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 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 10x10 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). These
Water Bodies products have been 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.
More details about the Small Water Bodies detection and the Seasonalilty assessment are available in the VGT4AFRICA User Manual, which can be downloaded with the products files. A technical documentation is also provided.
References
Gond, V., E. Bartholomé, F. Ouattara, A. Nonguierma, and I.
Bado,
Surveillance et cartographie des plans d'eau et des zones humides et
innondables en régions arides avec l'instrument VEGETATION
embarqué sur
SPOT4, International Journal of Remote Sensing, 25 :5,987-1004,
2004.
