Spatial soil loss risk and reservoir siltation in semi-arid Tunisia

Abstract

Soil erosion vulnerability and extreme rainfall characteristics over the Mediterranean semi-arid region of Tunisia are crucial input for estimation of siltation rate in artificial reservoirs. A comprehensive high resolution database on erosive rainfall, together with siltation records for 28 small reservoirs, were analysed for this region, the Tunisian Dorsal (the easternmost part of the Atlas Mountains). The general life-span of these reservoirs is only about 14 years. Depending on the soil degradation in the different catchments, the corresponding reservoirs display a wide range of soil erosion rates. The average soil loss was 14.5 t ha-1 year-1but some catchments display values of up to 36.4 t ha-1 year-1. The maximum 15-min duration rainfall intensity was used to determine the spatial distribution of rainfall erosivity. The northwestern parts of the Tunisian Dorsal display the most extreme rainfall erosivity. Spatial erosion patterns are to some extent similar; however, they vary greatly according to their location in the "soil degradation cycle?. This cycle determines the soil particle delivery potential of the catchment. In general, the northwestern parts of the Dorsal display modest soil erosion patterns due to the already severely degraded soil structure. Here, the soil surface is often the original bedrock. However, the greatest soil erosion occurs in the mid-eastern parts of the Dorsal, which represents the "degradation front?. The latter corresponds to the area with highest erosion, which is continuously progressing westward in the Dorsal. The large variation between the erosive rainfall events and the annual soil loss rates was explained by two important factors. The first relates to the soil degradation cycle. The second factor corresponds to the degradation front with the highest soil loss rates. At present this front is located at 300 m altitude and appears to be moving along an 80-km westward path starting from the east coast. A better understanding of the above can be used to better manage soils and soil covers in the Tunisian Dorsal area and, eventually, to decrease the soil erosion and reservoir siltation risk

Erosion in Mediterranean landscapes: Changes and future challenges

“…Nevertheless, some crops are more prone to soil erosion than others. There is a paucity of quantitative information on soil erosion under various crops in the Mediterranean, but the available data confirm that cereal cultivation can be affected by rilling and gullying, even on smooth gradients (Casalí et al, 1999;Kosmas et al, 2002), with increasing soil loss as mean precipitation increases (Kosmas et al, 1997;Jebari et al, 2010). In cereal cultivation the soil is bare during autumn and part of winter, which coincides with one of the rainiest periods in the Mediterranean region and the season having the most intense rainstorms, particularly in the western Mediterranean.…”

Historical aspects of soil erosion in the Mejerda catchment, Tunisia

“…Recent research has shown that semi-arid areas are very vulnerable to this phenomenon (e.g. GarciaRuiz et al 2010, Jebari et al 2010, Zucca et al 2010, Barbayiannis et al 2011. In fact, water erosion processes result in decrease in crop yields, loss in water potential, and are a cause of recurrent natural catastrophes (e.g.…”

The Use of Modified Annandale’s Method in the Estimation of the Sediment Distribution in Small Reservoirs—A Case Study

“…Much more diverse silting intensity is characteristic of small reservoirs in Tunisia. The data presented in the paper by Jebari et al [47] indicate that some of the reservoirs after ten years of operation are silted up 30%-40%, but among them there are some reservoirs whose silting degree is only a few percent. Among 23 reservoirs analyzed in the studies of Jebari et al [47], two of them were silted in ca.…”

“…The reservoir Krempna is located in a mountainous area with intensive surface erosion, and the catchment of the reservoir Zesławice, whereby 78% of the area is used for agricultural purposes, is covered with loess. It can be observed that a small reservoir becomes silted up faster if the siltation rate reaches 20% over a dozen years of operation (Table 3), in contrast to large reservoirs that achieve that level over centuries [46][47][48][49]. Krempna-1, Krempna-2, Zesławice, Głuchów reservoirs are characterized with such fast silting up, as well as Cierpisz reservoir in the period after its desilting and rebuilding i.e., from 1990 to present (Table 3).…”