Soil erosion is often regarded as one of the main processes of desertification. This has led to the use of various desertification indicators that are related to soil erosion. Most of these indicators focus, however, on small spatial units, while little attention has been given to the amount of sediment exported at the catchment scale. Such a small spatial unit approach neglects the transfer of sediment through catchments as well as the scaledependency of erosion processes. Furthermore, this approach does not consider important off-site impacts of soil erosion, such as sediment deposition in reservoirs, flooding as well as ecological impacts. This study aims to illustrate the importance of also considering catchment sediment yield (SY, t km-2 y-1) in desertification assessment studies. Based on recently established databases of SY and soil loss rates in Europe and examples from previous studies, we illustrate that soil erosion rates at the plot scale are not representative for catchment SY, as they are often several orders of magnitude smaller. Also, the erosion response of catchments to changes in land use or climate often differs strongly from responses to those changes at the plot scale. We further discuss several of the impacts of SY and their link with desertification: i.e. the sedimentation of reservoirs, problems related to flooding, catchment hydrology, export of nutrients and ecological implications. Using earlier established criteria we evaluate the potential for using catchment SY as a desertification indicator and conclude that this could give an important added value to desertification studies. SY, used in combination with other indicators, allows the identification of other sediment sources than those considered at the plot scale and can reflect the results of desertification processes over longer time periods than periods over which assessments at the plot scale have been made. We argue therefore, that SY is a strong complementary indicator of desertification providing valuable information on the catchment response to changes in drivers of desertification. © 2011 Elsevier B.V. All rights reserved.

Current understanding of the regional variation in sediment yield (SY) and its scale dependence is limited for Europe. Based on an extensive literature review, a SY-database was assembled to bridge this gap. Measured SY-data from 1794 different locations throughout Europe were collected, representing a minimum of 29 203 catchment-years of records and comprising a wide range of catchment areas (0.01 km2 to 1 360 000 km2). Clear differences were observed between the temperate regions of Europe (low SY-values, i.e. <50 t km-2 year-1) and the Mediterranean and mountainous regions of Europe where SY-values are generally higher (i.e. >300 t km-2 year-1). Furthermore, for most temperate regions a negative relationship was found between catchment area and SY. For mountainous and Mediterranean regions, this was generally not the case. A comparison of catchment SY with rates of sheet and rill erosion also points to clear regional differences. Whereas soil erosion rates are generally higher than SY for temperate regions, this is not the case for the Mediterranean region. This indicates the importance of other erosion processes (i.e. landslides, riverbank erosion, and gullies). The results illustrate important regional differences in the scale dependence of SY and emphasize the need for an integrated modelling approach considering various types of sediment source and sink.

Our understanding about the regional variation of Sediment Yield (SY) in Europe and its scale dependency currently relies on a limited number of data for mainly larger river systems. SY is the integrated result of all erosion and sediment transporting processes operating in a catchment and is therefore of high value for environmental studies and monitoring purposes. Most global assessments of SY consider catchment area (A), climate and topography as the main explanatory variables. However, it is still unclear if these factors also control regional variations of SY within Europe. This paper aims at bridging this gap. Therefore, we i) present a large database of SY-values which was constructed through an extensive literature review; ii) describe the spatial patterns of SY across Europe; and iii) explore its relation with A, climate, and topography. In total, sediment yield data from 1794 different locations throughout Europe were collected (507 reservoirs and 1287 gauging stations), representing a minimum of 29,203 catchment-year data. Only SY-data measured at gauging stations or derived from reservoir siltation rates over a period of a minimum of one year were included in the database. This database comprises a large range of catchment areas (A): i.e from small upland catchments (=0.01 km²) to major European river basins (=1,360,000 km²). An overview of the collected SY-data is provided and sources of uncertainty on the available data are discussed. Despite potentially large uncertainties on several of the individual SY-values, analysis of this database indicates clear spatial patterns of SY in Europe. The temperate and relatively flat regions of Western, Northern and Central Europe generally have relatively low SY-values (with ca. 50% of the SYb40 tkm^-2 yr^-1 and ca. 80% of the data b200 tkm^-2 yr^-1), while Mediterranean and Mountainous regions generally have higher SY-values (with around 85% of the SY-data N40 tkm-2 yr-1 andmore than 50% of the data N200 tkm^-2 yr^-1). These differences are attributed to a combination of factors, such as differences in climate, topography, lithology and land use. Although larger differences in SY were found between the climatic regions than between topographic zones, it is currently difficult to identify the individual importance of the various controlling factors of SY. SY–A relationships were calculated for the entire dataset and for subgroups stratified according to the measurement method (gauging stations or reservoir surveys), range of the catchment area, climatic region, topographic zone of the river outlet, and major European river system. Although typically a negative relationship between SY and A is expected due to a decrease in topsoil erosion rates on more gentle slopes and an increase in sediment deposition with an increase in catchment size, this relationship was found to be generally very weak and subject to a lot of scatter. Furthermore, results illustrate important differences in scale dependency: whereas a weak but significant negative trend is generally observed for the temperate and relatively flat regions, no significant or even positive trends were observed in mountain regions and Mediterranean Europe. When only larger river catchments (i.e. N100 km² and especially N10,000 km²) are considered, catchment area exerted a larger control on SY. Thesefindings confirmprevious studies and indicate that the relationshipbetween SY, spatial scale and other controlling factors is often complex and non-linear. © 2011 Elsevier B.V. All rights reserved.

