Spatial variability of rainfall on a sub-kilometre scale

Abstract

The variability of rainfall in space and time is an essential driver of many processes in nature but little is known about its extent on the sub-kilometre scale, despite many agricultural and environmental experiments on this scale. A network of 13 tipping-bucket rain gauges was operated on a 1·4 km2 test site in southern Germany for four years to quantify spatial trends in rainfall depth, intensity, erosivity, and predicted runoff. The random measuring error ranged from 10% to 0·1% in case of 1 mm and 100 mm rainfall, respectively. The wind effects could be well described by the mean slope of the horizon at the stations. Except for one station, which was excluded from further analysis, the relative differences due to wind were in maximum ±5%. Gradients in rainfall depth representing the 1-km2 scale derived by linear regressions were much larger and ranged from 1·0 to 15·7 mm km-1 with a mean of 4·2 mm km-1 (median 3·3 mm km-1). They mainly developed during short bursts of rain and thus gradients were even larger for rain intensities and caused a variation in rain erosivity of up to 255% for an individual event. The trends did not have a single primary direction and thus level out on the long term, but for short-time periods or for single events the assumption of spatially uniform rainfall is invalid on the sub-kilometre scale. The strength of the spatial trend increased with rain intensity. This has important implications for any hydrological or geomorphologic process sensitive to maximum rain intensities, especially when focusing on large, rare events. These sub-kilometre scale differences are hence highly relevant for environmental processes acting on short-time scales like flooding or erosion. They should be considered during establishing, validating and application of any event-based runoff or erosion model. © 2009 John Wiley & Sons, Ltd.