A watershed is the upslope area that contributes flow - generally water - to a common outlet as concentrated drainage. It can be part of a larger watershed and can also contain smaller watersheds, called sub-basins.

The Catchment Delineation Tool (CDT) is used to calculate the direct catchment area or sub-basin from a user selected input location or pour point. The tool also calculates the following catchment properties:

• area (hectares)
• mean annual precipitation (MAP) (mm/a)
• mean annual runoff (MAR) (mm/a and m³)
• max flow length (meters)

The CDT is based on an ArcGIS model using a combination of the Esri Spatial Analyst Extension tools, which is accessed through an ArcGIS Server Geoprocessing Service hosted by the Western Cape Department of Agriculture. More information about the Esri Watershed calculation process can be found here.

The tool utilises the 20m SUDEM (Stellenbosch University DEM) hydrologically correct elevation model and derived data sets e.g. flow accumulation and flow direction. The extent of the data and processing is however limited to the Western Cape.

• Navigate to the area of interest on the map
• Use the Flow Accumulation and/or Contour layers (top right corner) to determine the pour point location - a point on the flow accumulation lines would render the best results.
• Click the pour point location in the map
• Confirm the location and run the model (± 1 minute)

The catchment result will appear as a blue boundary on the map canvas and the catchment information will be added to the Results tab. A larger combined catchment area can be calculated by running the tool for multiple adjacent catchments along a major flow accumulation line.

The Map Export tool can be used to create a JPEG map of a selected area.

Combined Catchment Information

Total Catchment Area:	0 Ha
Total MAR Volume:	0 m³

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Extent

Current Extent
Catchments

Layout

Landscape
Portrait

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A4
A3
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Digital Elevation Model

Description:
20x20m hydrologically correct elevation model for the Western Cape.

Custodian/Source:
Centre for Geographical Analysis, Stellenbosch University

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Mean Annual Run-off

Description:
Mean annual runoff for South Africa (mm/year for each 1 x 1 minute grid cell), based on disaggregating the Water Resources Assessment 2005 data (Middleton and Bailey 2009), which represents the most commonly-used national mean annual runoff data used by the Dept of Water Affairs for water resources planning and management.

Custodian/Source:
CSIR

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Mean Annual Precipitation

Description:
Long term daily rainfall values from over 9 600 stations each with >= 15 years of daily records, interpolated to a arc minute (1' x 1' latitude/longitude, ~ 1.7 km x 1.7 km) grid.
Custodian/Source:
South African Atlas of Climatology and Agrohydrology, WRC.

Quaternary Catchments

Description:
The new SRTM Quaternary Catchment as prepared for the Water Reaseach Commission project K5/1908 was used to produce a dataset with DWA political boundaries in stead of the "hydrological political boundaries" by replacing the "hydrological political boundaries" with the new DWA landmass coastline and the northern boundary of South Africa from the 2011 Demarcation Board release. By doing this a new zone refered to as "No Impact Zone (NIZ)" was added to coastal areas where dunes caused water to flow directly to the sea. These NIZ zones where omited from the original dataset.

Custodian/Source:
Department of Water Affairs, ARC-Institute for Soil Climate and Water

Western Cape Department of Agriculture
Website:	www.elsenburg.com
Tel:	021 808 5111
Email:	GIS@elsenburg.com