This study investigates how medium-term gully-development data differ from short-term data, and which factors influence their spatial and temporal variability at nine selected actively retreating bank gullies situated in four Spanish basin landscapes. Small-format aerial photographs using unmanned, remote-controlled platforms were taken at the gully sites in short-term intervals of one to two years over medium-term periods of seven to 13 years and gully change during each period was determined using stereophotogrammetry and a geographic information system. Results show a high variability of annual gully retreat rates both between gullies and between observation periods. The mean linear headcut retreat rates range between 0·02 and 0·26m a^–1. Gully area loss was between 0·8 and 22 m² a^–1 and gully volume loss between 0·5 to 100 m³ a^–1, of which sidewall erosion may play a considerable part. A non-linear relationship between catchment area and medium-term gully headcut volume change was found for these gullies. The short-term changes observed at the individual gullies show very high variability: on average, the maximum headcut volume change observed in 7–13 years was 14·3 times larger than the minimum change. Dependency on precipitation varies but is clearly higher for headcuts than sidewalls, especially in smaller and less disturbed catchments. The varying influences of land use and human activities with their positive or negative effects on runoff production and connectivity play a dominant role in these study areas, both for short-term variability and medium-term difference in gully development. The study proves the value of capturing spatially continuous, high-resolution three-dimensional data using small-format aerial photography for detailed gully monitoring. Results confirm that short-term data are not representative of longer-term gully development and demonstrate the necessity for medium- to long-term monitoring. However, short-term data are still required to understand the processes – particularly human activity at varying time scales – causing fluctuations in gully erosion rates. Copyright © 2011 John Wiley & Sons, Ltd.

Eroding channels can usually be characterized by a power relationship between channel width (W) and channel discharge (Q). This paper examines the WQ relation using a recently developed channel junction approach to extend the validity of the WQ relation and to develop a procedure for estimating the WQ exponent and proportionality coeffi cient. Rill and gully channel data from the literature, and new data collected in different badland areas and in a few forest mountain streams, are analysed. Analysis shows that the WQ relation for channel width collected in badlands and forests agrees with trends observed for cropland. The exponent increases with increasing channel width in a continuous fashion rather than in a step-like way and tends to a maximum whose value ranges between 0·5 and 0·6. The proportionality coeffi cient can be split into two terms, one expressing the case in which an eroding channel can broaden, the other refl ecting the diffi culties in removing the less erodible clods or rock fragments from the channel bed. Its splitting allows the development of a more correct form of the WQ relation in agreement with modern approaches of channel geometry: one part has the dimension of a discharge and makes the power base dimensionless, while the other brings the dimension of a length, needed for the channel width, into the WQ relation. The interpretation of the two constants is supported by data collected in rainfall-runoff simulation experiments conducted in the fi eld. Values characterizing the two constants in some environments are also given. Nevertheless the approach is not suffi ciently parameterized yet to be of practical use (e.g. in models or for estimating peak discharge in areas where rill channels have formed). Copyright © 2009 John Wiley & Sons, Ltd.

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