Demarcation of endorheic basins in ArcGIS

Endorheic basins (internally drained) have no surface outlet (to a river) and lose water through evaporation and sometimes underground. In the traditional method of watershed delimitation, these internal drainage areas are filled to the point where they are able to be poured into neighboring cells.

But in the case of truly endorheic basins, this is clearly not the case. The method we use to resolve this (to prevent areas from filling up, and to produce a hydrologically correct DEM that can determine the direction of flow) is to create a hole in the DEM using the outlet point of the endorheic basin.

This tutorial provides a step-by-step guide to delineating endorheic basins in ArcGIS. In this exercise we have a DEM and a lake layer where the watershed drains (download example data).

Preparing the DEM

The central idea is to make a hole in the DEM using the polygon (of the lake where the water drains) of the basin. To do this after having drawn the polygon of the lake, in the attributes table we create a new field and assign a value preferably lower than the minimum value of the DEM, in this case we use -1000.

The next step is to convert the polygon to raster using the field containing the value of -1000, for this we will use the tool Polygon to Raster located in:

ArcToolbox > Conversions Tools > To Raster

A dialog box appears in Input Features where the lake layer is used, in Value field the data field containing the value -1000, and in Cellsize place the cell size of the DEM (right click on DEM > Properties > Source).

Now you need to join both raster layers, in this case we are going to use the Mosaic to New Raster tool located in:

ArcToolbox > Data Management Tools > Raster > Raster Dataset

For the Input Rasters field add first the DEM, then the lagoon raster. In Output Location, select a directory. In Raster Dataset Name with Extension the name of the output raster with extension (tif, img, bil, etc). In Pixel Type select 16_BIT_SIGNED (very important). In Number of Bands select 1.

Continuing with the DEM preparation, we are going to replace the value -1000 by NoData, for it we use the Raster Calculator tool located in:

ArcToolbox > Spatial Analyst Tools > Map Algebra

Within the expression SetNull(“DEM_hole.tif” == -1000,”DEM_hole.tif”) the DEM_hole.tif is used and the value -1000 represents the data that will be converted to NoData, in case of having used another value to make the change.

In this way, the DEM is ready to start with the delimitation of the endorheic basin.

Automatic delineation of a watershed

From here on, the process is the same as that normally used to demarcate a watershed. The first step is to clean the DEM with the Fill tool located at:

ArcToolbox > Spatial Analyst Tools > Hydrology

As input raster the DEM with NoData values for the lake or drainage area is used.

The direction raster is then constructed with the Flow Direction tool located at:

ArcToolbox > Spatial Analyst Tools > Hydrology

For input information the previously corrected raster (with Fill) is used, now the accumulation raster is generated with the Flow Accumulation tool located at:

ArcToolbox > Spatial Analyst Tools > Hydrology

As input information is used the direction raster, then from the drainage polygon (lake) is established the area (or point) of origin to delineate the basin using the Snap Pour Point tool located at:

ArcToolbox > Spatial Analyst Tools > Hydrology

If you configure this window, Input raster selects the polygon (of the lake), and Input accumulation raster selects the accumulation raster. Now to delineate the watershed use the Watershed tool located at:

ArcToolbox > Spatial Analyst Tools > Hydrology

Select in Input flow direcction raster the direction raster, and in Input raster or feature pour point data the raster of the drainage point. In this way the endorheic basin can be delimited.

The Raster To Polygon tool (ArcToolbox > Conversions Tools > From Raster) can be used to transform the raster of the delineated basin into a vector.

In order to generate the water network, we are going to start by defining a conditional with the Con tool located in:

ArcToolbox > Spatial Analyst Tools > Conditional

For Input conditional raster, select the accumulation raster. In Expression use the expression value > 2500 (if the value is increased the water network will be less dense or vice versa), and in Input true raster or constant value assign 1.

Now to establish links in the water network use the Stream Link tool located at:

ArcToolbox > Spatial Analyst Tools > Hydrology

In the Input stream raster field place the previously generated raster with the conditional function, in Input flow direction raster selects the direction raster.

The order of the water network is then created with the Strem Order tool located at:

ArcToolbox > Spatial Analyst Tools > Hydrology

In the Input stream raster field select the link raster, in Input flow direction raster select the address raster. Under Method of stream ordering optionally select the method of your preference. Finally to convert the water network from raster to vector use the Stream to Feature tool located at:

ArcToolbox > Spatial Analyst Tools > Hydrology

Now in the Input stream raster field select the command raster, in Input flow direction raster select the direcction raster.

This would be the full process of delimiting an endorheic watershed.

Source: Lakebasin