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

Basic Surface Hydrology

First a caveat: Hydrologic analysis is a complex subject. The concepts and tools presented to you here are, in themselves, not sufficient to undertake hydrologic analysis or modeling. Real-world situations frequently do not conform to the assumptions and conditions that underlie the examples presented here. However, the concepts discussed here will help you understand the basic principles of surface hydrologic analysis.

Surface hydrologic analysis (as opposed to underground hydrologic or groundwater analysis) seeks to describe the behavior of water as it moves over the surface of the Earth. Most simply, this type of analysis includes the following:

• Obtaining a mathematically correct representation of the surface of the area to be analyzed, considering the elevation of the surface at a given point to be the value of a raster cell at that point
• Determining the direction water would flow from each cell on the surface
• Determining to which adjacent cell water would flow when each cell is doused with a given amount of water
• Finding those cells that get considerable flow accumulation and delineating them as creeks, streams, and rivers, either persistently or when flooding occurs
• Developing a network of these creeks, streams, and rivers; determining a hierarchy of them; and classifying them as to volume, relative to their upstream tributaries
• Determining the areas (watersheds) that feed into given creeks, streams, and rivers and determining the outlets (pour points) ...