A small pond can be defined as a surface ranging between 1 km and few tens of km² of either free water or water with vegetation. Monitoring small ponds or water bodies is important for economic activities and is of great environmental value.

A method to map and monitor small water bodies in arid and semi-arid regions in northern Africa has been developed by the Joint Research Centre (Ispra) in the frame of EU/FP5. Algorithm has been applied to SPOT-VEGETATION 10-daily composites in the frame of the GEOSUCCESS project. Small ponds can be easily recognized by visual analysis because of the local contrast. Therefore, an index of local contrast (0-1) is derived by computing the difference between the "regional average" and the actual pixel value either of original or derived channels. This regional average is the value obtained with a moving window of 45 by 45 pixels. Simple thresholds are then used to select surfaces throughout the image. To extract free water, differences between the average and the original value of (NDVI-NDWI) are computed and compared with a threshold (Gond et al., 2004). In parallel, the same procedure is applied on the SWIR channel. If both procedures indicate the presence of of water, the pixel is classified as such. To extract inundated areas with vegetation, the same procedure is applied but for NDVI and NDWI independently. To avoid misclassification of clouds as being water bodies, a specific cloud mask has been developed based on a simple threshold on the blue channel. A dilation filter was then applied with a 10 x 10 buffer in order to mask also the shadows.
For each date of observation, the time series of each pixel is analysed to assess the dates of replenishment and end of drainage. Theses dates are provided per pixel.

In the framework of the GEOLAND project, these methodologies were applied to the whole Africa for applications of the Land Cover and Forest Change Observatory (OLF). The Water Bodies products are produced at VITO for the period March 2000-May 2004.

In addition, the history of occurence of detection was checked against a set of data : the Global Lake and Wetland data base (GLWD), the Shuttle Radar Topography Mission (SRTM), Landsat quicklooks images and SPOT/VEGETATION multispectral images. In the current version, the CILSS region was investigated ; this work is still in progress for the rest of the continent. This map of "manually validated water bodies" is provided in the Small Water Bodies products.

These products are available through the geoland/CSP website. More details about the Small Water Bodies detection and the Seasonalilty assessment are available in the VGT4AFRICA User Manual. A technical documentation is also provided with the products.