This is a review of worldwide land degradation problems. Four themes are emphasized: delineating and estimating the magnitude of soil erosion, quantifying erosion and sedimentation impacts on land productivity, establishing quantitative values for erosion-causing parameters, and implementing global and regional soil and water conservation programs. Papers deal with both developing and developed countries and illustrate how erosion control techniques used in developed countries can or cannot be applied in developing countries.

https://gendocs.ru/docs/29/28948/conv_1/file1.pdf

Soil Erosion and Conservation

Assessing the impacts of climatic and, in particular, land use changes on rates of soil erosion by water is the objective of many national and international research projects. However, over the last decades, most research dealing with soil erosion by water has concentrated on sheet (interrill) and rill erosion processes operating at the (runoff) plot scale. Relatively few studies have been conducted on gully erosion operating at larger spatial scales. Recent studies indicate that (1) gully erosion represents an important sediment source in a range of environments and (2) gullies are effective links for transferring runoff and sediment from uplands to valley bottoms and permanent channels where they aggravate off site effects of water erosion. In other words, once gullies develop, they increase the connectivity in the landscape. Many cases of damage (sediment and chemical) to watercourses and properties by runoff from agricultural land relate to (ephemeral) gullying. Consequently, there is a need for monitoring, experimental and modelling studies of gully erosion as a basis for predicting the effects of environmental change (climatic and land use changes) on gully erosion rates. In this respect, various research questions can be identified. The most important ones are: (1) What is the contribution of gully erosion to overall soil loss and sediment production at various temporal and spatial scales and under different climatic and land use conditions? (2) What are appropriate measuring techniques for monitoring and experimental studies of the initiation and development of various gully types at various temporal and spatial scales? (3) Can we identify critical thresholds for the initiation, development and infilling of gullies in different environments in terms of flow hydraulics, rain, topography, soils and land use? (4) How does gully erosion interact with hydrological processes as well as with other soil degradation processes? (5) What are appropriate models of gully erosion, capable of predicting (a) erosion rates at various temporal and spatial scales and (b) the impact of gully development on hydrology, sediment yield and landscape evolution? (6) What are efficient gully prevention and gully control measures? What can be learned from failures and successes of gully erosion control programmes? These questions need to be answered first if we want to improve our insights into the impacts of environmental change on gully erosion. This paper highlights some of these issues by reviewing recent examples taken from various environments.

Gully erosion and environmental change: importance and research needs

Sampling and analysis or visual examination of soil to assess its status and use potential is widely practiced from plot to national scales. However, the choice of relevant soil attributes and interpretation of measurements are not straightforward, because of the complexity and site-specificity of soils, legacy effects of previous land use, and trade-offs between ecosystem services. Here we review soil quality and related concepts, in terms of definition, assessment approaches, and indicator selection and interpretation. We identify the most frequently used soil quality indicators under agricultural land use. We find that explicit evaluation of soil quality with respect to specific soil threats, soil functions and ecosystem services has rarely been implemented, and few approaches provide clear interpretation schemes of measured indicator values. This limits their adoption by land managers as well as policy. We also consider novel indicators that address currently neglected though important soil properties and processes, and we list the crucial steps in the development of a soil quality assessment procedure that is scientifically sound and supports management and policy decisions that account for the multi-functionality of soil. This requires the involvement of the pertinent actors, stakeholders and end-users to a much larger degree than practiced to date.

Soil quality – A critical review

The European Soil Erosion Model (EUROSEM) is a dynamic distributed model, able to simulate sediment transport, erosion and deposition over the land surface by rill and interill processes in single storms for both individual fields and small catchments. Model output includes total runoff, total soil loss, the storm hydrograph and storm sediment graph. Compared with other erosion models, EUROSEM has explicit simulation of interill and rill flow; plant cover effects on interception and rainfall energy; rock fragment (stoniness) effects on infiltration, flow velocity and splash erosion; and changes in the shape and size of rill channels as a result of erosion and deposition. The transport capacity of runoff is modelled using relationships based on over 500 experimental observations of shallow surface flows. EUROSEM can be applied to smooth slope planes without rills, rilled surfaces and surfaces with furrows. Examples are given of model output and of the unique capabilities of dynamic erosion modelling in general. © 1998 John Wiley & Sons, Ltd.

https://gruenland.wzw.tum.de/fileadmin/auerswald/1998_Morgan-ESPL.pdf

The European Soil Erosion Model (EUROSEM): A dynamic approach for predicting sediment transport from fields and small catchments.

