ALCOM SADC Watershed Model

SADC Watershed Model and Database

developed in collaboration with WWF

SADC watershed mapIntroduction

The SADC Digital Watershed Model is a joint project between the ALCOM and WWF Zimbabwe. The model was derived from the 30 arc second digital elevation model (DEM) of Africa (1). Original purpose of the model was the development of digital species distribution maps and hydrological monitoring but ultimate applications reach far beyond this.

Development of the watershed model and naming

The delineation of the sub-basins was done base on a DEM and the DCW river layer (digitised from the ONC’s) in Arc Info (see procedure) by a consultant. Approximately 1100 sub-basins were delineated for subequatorial Africa. After the delineation, naming and determination of each downstream basin was done manually based on the DCW drainage and topographical layer by ALCOM.

Classification system of the watersheds

ALCOM developed a classification system which allows to determine within seconds and without the use of proper GIS software for each individual watershed:

  1. the megabasin to which the watershed belongs (e.g Ruvuma basin)
  2. all downstream basins (e.g. Zambezi from source to the ocean)
  3. all upstream basins or part of a mega-basin (e.g.Vaal catchment)
  4. name of primary and secondary rivers in the watershed (e.g. Vaal catchment)

The basis of this classification system is a database which holds names and the identifier of the downstream basin for each sub-watershed.

Use of the watershed model and database

With the help of the classification system, mapping of downstream basins, upstream basins, megawatersheds or parts of watersheds becomes very easy. After the query, a macro button in the database allows export of a simple worksheet file which holds the ID’s of the watersheds together with the most relevant data such as names and color codes. This sheet is then loaded into a relatively inexpensive mapping programme (Mapviewer) which visualises automatically the query through a hatch map with predefined colors. A small custom-written programme allows extraction of the selected basins from the complete basin-file (in BNA format).

A poster with all the subwatersheds in subequatorial Africa and their primary names is currently in press and will be made available very soon by ALCOM and WWF. A simplified version with the main watersheds in Southern Africa is available here as a GIF file.

The watershed model and database is currently being used to develop aquatic species distribution maps based on point locations. Monitoring of downstream spread of aquatic pests such as Water Hyacinth is equally one of the applications. Other programmes are already using the data for flow analysis to monitor filling of dams based on the rainfall in the catchment area. Applications of the model however will go far beyond this and the database is also extremely useful for educational purposes.

(1) The DEM of Africa used had been developed by U.S and other international cooperators at the USGS EROS Data Center in Sioux Falls,SD,USA. This DEM was based primarily on elevation data taken from the Digital Chart of the World (DCW) which had been digitized from the 1:1,000,000 Operational Navigation Charts by the Environmental Systems Research Institute (ESRI) for the U.S. Defence Mapping Agency (DMA). Where incomplete elevation data were indicated on the ONC Charts courser resolution data from the DTED DEM, also from the DMA, were used to fill elevation gaps in the USGS DEM. The interpolated resolution of the DEM of Africa corresponds to 0.0084 decimal degrees which equates to roughly one square kilometer pixels. The DEM provided by the FAO/FSTAU to ALCOM for the project was in IDRISI format.

For more information contact:

The Fisheries Officer, ALCOM
Mail:       PO Box  3730, Harare, Zimbabwe
Location :  Fisheries Research Unit, National Parks Complex,
            Sandringham Drive, Harare, Zimbabwe
Telephone: 263-4-734797, 724985
Fax:       263-4-736846
Telex:      260-40 FAO ZW

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