Extreme weather events in the Sneeuberg, Karoo, South Africa: a case study of the floods of 9 and 12 February 2011

Abstract. Two destructive flood events occurred in rapid succession in the semi-arid Sneeuberg Mountains of the Karoo, South Africa in February 2011. The temporal and spatial characteristics of these two extreme events are examined in this paper through analysis of data from an unusually dense, and reliable, network of farm rain gauges. These analyses add to our understanding derived from existing rain gauge information. Comparisons are then made with patterns from a range of modeled products derived from remote sensed information: the Modern-Era Retrospective Analysis for Research and Applications (MERRA), the Tropical Rainfall Measuring Mission (TRMM) and the Global Land Data Assimilation System (GLDAS). We found that the first flood event was widespread and precipitation was related strongly to altitude. The second was highly localised, with no relationship to altitude. Both had very sharply peaked rainfall intensities. These findings are of significance to the studies of flooding and landscape change in the area as such events have become more pronounced over the past 50 yr and it is likely that this trend will accelerate. The modeled patterns are derived largely from remote sensing and we found that they are reliable for drawing out monthly and annual variations but they make noticeable underestimates. They are poor estimates, however, both for the spatial distribution of precipitation, and the short term trends as they struggle to estimate the impact of topography and other local forcing factors. This finding corroborates information derived from other analyses at broader spatial scales using more widely spread, established rain gauge stations. Ten percent of southern Africa has been classified as mountainous and these areas provide much of our water resources so our findings are significant to water managers throughout this and similar mountainous regions.