A watershed is an area where water that falls or flows across the surface of the land drains into a stream or river, and eventually flows through a single point or outlet.
ArcGIS Pro has a set of hydrology tools to automatically delineate a watershed (through the Spatial Analyst extension), this entry shows the step-by-step procedure to execute this process, it is recommended to download the DEM for practice.
In a new ArcGIS Pro project you need to add the DEM, then go to ArcToolbox and run the hydrology tools in the following sequence:
- Fill
- Flow Direction
- Flow Accumulation
- Con
- Stream Link
- Stream Order
- Stream To Feature
- New Feature Class
- Snap Pour Point
- Watershed
- Raster to Polygon
The Fill tool is used to remove imperfections (holes and drains) from the DEM.
ArcToolbox > Spatial Analyst Tools > Hydrology > Fill
Set the hydrological flow direction of the slope with the Flow Direction tool.
ArcToolbox > Spatial Analyst Tools > Hydrology > Flow Direcction
Determine the flow accumulation of the cells flowing into each cell by descending the slope using the Flow Accumulation tool.
ArcToolbox > Spatial Analyst Tools > Hydrology > Flow Accumulation
If all goes well we should have a raster as shown in the image below.
Now, to build the water network automatically, use the conditional tool Con. The density of the water network depends on the size of the raster: in “Input true raster or constant value“, set the unit 1, in “Expression” use the expression VALUE > 2500 (this value depends on the size of the pixel and the raster, if you need to decrease the density you must increase the value or vice versa, this can also be done with Raster Calculator), i.e. the conditional allows you to classify the cells with flow accumulation above a threshold specified by the user, the Con tool is located in the following direction:
ArcToolbox > Spatial Analyst > Conditional > Con
To obtain the water network in vector format, it is first necessary to prepare the accumulation raster modified with the conditional.
Use the Stream Link tool to establish the links in the water network.
ArcToolbox > Spatial Analyst Tools > Hydrology > Stream Link
Proceed to set the order of the water network (either by the Stralher or Shreve method) using the Stream Order tool.
ArcToolbox > Spatial Analyst Tools > Hydrology > Stream Order
To convert the water network from raster to vector use the Stream to Feature tool.
ArcToolbox > Spatial Analyst Tools > Hydrology > Stream to Feature
The next step is to establish the outlet or drainage point of the basin. To do this, a new vector file (point type) is created, the editing mode is activated and the exact outlet or drainage point is drawn on the water network of the basin to be delimited.
To set the point or cell from which the basin is required to be delimited use the Snap Pour Point tool.
ArcToolbox > Spatial Analyst Tools > Hydrology > Snap Pour Point
Para culminar la delimitación hacerlo con la herramienta Watershed usar el ráster creado con Flow Direction y el punto de salida generado con Snap Pour Point.
To finish the delimitation do it with the Watershed tool using the raster created with Flow Direction and the exit point generated with Snap Pour Point.
ArcToolbox > Spatial Analyst Tools > Hydrology > Watershed
Finally, to obtain the polygon of the micro-basin, all you have to do is convert the raster to polygon shapefile with the Raster to Polygon tool.
ArcToolbox > Conversion Tools > From Raster > Raster to Polygon
The result of the delimitation of the micro-basin is shown in the following image.
