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For large-scale catchment hydrology, the crucial importance of a good estimate of spatial rainfall variability is generally admitted. However, the assumption of uniform rainfall is still applied for small areas, whether they are studied as individual catchments or represent an elementary area in a distributed model. To investigate the validity of this assumption, an experiment was conducted in a small catchment (4.4 ha) in the semiarid USDA-ARS Walnut Gulch Experimental Watershed. Measurements were made with recording and non-recording raingages as well as vectopluviometers for a range of events during the 1990 monsoon season (July-September). Geostatistical analysis of the data indicated the presence of first-order drift with corresponding rainfall gradients ranging from 0.28 to 2.48 mm per 100 m with an average of 1.2 mm per 100 m. These gradients represent a 4-14% variation of the mean rainfall depth over a 100 m distance. Given these observations, the assumption of spatial rainfall uniformity in this and similar convective environments at the small watershed scale of 5 ha appears to be invalid.
