Rainfall, topography and primary production relationships in a semiarid ecosystem

A series of 15 Satellite Pour l'Observation de la Terra XS images, which were acquired during a single growing season, were used to study the spatial and temporal relationships between topography, rainfall and aboveground net primary production (ANPP) of annuals in a semiarid environment. A digital elevation model was used to locate five physiographic units: interfluve, shoulder, backslope, foot slope, and channel, along the slope catena. ANPP values were then surrogated by the normalized difference vegetation index (NDVI). Four phenological phases were interpreted from the seasonal NDVI profiles: germination, green-up, drying, and senescence. A significant difference between NDVI values of all physiographic units was found, mainly during the green-up phase. NDVI values at peak season were characterized as typical of semiarid ecosystems, except those of the footslope and the channel, which were found to have meaningfully high NDVI values, due to run-off distribution and a thick soil profile. The seasonal NDVI integral was related positively and linearly to model-derived water availability values for all physiographic units and the NDVI was correlated to multiple-timescale rainfall data and the length of dry spells. The 1-month rainfall data were found to have the highest correlation with the NDVI, indicating lag and cumulative effects of rainfall on production. This implies that the time required for the plants to use the water for production is around a month. It is concluded that studies of ecosystem functioning and capabilities in semiarid environments should consider not only mean annual rainfall amounts, but also the temporal rainfall distribution, mainly on a monthly scale, and the effect of physiographic units. Copyright (C) 2010 John Wiley & Sons, Ltd.