For automatically define a watershed, it is necessary to properly define their boundaries, considering upstream from an outlet point, and are usually demarcated by mountain ridges. A good advantage is that ArcGIS permit to run hydrologic analysis tools, then makes work easier, a good advantage is that ArcGIS permit to run hydrologic analysis tools, making job easier, for this example you can download the files for practice, other option is it download a DEM from Aster GDEM, NASA Reverb or LP DAAC Global Data Explorer, if you can make your own DEM also it is possible.
Then we will show you step by step how to automatically delineate a watershed.
ArcToolbox > Spatial Analyst Tools > Hydrology > Fill
ArcToolbox > Spatial Analyst Tools > Hydrology > Flow Direcction
Determine the accumulation flow cells, each cell flowing down the slope through of the tool Flow Accumulation.
ArcToolbox > Spatial Analyst Tools > Hydrology > Flow Accumulation
If everything goes right we have a raster as shown in the figure below.
Now we will build the drainage network automatically, through of a conditional (this depends on the size of the raster in “Input true raster or constant value” place the unit 1 point output directory is now very important in “Expression” using the expression value> 400, this value depends on the size of the pixel and the raster, the larger the watershed to use a higher value, another option is to use Raster Calculator) using the Con conditional to classify the cells with flow accumulation greater than a threshold specified by the user, for example to enter a low value results show small tributaries drain network, but the higher the value the drainage get larger.
ArcToolbox > Spatial Analyst > Conditional > Con
As a next step to generate a vector of the result from the raster of accumulated flow and conditional, using the Stream to Feature tool.
ArcToolbox > Spatial Analyst Tools > Hydrology > Stream to Feature
The next step to determine the drainage point of the watershed, this can be done by creating a point shapefile type, then interpolate a digital elevation model (TIN or DEM) to obtain three-dimensional coordinates.
ArcToolbox > 3D Analyst Tools > Functional Surface > Interpolate Shape
To conclude with the Watershed tool, using the Flow Direction raster created and interpolated discharge point.
ArcToolbox > Spatial Analyst Tools > Hydrology > Watershed
Finally to obtain the watershed polygon just enough to convert the raster to polygon shapefile type.
ArcToolbox > Conversion Tools > From Raster > Raster to Polygon
The result of the delineation of the watershed is shown in the figure below.