Module 1 - Applications in GIS

In this week's Applications in GIS lab, we delineated streams and watersheds using a DEM, or digital elevation model.
This map shows the Hanamaulu Stream and its watershed in eastern Kauai, Hawaii.  The dark gray stream system and the red-bordered watershed area have been modeled from the Digital Elevation Model.  The blue stream system and light green polygon feature are drawn from analysis of aerial photos of the actual area.

 In this procedure, the theoretical flow of water is modeled on the raster of elevation data, and based on the D8 mode of analysis.  Water flows from each 30' x 30' plot on the ground, to the lowest of its eight neighboring cells. In this way, a model of flow patterns across a landscape is established.  From the flow direction raster, we then produce a flow accumulation raster, in which the water from all uphill cells progressively accumulates in the downhill cells.  The accumulation does not have real units; each cell's contribution to downhill cells is assigned a value of 1.  The cells along the top of drainage divides have an accumulation of zero.
If we set a minimum threshold of accumulation for cells, for example, 100, we can model a network of stream channels based on theoretical flow and accumulation of water as a factor of elevation.  It is then possible to define the theoretical watershed for all drainage above a particular outlet, or pour point.

It is interesting to compare the modeled stream system and watershed with the actual existing features in the same area, which were digitized from aerial photos.  We can see that the Hanamaulu watershed (with red outline), derived from the National Hydrologic Database and based on existing features, has a large area to the northeast that is not included in the light green modeled watershed.  A modeled stream system was created in this area (dark gray lines) from the analysis, but according to the existing stream data, there are very few stream channels (blue lines) in most of that area.  Because this area is very flat, based on the DEM, it is likely that runoff across that area disperses itself across many ephemeral channels which may not be visible in aerial imagery.