INLAND CATCHMENTS - EXERCISE

The purpose of this exercise is to illustrate how to use the inland catchment AML. It is assumed that you are familiar with Arc/Info-Grid, with the standard methodology for stream and watershed delineation, and with the use of ArcView for preparing maps. The methodology used by this AML can be found at Inland Catchments, and the AML code at Inland Catchments - AML Code. You will need Arc/Info-Grid to run the AML.

Study Site and Data

The North-Western part of the African continent embodies two large terrain depressions: one is the Lake Chad basin, and the other comprises primarily the Algerian Sahara. Much attention has been paid to the Lake Chad basin because of its distinct hydrologic characteristics (i.e., the lake does not have an outlet river); on the contrary, the Algerian Sahara depression is essentially a dry region. For this exercise, it has been assumed that depressions bigger than 300,000 Km² and deeper than 80 m are real terrain depressions and not just DEM errors. This figures, though, should be determined based on the geography and the reliability of the terrain data. In this exercise you will identify the inland catchments, delineate watersheds, and compare your results with those obtained when inland catchments are ignored in the delineation process.

A digital elevation model (DEM) prepared by M.F. Hutchinson of the Center for Resource and Environmental Studies (CRES) of the Australian National University is used in this exercise. This DEM, originally in Geographic Projection with cell size of 3 arc-minutes, was projected into Flat Polar Quartic Projection with cell size 5000 m. The elevation values are given in tenths of meters.

Running the AML

If you are working from the GISHydro97 CD-ROM copy the entire directory gisfiles located at /terrain/inland to your hard drive; otherwise, download the entire directory inland using anonymous ftp from ftp.crwr.utexas.edu/pub. Change the working directory to gisfiles (inland if you downloaded the files). Once you have the files in your hard drive and are located in the correct working space, you are ready to start working with the inland catchment AML. Follow the instructions shown below:

1. Open the AML with a text editor and edit the line:
   fp01 = <your DEM path/name>
   so that it reads:
   fp01 = wafr_dem
2. At the UNIX prompt type
   %: Arc
3. At the Arc prompt type
   Arc: &run inland.aml
4. At the prompt "Minimum inland catchment area:" type
   Minimum inland catchment area: 300000000000
   which equals 300,000 Km².
5. At the prompt "Minimum inland catchment depth:" type
   Minimum inland catchment depth: 800
   which equals 80 m
6. At the prompt "Threshold area for watershed delineation (in grid cells):" type
   Threshold area for watershed delineation (in grid cells): 40000
   which equals 1'000,000 Km².

The output of the AML are an arc coverage of streams called acsplus, and a polygon coverage of watersheds called pcwplus.

Checking your work

A duplicate of all grids generated during the process of running the AML (some of them killed by the AML itself) are included in the directory gisfiles (inland if you downloaded the files). The names of these duplicated grids have the prefix x to differenciate them from those created by the AML. You may want to check your work by comparing your grids with the duplicated ones.

Coverages and grids obtained when delineating streams and watersheds without identifying inland catchments are also included in the directory gisfiles (inland if you downloaded the files).

The following Table lists all the grids and coverages included in the directory gisfiles (inland if you downloaded the files).

The figure below shows the watershed delineation of the Niger River basin with and without inland catchment identification. Note that, when the inland catchments are not identified, the Lake Chad basin and part of the Sahara desert (orange areas) drains towards the Niger river basin (green area).

Using ArcView, display in the same View the coverages pcw and pcwplus (which should be equal to xpcwplus) to check if your are getting the same results.

Please e-mail your questions and comments to folivera@mail.utexas.edu

These materials may be used for study, research, and education, but please credit the authors and the Center for Research in Water Resources, The University of Texas at Austin. All commercial rights reserved. Copyright 1997 Center for Research in Water Resources.

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