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Discuss., 10,Hydrology 10809–10844, and 2013 Hydrology and www.hydrol-earth-syst-sci-discuss.net/10/10809/2013/Earth System Earth System doi:10.5194/hessd-10-10809-2013 © Author(s) 2013. CC Attribution 3.0 License.Sciences Sciences Discussions Open Access Open Access Ocean Science This discussion paper is/hasOcean been under Science review for the journal Hydrology and Earth System Sciences (HESS). Please refer to the corresponding final paper in HESS if available.Discussions Open Access Open Access Solid Earth Extreme weatherSolid Earth events in the Sneeuberg, Karoo, South Africa: a case study ofDiscussions the floods of 9 and 12 FebruaryOpen Access 2011 Open Access The Cryosphere The Cryosphere Discussions R. C. Fox and K. M. Rowntree Department of Geography, Rhodes University, 6140 Grahamstown, South Africa Received: 25 July 2013 – Accepted: 31 July 2013 – Published: 19 August 2013 Correspondence to: R. C. Fox ([email protected]) and K. M. Rowntree ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 10809 Abstract Two destructive flood events occurred in rapid succession in the semi-arid Sneeu- berg Mountains of the Karoo, South Africa in February 2011. The temporal and spa- tial characteristics of these two extreme events are examined in this paper through 5 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 10 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 15 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 20 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. 10810 1 Introduction The Gats and Wilgerbosrivier rivers, two tributaries of the Sundays River that drains the Sneeuberg Mountains in the Eastern Cape of South Africa, experienced two extreme rainfall events on 9 and 12 February 2011. Severe flooding occurred in this semi-arid 5 mountain catchment, destroying weirs, bridges and irrigation infrastructure, inundating meadowlands and drowning livestock on the downstream farm of Ganora. Figure 1 (left panel) shows the main road bridge over the Wilgerbosrivier above Ganora Farm com- pletely submerged under a wave of flood water. Figure 1 (right panel) was taken 90 min later immediately downstream of the bridge showing the flood water surging through 10 the canyon where Ganora’s farm buildings and associated irrigated lands lie. The storm of 9 February 2011 had a peak flow that we estimated at 600 m3 s−1. The wall of a two meter high diversion weir was breached; it is completely submerged by several me- ters of water in Fig. 1 (right panel). Two other weirs were also significantly damaged, siphons carrying water under the river bed were destroyed and furrow systems dam- 15 aged. A second flood hit on 12 February 2011 generating substantial amounts of storm runoff. The impact these floods was felt over the wider Sundays catchment and caught the attention of the local media. It was reported that one farmer in the nearby town of Nieu Bethesda estimated a cost of R 500 000 (c. US$ 50 000) to rebuild his dam 20 and that it could take some farmers a decade to recover (S.A. Weather and Disaster Information Service, 2011). The Cqwebe Dam, upstream of the town of Graaff-Reinet and approximately 50 km downstream of Ganora Farm and Nieu Bethesda, overflowed for the first time since 1974. These events have happened before in this part of the Eastern Cape Province of 25 South Africa. Ganora Farm’s main weir was swept away previously in 1961. In 1974 the nearby town of Nieu Bethesda’s diversion weir was similarly destroyed. This weir feeds the furrow system that the residents of the old areas of the town use for mar- ket gardening of vegetables. Downstream from the town, small scale farmers have 10811 flood-irrigated lands on which they depend for growing Lucerne and feeding sheep and cattle. It usually takes a minimum of two years for the weir and furrow systems to be reconstructed and so these events are a major disruption to local lifestyles. Mason et al. (1999) have shown that the rainfall stations in proximity to our study area 5 have experienced significant increases in extreme events between 1931 and 1990. They estimate that the area experienced a 20–30 % increase in the intensity of 10 yr rainfall events. A recent analysis of rainfall patterns from two well established stations with reliable records (Cranemere 90 km South-East and Middleburg 50 km North-East of Ganora) by Foster et al. (2012) shows evidence of a shift in the magnitude–frequency 10 relationship of daily rainfall since 1950. At Cranemere the magnitude of the 10 yr daily rainfall increased from 54 to 64 mm and the 50 yr daily rainfall from 71 to 84 mm. Mid- delburg showed greater changes with a shift of the 10 yr daily rainfall from 50 to 82 mm and the 50 yr daily rainfall from 70 to 115 mm. Despite these greater extremes, the long term mean rainfall appears to be constant. These shifts in the character of the rainfall 15 can have important consequences not only for farming enterprises but also for land degradation, biodiversity and the wider rural economy. The Sneeuberg Mountains to the north of the Cambedoo Plains between Pearston and Graaff-Reinet have been the focus of geomorphological research since the late 1990s. Holmes (1998, 2001) investigated palaeo-environments, with special attention 20 to valley fills. Boardman et al. (2003) and Keay-Bright and Boardman (2006, 2007, 2009) shifted the research focus to soil erosion during the period of historical records. Foster et al. (2012), Foster and Rowntree (2012) and Rowntree and Foster (2012) have added studies of catchment sediment yield based on sediment deposits in small farm dams. The consensus arising from this research is that catchment scale erosion is re- 25 sponding to a complex set of drivers including land management (pressures from live- stock grazing and cultivation), climate and intrinsic structural changes to connectivity (Foster et al., 2012). The sediment record from farm dams indicates elevated sediment delivery since the 1950s that is coincident with the increased severity of storm rain- fall. The flood of 2011, described in this paper, caused significant geomorphological 10812 change in the channel downstream of the weir and a monitoring programme has been set up to study the post flood response. There are five farms in the 277 km2 catchment above Ganora that keep daily rainfall records and we ourselves installed an autographic gauge at Ganora Farm in April 2009. 5 Additional daily records are available from farms immediately outside the catchment area. This relatively dense rain gauge network presented an opportunity to investigate the spatial and temporal pattern of storm rainfall in February 2011. While the traditional source of precipitation data is from a rain gauge, new sources of data derived from satellite imagery are becoming readily available. These provide a continuous spatial 10 cover that is lacking from the point coverage provided by a rain gauge network. Fur- thermore, the number of active stations is becoming depleted (Lynch, 2004). These satellite derived data have become increasingly accessible through servers such as Giovanni: the Goddard Earth Science Data and Information Services Centre’s web tool (Kempler, 2013a). A question that we ask in this paper is: does this new data source 15 have potential to aid the investigation of extreme events such as the one observed in February 2011? We make a comparison of recorded precipitation and modeled data from three programmes accessed from Giovanni through examining both the short term rain series (daily and hourly rainfall) over the period of the extreme events from 9–12 February 2012 and the long term (annual and monthly rainfall) over the past 30 yr.