Information on sediment and nutrient export from catchments and about related erosive processes is required by catchment managers and decision-makers. Many models exist for the consideration of these processes. However, these models differ greatly in terms of their complexity, their inputs and requirements, the processes they represent and the manner in which these processes are represented, the scale of their intended use and the types of output information they provide. This paper reviews several different erosion and sediment and sediment-associated nutrient transport models with regard to these factors. The review of models is limited to those models with explicit considerations of either the sediment generation or transport process.

A review of erosion and sediment transport models

This paper presents two new definitions of sediment and water flux connectivity (from source through slopes to channels/sinks) with examples of applications to sediment fluxes. The two indices of connectivity are operatively defined, one (IC) that can be calculated in a GIS environment and represents a connectivity assessment based on landscape's information, and another that can be evaluated in the field (FIC) through direct assessment. While IC represent a potential connectivity characteristic of the local landscape, since nothing is used to represent the characteristics of causative events, FIC depend on the intensities of the events that have occurred locally and that have left visible signs in the fields, slopes, etc. IC and FIC are based on recognized major components of hydrological connectivity, such as land use and topographic characteristics. The definitions are based on the fact that the material present at a certain location A reaches another location B with a probability that depends on two components: the amount of material present in A and the route from A to B. The distance to B is weighted by the local gradient and the type of land use that the flow encounters on its route to B, while the amount of material present in A depends on the catchment surface, slope gradient and type of land use of said catchment. Although IC and FIC are independent from each other, and are calculated using different equations and different inputs, they complement each other. In fact, their combined use improves IC's accuracy. Hence, connectivity classes can afterward be rated using IC alone. This procedure has been applied in a medium-size watershed in Tuscany (Italy) with the aim of evaluating connectivity, identifying connected sediment sources and verifying the effects of mitigation measures. The proposed indices can be used for monitoring changes in connectivity in areas with high geomorphological or human induced evolution rates.

Prolegomena to sediment and flow connectivity in the landscape: A GIS and field numerical assessment

This paper reviews the various effects of rock fragments on soil erosion by water. Since these effects are scale dependent, they are investigated at three different nested spatial scales: the microplot (4 × 10−6–100m2), the mesoplot (10−2–102 m2) and the macroplot (101–104 m2). For each scale the corresponding process mechanisms are discussed. Particular attention is paid to the effects of rock fragment cover on the intensity of soil erosion processes. At the mesoplot scale, i.e. on interrill areas, rock fragments at the soil surface can have negative as well as positive effects on sediment yield. These ambivalent effects are conditioned by the type of fine earth porosity, soil surface slope, vertical position and size of rock fragments and by the occurrence of horseshoe vortex erosion. At the microplot scale, i.e. the soil surface area which is covered by a single rock fragment, and at the macroplot scale, i.e. upland areas where both interrill and rill erosion takes place, rock fragments at the soil surface have a negative effect on sediment yield. In these two scales rock fragments can thus be considered as natural soil surface stabilizers. At the macroplot scale the mean decrease of relative interrill and rill sediment yield with rock fragment cover can be expressed by an exponential decay function. The scatter of the data indicates that a given rock fragment cover can have different efficiencies in reducing interrill and rill sediment yield depending on the varying intensities of the hydrological and erosion subprocesses. These findings have implications for erosion modelling and soil conservation.

Effects of rock fragments on soil erosion by water at different spatial scales: a review

A review of topographic threshold conditions for gully head development in different environments

Summary A set of laboratory experiments has been carried out in order to investigate the importance of soil cohesion and the interference due to the depth of the surface runoff on the detachment rate of soil particles and aggregates. Experimental data show that the soil detachment rate decreases as the runoff depth increases. This indicates that the detachment power of the raindrops is partially dispersed by the water layer. An inverse relationship between soil detachment and soil cohesion has been confirmed. Nevertheless the grain-size distribution must also be used in order to define soil detachability.

Dino Torri

Papers

The European Soil Erosion Model (EUROSEM): A dynamic approach for predicting sediment transport from fields and small catchments.

Prolegomena to sediment and flow connectivity in the landscape: A GIS and field numerical assessment

Effects of rock fragments on soil erosion by water at different spatial scales: a review

A review of topographic threshold conditions for gully head development in different environments

Splash detachment: Runoff depth and soil cohesion