Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

A Major Application of Raster Processing: Hydrology

Because of the regular nature of the areal unit in raster processing, a number of applications, such as a simulation of a forest fire or an oil spill, are made much simpler. Hydrologic modeling is one such application, so we look at it in some detail, both for its own sake and because it illustrates the power of spatial analyst. An important proviso: If you want to do serious hydrologic investigations, you should use packages designed for the purpose, including those provided as separate software by Esri.

You may not have thought a lot about how water flows over the surface of the Earth, or how to simulate it with a computer, so let’s explore the following:

• The fundamental elements of hydrologic modeling and analysis
• How to determine the direction of water flow from cells in a study area
• The importance of eliminating sinks from the drainage area
• The importance of including the entire drainage area in the study area
• How to determine flow accumulation in the cells of a raster
• How to find the distance, both upstream and downstream, from a given cell
• The methods used to delineate and assign order numbers to segments
• How to generate watersheds for streams and points

We enter into a discussion of these topics in the text that follows. Then, in the Step-by-Step section of this chapter, you simulate a hydrological system.