Base Maps: SRTM Derivatives

AfSIS SRTM derivatives described below can be viewed and downloaded using the Interactive Map Tool.

Hydrologically Corrected / Adjusted SRTM Digital Elevation Model (AfrHySRTM)

AfrHyShuttle Radar Topography Mission (SRTM) is an adjusted elevation raster in which any depressions in the source Digital Elevation Model (DEM) have been eliminated (ﬁlled), but allowing for internal drainage since some landscapes contain natural depressions. These landscapes have their own internal drainage systems, which are not connected to adjacent watersheds. Null cells (drains) were placed in depressions exceeding a depth limit of 20 m and with no less than 1000 cells (pixels) during the DEM adjustment process. After ﬁlling depressions in the DEM, ﬂowpaths can also be generated. This dataset was produced at the World Agroforestry Centre (ICRAF) in Nairobi, Kenya, for the Africa Soil Information Service.

Metadata for the AfHySRTM dataset are available here.

Geomorphologic/Hydrologic Characteristics

1. Speciﬁc catchment area

The Africa Soil Information Service: Specific Catchment Area (SCA) is a 90m raster dataset showing local flow accumulation and flow direction using the formula:

SCA = A/I

where A is unit contributing area of land upslope of a length of contour I. Unit ﬂow width was calculated from the DEM cell/pixel dimensions and varies depending on whether ﬂow direction in horizontal vertical or diagonal through the cell. The specific catchment area contributing to flow at any given location can be used to determine relative saturation and water runoff and, together with other topographic factors, can be used to model soil erosion, sediment yield, and landslide risk. The digital elevation model used to construct this dataset was AfHydSRTM, based on the CGIAR-SRTM 90m Version 4. This dataset was produced at the World Agroforestry Centre (ICRAF) in Nairobi, Kenya, for the Africa Soil Information Service.

Metadata for the AfSIS SCA dataset are available here.

2. Topographic wetness index

The Africa Soil Information Service: Topographic Wetness Index (TWI) is a 90m raster dataset showing zones of increased soil moisture where the landscape area contributing runoff is large and slopes are low. The topographic wetness index, originally developed by Beven and Kirkby in 1979, provides a measure of wetness conditions at the catchment scale. Local upslope contributing area and slope are combined to determine the wetness index:

WI = ln (A_s / tan(b) )

where As is ﬂow accumulation or effective drainage area and b is slope gradient. Methods of computing this index differ primarily in the way the upslope contributing area is calculated. The use of effective drainage area gives a quasi-dynamic index overcomes assumptions of of steady-state (i.e. uniform soil properties). This index predicts zones of increased soil moisture where the landscape area contributing runoff is large and slopes are low and is commonly used in soil landscape modeling and in the analysis of vegetation patterns. The digital elevation model used to construct this dataset was AfHydSRTM, based on the CGIAR-SRTM 90m Version 4. This dataset was produced at the World Agroforestry Centre (ICRAF) in Nairobi, Kenya, for the Africa Soil Information Service.

Metadata for the AfSIS TWI are available here.

Click here for further details on AfSIS SRTM processing (pdf).