Satellite evidence of a topographic sink-source system in a small semi-arid watershed

Water is a limiting factor for biological processes in drylands and consequently it is expected that vegetation cover along slopes will be affected by runoff flow regime and shape of topography. Nevertheless, spaceborne remote sensing data have hardly been used to study the effect of hydrological processes on vegetation pattern at the slope scale. This Letter reports on the spatio-temporal variation in spaceborne-derived Normalized Difference Vegetation Index (NDVI) data, in physiographic units of unique pedo-hydrological characteristics, in a semi-arid watershed. It was found that NDVI values in the footslope and shoulder physiographic units are significantly (p<0.0001) higher than those of the interfluve and the backslope. This difference, observed during the entire phenological cycle, was enhanced towards the peak season but was of less significance during the early season and towards senescence. These results support the hypothesis that water redistribution can significantly increase plant production in sink areas, also in the slope-scale.