Geography Reference
In-Depth Information
The importance of eliminating sinks from the drainage area
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The importance of including the entire drainage area in the study area
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How to determine flow accumulation in the cells of a raster
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How to find the distance, both upstream and downstream, from a given cell
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The methods used to delineate and assign order numbers to segments
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How to generate watersheds for streams and points
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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.
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, consider-
ing the elevation of the surface at a given point to be the value of a raster cell at that point
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Determining the direction water would flow from each cell on the surface
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Determining to which adjacent cell water would flow when each cell is doused with a given amount
of water
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Finding those cells that get considerable flow accumulation and delineating them as creeks, streams,
and rivers, either persistently or when flooding occurs
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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
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Determining the areas (watersheds) that feed into given creeks, streams, and rivers and determining
the outlets (pour points) of these watersheds
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Determining into which watershed and which water entities a given quantity of liquid (such as a
polluting spill) might flow
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What we will discuss here are the basic hydrologic tools available in Spatial Analyst. As mentioned there
are also Hydrologic Modeling tools available in other Esri products.
In Spatial Analyst, most hydrologic analysis is done by generating new rasters. This operation is usually
accomplished by entering formulas in the Raster Calculator or by specific tools in ArcToolbox.
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