This study aimed to (1) increase understanding of the relation between sediment yield and environmental variables at the catchment scale; (2) test and validate existing and newly developed regression equations for prediction of sediment yield; and (3) identify how better predictions may be obtained. Materials and methods A correlation and regression analysis was performed between sediment yield and over 40 environmental variables for 61 Spanish catchments. Variables were selected based on availability and expected relation with diverse soil erosion and sediment transport processes. For comparison, the Area Relief Temperature (ART) sediment delivery model was applied to the same catchments. Sediment yield estimates obtained from reservoir surveys were used for model calibration and validation. Results and discussion Catchment area, catchment perimeter, stream length, relief ratio, Modified Fournier Index, the RUSLE’s R factor, and catchments percentage with poor vegetation cover showed highest correlations with sediment yield. Stepwise linear regression revealed that variables representing topography, climate, vegetation, lithology, and soil characteristics are required for the best prediction equation. Although calibration results were relatively good, validation showed that the models were unstable and not suitable for extrapolation to other catchments. Reasons for this unstable model performance include (1) lack of detail and quality of the data sources; (2) large variation in catchment characteristics; (3) insufficient representation of all relevant erosion and sediment transport processes; and (4) the presence of nonlinear relations between sediment yield and environmental variables. The nonlinear ART model performed relatively well but systematically overpredicted sediment yield. A model reflecting human impacts, including dams and conservation measures, is expected to provide better results. This, however, requires significantly more input data. Conclusions Although important insight is obtained into the relation between sediment yield and environmental factors, prediction of sediment yield at the catchment scale requires alternative approaches. More detailed information is required on land cover (change), and the effect of soil conservation measures. Validation of regression equations is a necessity, and better predictions are obtained by nonlinear models.© Springer-Verlag 2011

While much attention has been given to erosion processes in badlands, an integrated analysis of sediment production and export rates in badland areas at various spatial scales is currently lacking. This study reviews area-specific sediment yield (SY) from badlands in the Mediterranean measured at different spatial scales, using various measuring techniques, in order to investigate the relationship between size of study area (A) and SY. A database representing 16 571 plot-year and catchment-year data on SY at 87 Mediterranean study sites was compiled. Themost commonly reported lithologies associated with badlands aremarls, clay rocks and mudstones, and to a lesser extent shales. A high variability of SY from badlands in the Mediterranean region is observed. The relation between A and SY for Mediterranean environments with badlands is significantly different from that reported for Mediterranean environments without badlands. A complex ASY relationship is identified: for areas < 10 ha, SY is very high (mean SY¼475 t ha^–1 y^–1), whereas for areas > 10 ha, SY decreases non-linearly (power law) with increasingA(mean SY¼75 t ha^–1 y^–1 and drops from164.5 t ha^–1 y^–1 for 10 ha <A<200 ha to 9.3 t ha^–1 y^–1 for A>100 000 ha). This difference is explained by several factors. For A < 10 ha there is little or no sediment storage within badland areas, while for A > 10 ha progressively more sediment can be trapped in different sinks. Further, for A > 10 ha, area-specific erosion rates do not increase (or even decrease) due to decreasing average hillslope gradients and a decreasing fraction of erosion-prone (bare/badland) area. No significant relationships between SY, lithology, and mean air temperature nor mean annual precipitation were observed. © The author(s) 2011

Soils are commonly stony, especially in steep upland or heavily degraded sites. The hydrological effect of large stone contents has been previously investigated in wettable but not in water-repellent soils. For the latter, the focus has instead been on the impact of other soil characteristics (e.g. cracks and macropores) likely to promote water percolation. This paper investigates stone effects on water flow in water-repellent sand under laboratory conditions. Seventy-five experiments were performed on a water-repellent sand mixed with a range of quantities of different-sized wettable and water-repellent stones. The time taken for water to pass through each sand–stone mix, the percolated water volumes and numbers of dry and wet stones following each 60-minute experiment were recorded. At large stone contents (> 55% or > 65% by weight, depending on stone wettability), percolation occurred relatively quickly and in comparatively large quantities. At intermediate stone contents (45–65%) percolation response was variable and at stone contents < 45% for wettable and < 55% for water-repellent soils no water percolation occurred. We argue that with large stone contents flow pathways develop along sand–stone interfaces and a continuous preferential flow path can form provided there are sufficient stone-to-stone connections. The distribution and alignment of the stones, especially at intermediate stone contents, are important for promoting water movement. Water repellency determinations based only on the fine sediment component in stony soils could therefore be misleading as regards determining their hydrological response: the influence of the clastic component must also be considered.