Using a DEM we can define a watershed in a series of steps.
Hydrologic modeling has important uses in
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- Eliminating "pits" in the DEM.
- Where are they? Undrained depressions prevent most watershed algorithms from working. Sinks are both natural, such as "sinkholes," and artificial ones caused by errors in datasets.
- Want to r emove them? Most GIS routines "fill" the sinks until it will drain across the sink to the next lowest point. Other GIS routine involve a combination of fill and breach, wherein the depression is filled and the cell in the steepest path downhill is abraded.
- ArcMap's approach: If you look up the process for ArcMap is says that it does a combination of the two. Filling tends to make stream profiles into steps. With the Hydrology toolbar turned on, you can execute a "fill pits" routine that iteratively calculates flow direction. But I think the ArcGIS routine does not "remove peaks."
Here is an example of a river profile with the DEM data containing pits (from Harbor et al., 2005, method on my web site).
Below is a surface with "pits" shown were flow direction produces an undrained depression. (note: the arrowheads for the flow direction were made using an older version of ArcMap. I haven't found this capability yet for version 10)
- Determining flow direction.
- There are several approaches to this in the literature. Some partition flow to only one cell, some divide it up based on the proportion of flow into each cell (usually 3 or less neighboring cells).
- From the ArcMap help files
"The direction of flow is determined by finding the direction of steepest descent from each cell. This is calculated as drop = change in z value / distance * 100. The distance is determined between cell centers. Therefore if the cell size is 1, the distance between two orthogonal cells is 1 and the distance between two diagonal cells is 1.414214."
- In ArcMap, here is the binary sequence that is used to label flow direction, using the "D-8 method". (Note that is will be different than the values for a CostDirection surface, which was values 1-8).
It is possible, using what is known as the "D-infinity" method developed by David Tarboton at Utah State, to partition the flow incrementally to many cells in the downslope direction.
- Computing Accumulation
- Accumulation is the total number of cells that drain into a given cell. This determined from sequential analysis of flow direction and is an intensive computational load on the processor. (the commands below are from ArcView3.2, but would be similar on the command line or Raster Calculator in ArcMap9.1)
These portions of the grid show the flow direction and accumulation values. Any cell where flow originates (no flow into it) has a value of 0.
- Accumulation is the total number of cells that drain into a given cell. This determined from sequential analysis of flow direction and is an intensive computational load on the processor. (the commands below are from ArcView3.2, but would be similar on the command line or Raster Calculator in ArcMap9.1)
- Determining streams and subwatersheds One needs to decide a "threshold" value of accumulation required to create a stream, and another separate value for the minimum size of a sub-basin.
Hydrologic modeling has important uses in
- flood warning (real-time rainfall estimates is derived from RADAR and delivered to a GIS-based watershed model).
- channel and navigation studys
- water quality modeling (nutrient loading to the Chesapeake Bay, for example)
- geomorphic study of erosion and uplift
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