Generating a HMS basin file for Africa

Hydrologic Modeling System (HMS) is a windows-based flow routing software developed by the Hydrologic Engineering Center of the US Army Corps of Engineers. HMS describes watershed characteristics using a schematic diagram with an associated basin file. A preprocessor for developing the basin file required by HMS has been developed at the Center for Research in Water Resources (CRWR) of the University of Texas at Austin. The preprocessor, CRWR PrePro, generates the basin file based on the Digital Elevation Model (DEM). For detailed instructions on how to use the preprocessor, see the exercise on Digital Watershed Delineation and CRWR PrePro.

A HMS model of Africa has recently been processed as part of an effort to achieve continental scale flow routing. The model was developed from 30" DEMs and flow direction grids developed by the EROS Data Center of the USGS. Each cell in this DEM is one kilometer by kilometer wide and hence 1 km2 in area. The data sets have been projected into a Lambert Azimuthal projection. Click here to see the projection file or a table of the projection parameters for various continents.

A flow accumulation grid was computed from the flow direction grid, and all cells with a flow accumulation of 10,000 or more were classified as being part of a stream. The resulting strings of cells were joined together to form continuous stream links each with a drainage area of at least 10,000 kilometers squared. The figure below shows a portion of the stream network including the Niger River in West Africa. The DEM of the region is displayed in the background.

The diagram below shows the corresponding HMS schematic for the same river basin.

The subwatersheds were also delineated at the same threshold as shown in the figure below. Each of the areas demarcated by the grey line is a subwatershed. There is one stream arc for each subwatershed in this figure though this is not a requirement for CRWR PrePro.  Interactive menus allow the user to modify the stream network and subwatersheds by merging existing subwatersheds, defining new outlets or creating additional streams as required.

The subwatersheds, junctions, outlets, sinks and sources were also identified and presented as points. In the figure below, a grey dot indicates a subwatershed. Junctions are shown as yellow dots while outlets are indicated by red dots. Sources are represented by bright green dots. Attributes of the subwatersheds include the drainage area, downstream reach, constant loss and lag.


HMS provides the user with various options for computing loss rates. Two of these methods, namely the Soil Conservation Service (SCS) curve numbers, and the approach which assigns an initial loss in the subwatershed followed by a constant loss rate for in-stream flow (Initial + Constant loss approach), are supported by CRWR PrePro. It also supports two flow routing methods; the Muskingum and Lag routing.

The Lag routing method was selected for flow routing while losses were determined using an Initial + Constant loss approach. Variables which are dependent on terrain such as flow length and the slope along the flow path are computed for the DEM by the preprocessor. Other flow routing variables required by HMS are provided by the user as text files. The preprocessor informs the user of the fields required for these text files based on the flow routing and loss computation methods selected. The variables required include the Muskingum 'X' coefficient, the initial and constant loss rate and the flow velocity for both subwatershed and stream reaches. The simulation time step is also a required input for the preprocessor.

For this delineation, a flow velocity of 0.3 m/s was used. This velocity was determined as being characteristic of flows in the Niger basin during a previous Water Balance of the Niger River. Initial and constant loss rates were both set to zero while the Muskingum 'X' coefficient was set to 0.2 for all river reaches in Africa. Since the resulting basin file can be opened in a text editor, the values of these parameters can be changed on a case by case basis or by using the replace function available in most text editors.


As can be seen from the figure below, all the elements in the delineated stream coverages are represented in the HMS schematic. The basin file associated with each of these river reaches contains a lag time computed by dividing the flow length (distance along the flow path from the furthest cell in the reach to the outlet) by the velocity. It also contains a grid code which links the river arc to the corresponding subwatershed. Connectivity between river reaches is established by a field which contains the id number of the next downstream element.


The basin file generated by Arcview was successfully imported into HMS as a basin model. The result is a a basin model of Africa containing over 3000 elements including 1500 subwatersheds and stream segments. HMS allows the basin file to be modified by creating additional elements or removing unwanted ones. Reservoirs, diversions and source can also be created as desired. Different precipitation events or scenarios can be simulated by creating the precipitation model and control files required to perform a simulation in HMS. An initial run using dummy data was successfully complete for the Congo basin. Work is currently continuing to create precipitation models based on Wilmott-Legates global climatology data sets for the whole continent.