Geographic Information Systems: An Introduction, 3rd Edition

15.7 Hydrologic Modeling

The use of a digital terrain model makes it possible to calculate a synthetic drainage system in the terrain, including the individual areas for drainage, drainage network, and nodes through which the water runs (Figure 15.24). This kind of hydrological model can be used to simulate different drainage situations, including the establishment of parameters for calculating the dimensions of the piping network in connection with the development of a housing complex.

Figure 15.24

A watershed consists of watershed boundaries, subbasin, stream network, and outlets.

A grid-based terrain model represents the continuous surface of the terrain. Drainage will always be in the direction with the lowest elevation. Each cell has eight adjoining cells and eight possible drainage directions (Figure 15.25). Comparison of the basis cell's elevation value with the values of the adjoining cells thereby determines the direction. The sum of the number of cells that run into each other gives the accumulated drainage and forms the basis for calculation of the network. Cells with a high accumulated value show where the water collects and create a drainage pattern. Cells with the lowest accumulated value represent the limit for individual watersheds. The cells that represent the network (and the boundaries for the drainage areas) can be vectorized and attributes linked to the different ...