorges Project on the Jing Nan river network area,clearing the key link of water shortage and allocating water resources[3].At present,in calculation of water resources supply and demand balance[4-6],the calculation unit is usually administrative area,irrigation area or river basin,and careless or unreasonable division of the calculation unit easily lead to part of the water shortage links being covered.In addition,in areas where rivers are used as water sources,the influence of upstream water users'getting water from the river on downstream water users is not considered,and the bendway of the river after river blanking is also not taken into account.Jing Nan river network area has features of multi-water users(refer to water for agriculture,industry,construction industry,the tertiary industry,life and ecology,etc.),multiwater source(refer to Yangtze River,Jingjiang river,reservoirs,lakes,canals,river bendways,groundwater and ponds,etc.),and multi-water intakes(refer to the diversion culverts,pumping stations,and intakes of water works,etc.).These features lead to the calculation water resources supply and demand balance in this area possess the unique complexity of plain river network.Therefore,traditional calculation methods and thoughts are not very applicable,and cannot really reflect the water shortage links in this area.In order to overcome all these obstacles,the water resources supply and demand balance model in plain water network with multi-water sources and multi-water intakes is set up,which is based on the characteristics of plain river network.And this model is utilized to calculate and analyze the water resources supply and demand balance in river networks on south bank of Jingjiang river reach.Relevant research results have important reference value in calculation and analysis of water resources supply and demand balance in plain river network.