The impact of rainstorms on floods in ephemeral channels in southeast Spain
Introduction
The conditions, position and connectivity of runoff and sediment source areas in semi-arid catchments have a profound effect on flood characteristics of ephemeral streams. The relationship between rainfall, the source areas of runoff and the flood hydrograph is complex and, because of the discrete nature of each convective cell, an individual storm is unlikely to affect the entire drainage basin and successive storms will affect different parts of a catchment (Schick and Lekack, 1987). Total annual precipitation for the region is less then 500 mm, and this is concentrated in a few winter months. Rainfall in the Mediterranean is usually very intense (Obled and Tourasse, 1994; IGN, 1995), exhibits an erratic temporal and spatial distribution, and can be very localized (Thornes, 1976; Alonso-Sarria and López-Bermúdez, 1994). The likelihood of very large floods is therefore small because the precise conditions needed to produce outflow from sub basins across the whole of a catchment rarely occur.
This paper presents partial records of rainfall and runoff for floods in two catchments in southeast Spain in September 1997. The recorded rainfall data are linked to estimates of peak flood discharge that were made throughout the catchments following the storms. These data are used to investigate variations in discharge throughout the ephemeral channel systems and to estimate rainfall intensities and return periods for previous large floods. These estimates of changing flood discharge are some of the first of their kind and allow us insight into conditions promoting floods.
Section snippets
Study area
The work was carried out in two catchments; the Rambla de Nogalte located on the border of the Provinces of Murcia and Almerı́a, and the Rambla de Torrealvilla located in the Province of Murcia, southeast Spain (Fig. 1). Characteristics of the two study basins are summarised in Table 1. The Rambla de Nogalte is the larger basin and drains mainly metamorphic rocks and conglomerates, dominated by red mica schist, but with localised outcrops of blue mica schist. The soils derived from the
Nogalte
Fig. 2 shows rainfall intensities from 11 March 1997 to 5 January 1998. The floods in the Nogalte were caused by the storms on the 29 September 1997. For this day, all the rain gauges registered three intense rain events. At most rain gauges, the first two storms had similar maximum intensities, approaching 200 mm h−1 and the third storm was much less intense at approximately 50 mm h−1. The maximum rainfall intensities were much higher than expected.
These data can be plotted at a much greater
Variations in discharge due to lithology
The argument that flood discharge is influenced by lithology is supported by the difference in lithology and discharges between the two study catchments, but also by variations locally within the basins. Differences in lithology between the two catchments results in a lower threshold rainfall for the Torrealvilla than that for the Nogalte. Initial estimates of the thresholds (as calculated above) are 50 and 150 mm h−1, respectively. Flood discharge is also influenced by transmission losses,
Conclusions
The data record from the rain gauges in the Rambla de Nogalte catchment showed that the network is too sparse to accurately relate rainfall to discharge. The difference in timing between flow and rainfall at the stage gauging site and rain gauge S1 showed how rainfall varies over distances as small as 2.5 km. Thus, the intense storm cells that result in flows in semi-arid channels may be much smaller than estimated by Renard and Keppel (1966)and Diskin and Lane (1972). If rain gauges were going
Acknowledgements
This work has been carried out as part of the MEDALUS III project. We would also like to thank Mark Newcombe in the Graphics Unit in the School of Geography, University of Leeds for Fig. 1, Fig. 3, Fig. 6, Dr. Roy Richardson for helping install the rain gauges and Fernando Garcia Robredo for the 30-year data record. Automatic monitoring equipment was funded by King's College London in the Nogalte catchment and by Leeds, Portsmouth and Leuven in the Torrealvilla.
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