RIVER ROUTING SCHEME

The topographic structure is extracted from DEM in Geographic Information System (Jenson and Domingue, 1988). If the realistic river way and watershed border are available, they can be used to revise the DEM and help to produce river network and river basin. First, find the grids on the river network according to the realistic river way map. Then "burn into" the river network by changing the elevation value of these grids to some units (e.g. 100 units) less. For all the grids out of the realistic watershed border, modify the elevation value of these grids to some units (e.g. 100 units) larger. The flow direction and downstream distance to the outlet along the flow direction is calculated from the modified DEM (Jenson and Domingue, 1988). The river network produced from the modified DEM will fit the realistic river way map. For the grids over the watershed border, the area fraction, which means the area fraction inside the realistic watershed, is considered. The modeled river basin area will fit the realistic watershed area.

Identification of sub-river basins is an indispensable step in large river basin modeling to route the river network and support water resource management. The Pfafstetter numbering scheme for delineation and codification of river basin is used which is based on topographic control and the topology of the river network. The system is founded upon concepts first articulated by Pfafstetter (1989) and detailed documented by Verdin and Verdin (1999). The numbering scheme is self-replicating, making it possible to provide identification numbers to the level of the smallest sub-basins extractable from DEM. The routing order of the sub-basins is implicated in the Pfafstetter code. Within a given smallest sub-basin, flow intervals are specified to represent the time lag and accumulating processes in river network according to the distance to outlet of the sub-basin. The approach to determine flow interval is illustrated in Figure 3. The numbers in the grids show the downstream distance to the outlet of the sub-basin along the flow direction. The grids are assembled into flow interval j:

j = 1 + Truncate(L^40#40/T_v) (20)

where L^40#40 is the downstream distance to the outlet of the sub-basin along the flow direction in times of grid size, T_v is a threshold value to determine flow intervals. The T_v should be larger than 54#54 times of grid size to guarantee there are grids in the first flow interval. A value of 3 times of grid size is used in this study. The function Truncate is used to drop all the digits after the decimal point. For each flow interval, a river section is allocated. The river channel length L_j of flow interval j is then estimated as:

L_j = k ^. L_j^40#40 = 55#5556#56 (21)

k = 57#57 (22)

where L_r is the river length of the main stream and m denotes all the flow intervals along the main stream. The slope of each grid is calculated from the 3×3 neighborhood grids using the average maximum technique (Burrough, 1986). The river bed slope S_r is then estimated as the averaged slope of all the grids in the given flow interval. All the river water recharge in the grids of the flow interval is accumulated to the river section and led to the outlet of the river basin following the river network.

Figure 3: Methodology adopted to determine flow intervals with a numeric example.

58#58