Importance of slope gradient and contributing area for optimal prediction of the initiation and trajectory of ephemeral gullies

Ephemeral gully erosion rates are controlled by various factors out of which topography plays an important role. This study investigates the possibility to predict the location of ephemeral gullies using two topographic attributes only: i.e., local slope gradient (S) and upslope contributing area per unit length of contour (A(s)). An inverse relationship between S and A(s), in which the relative importance of the area exponent was varied, was evaluated as to its performance in predicting the location of ephemeral gullies for three intensively cultivated catchments in the Belgian loess belt. This model is easily applicable as it only uses information on topography, which can be derived from a digital elevation model, and land use. Predicted locations of ephemeral gullies were confronted with the locations recorded over 5 years of field observations. For all catchments a high relative area exponent (relative to the slope exponent) was required to yield good results. The optimal relative area exponent ranged from 0.7 to 1.5; however, the quality of the prediction did not vary significantly when changing the exponent within this range. An average relative area exponent of ca. 1 is appropriate for all three study areas and may therefore be proposed for similar areas under similar conditions. A striking discrepancy was found between the high relative area exponent required to predict optimally the trajectory of the gullies and the low relative area exponent (0.2) required to identify the spots in the landscape where ephemeral gullies begin. This indicates that spots in the landscape where gullies start are more controlled by slope gradient, while the presence of concavities control the trajectory of the gullies until the slope gradient is too low and sediment deposition starts. No relationship was found between the frequency of the occurrence of ephemeral gullies and the percentage of predicted gully pixels for each frequency class. This indicates that frequently occurring gullies at the same location are not easier to predict than accidental gullies. (C) 1999 Elsevier Science B.V. All rights reserved.status: publishe

Tafoni in the El Chorro area, Andalucia, southern Spain

Using a combination of field, laboratory and micromorphological evidence, this study examines tafoni (singular, tafone) in the El Chorro area of Andalucia, southern Spain, and makes inferences concerning the processes responsible for their formation. Twenty-five tafoni were randomly selected for field examination. The morphology of these cavernous rock domes is characterized by a helmet-shaped outer roof and an arched-shaped cavern, often with a partially overhanging visor; measurements of height, width and depth of the caverns revealed marked variations in size. The presence or absence of lichen cover, surface varnish, overhanging visor, cavern backwall stripes, rock flaking, weathering pits and cavern floor sediments was also noted. Surface hardness values, obtained using a Schmidt hammer, are relatively low but significantly higher on the outer roof of the tafoni than on the inner cavern walls. Analysis of sediment samples collected from the cavern backwalls and floors indicates predominantly sandy textures, alkaline pH values and some base cation enrichment. Micromorphological analysis of thin sections, prepared from undisturbed blocks, reveals large quantities of pore-filling cement, consisting mainly of calcite, mineral grains affected by weathering and pseudomorphic replacement, and dark, rounded nodules with a metallic appearance. In terms of their formation, different processes appear to act on different parts of the landform. On the outer roof surfaces, case hardening, resulting from near-surface cementation and surface varnish development, is dominant. On the inner cavern surfaces, however, core softening, resulting from granular disintegration and flaking, dominates. Exfoliation weathering, running water and wind deflation also appear to play an important role in tafone formation. A phased model of tafone evolution is proposed whereby the features pass through four phases of development–initiation, enlargement, amalgamation and degradation; in the study area there are examples of tafoni in each of these phases. Much of the evidence suggests that the tafoni are actively developing under current environmental conditions

Modelling particle residence times in agricultural river basins using a sediment budget model and fallout radionuclide tracers

Contemporary patterns in river basin sediment dynamics have been widely investigated but the timescales associated with current sediment delivery processes have received much less attention. Furthermore, no studies have quantified the effect of recent land use change on the residence or travel times of sediment transported through river basins. Such information is crucial for understanding contemporary river basin function and responses to natural and anthropogenic disturbances or management interventions. To address this need, we adopt a process-based modelling approach to quantify changes in spatial patterns and residence times of suspended sediment in response to recent agricultural land cover change. The sediment budget model SedNet was coupled with a mass balance model of particle residence times based on atmospheric and fluvial fluxes of three fallout radionuclide tracers (7Be, excess 210Pb and 137Cs). Mean annual fluxes of suspended sediment were simulated in seven river basins (38-920 km2) in south-west England for three land cover surveys (1990, 2000 and 2007). Suspended sediment flux increased across the basins from 0.5-15 to 1.4-37 kt y-1 in response to increasing arable land area between consecutive surveys. The residence time model divided basins into slow (upper surface soil) and rapid (river channel and connected hillslope sediment source area) transport compartments. Estimated theoretical residence times in the slow compartment decreased from 13-48 to 5.6-14 ky with the increase in basin sediment exports. In contrast, the short residence times for the rapid compartment increased from 185-256 to 260-368 d as the modelled connected source area expanded with increasing sediment supply from more arable land. The increase in sediment residence time was considered to correspond to longer sediment travel distances linked to larger connected source areas. This novel coupled modelling approach provides unique insight into river basin responses to recent environmental change not otherwise available from conventional measurement techniques

Soil loss, productivity and cropland values GIS-based analysis and trends in the Basilicata region (Southern Italy) from 1980 to 2013

This paper concerns the trends assessment of the productivity values and croplands values of specific crops (cereals (arable cereals land), vineyards, olive-growing lands) in the Basilicata region at regional scale, from 1980 to 2013, in relation to the soil loss evaluated through the USPED method. The comparative analysis shows the interrelations between the soil loss by erosion and the economic value deriving from the erosive phenomenon affecting the croplands considered