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# Basic Surface Hydrology Concepts

The concepts are described in terms of the tools you can use to examine surface hydrology.

• The FlowDirection calculation determines the direction of flow from each cell of a surface raster. The raster generated by FlowDirection must be well behaved. The sort of analysis I am describing specifically excludes land areas that contain lakes or ponds. The assumption is that all the water placed on the raster will ultimately exit the raster at one or more low points on its edge.
• Assuming that the study area involved does not contain lakes or ponds, one of the ways the raster can be ill behaved is to contain a cell that is lower than its surrounding neighbors; such a cell is called a sink. Sinks distort the analysis; to find them, you will use the Sink calculation. (Editing rasters with sinks is beyond the scope of this lesson.)
• Another requirement of the raster is that the cells of primary interest—for example, the mouth of a river near a town that might flood—must include all the “uphill” cells. That is, all the cells that constitute the drainage basin for the cells of interest must be considered. The FlowAccumulation calculation may be configured to compute the amount of water that flows into each cell from all of the cells that are uphill from it.
• Stream networks are characterized by small creeks flowing into larger ones, these flowing into small streams, and so on. It is useful to speak of the “order,” or relative size, of such water entities. The ...

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