Locational Analyses 1

Watershed Analyses

Using a DEM we can define a watershed and the streams within it in a series of steps. If you want more detail on the hydrology tools in ArcGIS Prom, and the concepts they support, go to this help website and click through some of the topics.

  1. Eliminating “pits” in the DEM.
    1. 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.
    2. Want to remove them? Most GIS routines “fill” the sinks numerical water 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.
    3. 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. Using the tools in the Hydrology toolbox within the Spatial Analyst toolbox, 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).
  2. Determining flow direction (from original surface and/or “filled” DEM).
    1. 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.”
    2. For flow direction, ArcGIS uses a binary sequence to label flow direction, using the “D-8 method” and EAST as the value 1, doubling in a clockwise direction.
    3. Pits are easily found with a flow direction map, because flow is “trapped” because two or more cells point at each other, as shown below.
    4. There are several approaches to flow direction calculation 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).
    5. 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.
  3. Computing Accumulation
    1. Accumulation is the total number of cells that drain into a given cell. The value of zero is given to the ridge lines, where flow originates. This determined from sequential analysis of flow direction.

      Note the “accumulation” in the sink. Any cell where flow originates (no flow into it) has a value of 0.
  4. Determining streams and subwatersheds
    1. One needs to decide a “threshold” value of accumulation required to create a stream, here accomplished in one of two ways using the “raster calculator” tool.
      where SetNull( conditional statement, value if false) –if the conditional statement is true, the cell is set to “NoData” or “Null”
      and “Con(conditional statement, value if true, value if false). If you omit the “value if false” and the conditional statement is false, the cell is set to “NoData” or “Null”
    2. The “Basin” tool finds the watersheds draining to different end points in the DEM, the “Watershed” tool finds basins above selected points (help file)

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 studies
  • water quality modeling (nutrient loading to the Chesapeake Bay, for example)
  • geomorphic study of erosion and uplift