A review of daily soil erosion in Western Mediterranean areas
Introduction
Recent research suggests that climatic variability will increase as a consequence of global climate change, resulting in greater frequency and intensity of extreme weather events, which could increase rates of erosion (Nunes and Seixas, 2003, Nearing et al., 2005). This fact could be especially relevant in Mediterranean areas where precipitation is characterized by scarcity, torrential storms and extreme variability in space and time (Romero et al., 1998, Alpert et al., 2002). Daily storms of several hundred mm are common throughout the Mediterranean area (López-Bermúdez and Romero, 1993, Poesen and Hooke, 1997), and they have very short return periods (de Luis, 2000).
Several researchers have pointed out that this irregularity of precipitation is the main cause for temporal irregularity of erosion rates in Mediterranean landscapes (Tropeano, 1983, Tropeano, 1991, Zanchi, 1988, Ergenziger, 1988, Renschler et al., 1999, Vacca et al., 2000, Renschler and Harbor, 2002). Also, it has been found that there is no exact relation between extreme rainfall and extreme fluvial discharge (Lins and Slack, 1999, Osterkamp and Friedman, 2000, Nunes et al., 2005), and largest rainfall events do not necessarily produce the maximum soil erosion (González-Hidalgo, 1994, Romero et al., 1999). Moreover, in addition to the Mediterranean rainfall being highly variable in space there exist high diversities of soil and plant cover. As a consequence erosion rates greatly vary spatially.
In Mediterranean areas the relation between extreme rainfall events and erosion process has been difficult to assess until now (Milly et al., 2002, Michael et al., 2005) and there is little research into the effects of extreme events on slope geomorphology (Mulligan, 1998). In addition to being infrequent, during extreme events the field installations are usually flooded or destroyed (Torri et al., 1999, Coppus and Imeson, 2002), and simulations of the processes generally refer to normal events (see an exception in González-Hidalgo et al., 2004). Also, quantification of magnitudes of daily soil eroded can be affected by field methods. All these reasons explain the extreme disparity of erosion amounts at different times and places (Imeson and Lavee, 1998, Renschler et al., 1999, Bagarello and Ferro, 2004) and highlight the difficulty presented by extrapolating data obtained from experimental plots (Roels, 1985, Stroosnijder, 2005).
In this paper, we will review daily soil erosion (mainly by rainfall) in Western Mediterranean areas from published data, and study the relationship between total soil eroded and daily erosion events. The objective of this paper is to look for general characteristics of daily erosion and discuss the implications of largest annual events on total soil erosion.
Section snippets
Data sources, method
There are very few studies which include full data of soil erosion on a daily scale, and they usually provide data that has been summarized or averaged out. Notwithstanding, in Western Mediterranean areas we have found in published literature daily data base of soil erosion from 17 different sites. Many of them referred to rainfall erosion (i.e. wash, splash) from plots different in size, length and characteristics (Imeson, 1983, Marques and Roca, 1986, Albaladejo and Stocking, 1989, Marques,
Results
The amount of total soil eroded from research less than two years varied tremendously from one place to another (Table 1). However, the three most erosive events produced at least 50% of the total amount. In some places the three highest events contributed more than 75% of the total erosion.
In studies with a span of more than two years again three highest daily erosive events produced > 50% of the total soil eroded annually in each place, regardless of the amount of soil eroded (Table 2). The
Discussion
Reason for continuous soil erosion research have been recently revised, however the problems of analyzing these are still unresolved (Stroosnijder, 2005). On the other hand, erosion and desertification have been the subject of growing attention over the last few years, and pose the problem of whether they form part of the variability of the climate, or they are a sign that the climate is changing.
Our approach shows that the use of climatic average values, for example annual precipitation (cfr.
Conclusions
The review of daily soil erosion published in Western Mediterranean areas suggests that the phenomenon depends on a few daily events and consequently it is strongly compressed in time. In spite of the high variability between sites, years, and despite the different approaches, over 50% of soil eroded annually belongs to just three daily erosion events. Moreover, this characteristic remains in the upper time scales, regardless of the length of the study and proves the importance that a few daily
Acknowledgements
The researchers would like to express their thanks for financial support to the projects: REN2002-01023-CLI and REN2003-07453 (Spanish Ministry of Science and Technology) and the program for consolidated research groups (Climate, Water, Global Change and Natural Systems Group and the Quaternary Paleo-environmental Group-PALEOQ, BOA 20-04-2005), financed by the Government of Aragón. We especially thank Dr. Lasanta and Dr. García Ruiz (CSIC-IPE, Spain) for their help in providing unpublished data
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