Slide 7 of 7
Application of this methodology in a GIS system begins first with laying a fine mesh of grid cells over the landscape, typically 10,000 to 10 million cells. This range is governed by the capabilities of current GIS software and computers. Then the land type of each cell is determined and map algebra applied to determine the water characteristics of each cell using transform equations, such as the SCS runoff curve number equation employing the corresponding values of the transform coefficients.
The eight direction pour point model is a way of connecting each grid cell to one of its eight neighbors (four on the axes, four on the diagonals) according to the direction of steepest descent of flow. Once the land surface terrain is classified in this way, a weighted flow accumulation function can be defined where the weight grid is the water characteristic defined on each land cell, and the flow accumulation counts the sum of all such weights in cells upstream of a given cell. Thus, one can determine the mean annual flow and pollutant load of each cell on a river system. If more detail is required, such as the time distribution of flood discharge or water quality, dynamics models are needed.