Watershed Routing Network Modeling In The Hilly Region Of South China

Watershed routing network is an essential input of watershed process model.Because of the special natural and anthropogenic factors in a region,watershed routing network usually has different characteristics in different region,and those characteristics must take into considerate when describing the watershed routing network.The watershed routing network in the hilly region of South China has its unique spatial structure and is drastically influenced by anthropogenic factors,which is represented by rice terraces and ponds in the basin and their irrigation system.This kind of routing network cannot be reasonably modeled at a fine scale using existing methods,those methods seldom support this kind of spatial structure and anthropogenic factors and thus hard to satisfy the need of small basin watershed simulation scenario as well as the decision support.In order to settle those issues,firstly,a new spatial discretization method is proposed in this paper.In this method,the watershed spatial structure in this region is studied,and guided by this study results,the watershed is discretized into irregular polygons assisted by point and polyline in different district.The district is divided by the land use types of the region,in different district use corresponding discrete method according to the characteristics of the land use type.Then the flow relationships among spatial units can be established:step 1,the flow relationships among units with same type of land use are determinate;step 2,the flow relationships among units with different types of land use are built according to the mechanism of how those anthropogenic factors affect flow;step 3,settle the loops among the whole routing network,aiming at each unit can flow to river or out of watershed.Finally,this watershed routing network is combined with a distributed watershed model named SEIMS(Spatially Explicit Integrated Modeling System)and the irrigation and drainage are implemented in the model to realize the fully distributed watershed simulation scenario.A typical small watershed in the hilly region of South China,ZhongTianShe watershed in TaiHu basin is selected as the study area.The method is applied in this area to model the watershed routing network and run distributed watershed simulation scenario.Here comes the conclusions.(1)Both in the shape of surface features(e.g.paddy field,pond)and the flow relationship between features,the routing network creates in this paper is more tally with the actual situation than the raster based one in this region.For instance,a paddy field or a pond usually has same inner elevation,an irregular polygon based discretion can represent shape and elevation information of paddy field or pond more flexible and accurate.Meanwhile,the flow in and flow out relationship of a pond can be more clear and efficient since pond is settled as an independent entity instead of a series of raster units.(2)A distributed watershed model named SEIMS is modified to fit the routing network created in this paper.Then distributed watershed simulation scenario is applied in this research area.The daily runoff result in validation period(2015 and 2016)shows that modified SEIMS(NSE=0.60,R2=0.65)is superior to the original SEIMS result(NSE=0.43,R2=0.51).(3)The mutual flow relationship between spatial units can be described more clearly in modified SEIMS model.For instance,the effect of pond in overland flow process is considered:the flow of upstream units of pond add to pond volume first,only if current volume exceed the max volume could the pond flow to the river or downstream pond.The change of water volume in paddy field,pond and river caused by irrigation and drainage is also considered:when the paddy field is water shortage,modified model will irrigate them from nearby pond or river,when the volume of paddy field exceed its max volume,drainage water to the river or downstream pond,etc.