John Boardman

Socio-economic factors in soil erosion and conservation

Farmer decisions with regard to production and land use are strongly influenced by socio-economic factors. In the developed world, the role of agricultural subsidies, quotas and guaranteed prices is especially important. In the past there have been many examples of economic signals which led to unfortunate and unforeseen environmental consequences (‘perverse subsidies’), including soil erosion. The problems were neglected because of an emphasis on increases in productivity and the fact that many of the costs were hidden or were external to the farm and were borne by society. In recent years ‘agri-environmental measures’ have begun to reverse the trend towards environmental degradation.

Soils And Quaternary Landscape Evolution

This book examines a period of spectacular environmental change, the Quaternary, with respect to soil-landscape relationships in Western Europe, the British Isles and North America. The possibilities for using soils as climatic indicators in the same way as fossils are discussed. The major themes emerging from the book are: the importance of soil stratigraphy in understanding Quaternary sedimentary sequences; the problems of soil-dating and the significance of soil colour; and the growing realisation of the importance of cold-climate processes in soil formation in Western Europe.

Modelling Soil Erosion by Water

This volume is the Proceedings of the NATO Advanced Research Workshop ‘Global Change: Modelling Soil Erosion by Water’, which was held on 11–14th September 1995, at the University of Oxford, UK. The meeting was also one of a series organised by the IGBPGCTE1 Soil Erosion Network, which is a component of GCTE’s Land Degradation Task (3.3.2) (Ingram et al., 1996; Valentin, this volume).

Development of badlands and gullies in the Sneeuberg, Great Karoo, South Africa

Abstract The study aims to examine the origin and development of land degradation with particular emphasis on badland and gully systems in the Sneeuberg uplands of the Great Karoo. This is an area of semiarid extensive stock farming where land degradation in the form of rill and gully erosion has accompanied the replacement of grassland by shrub vegetation. Species diversity has declined and ground cover has been reduced, leading to a positive feedback loop which exacerbates the degradation. Many foot slopes developed in shales, clays and colluvium have extensive, incipient badland development with closely spaced gullying up to 1.5 m deep. In valley-bottom and valley-side depression locations gullies up to 8 m deep have developed, usually cut to bedrock through valley fills of mainly Holocene colluvium. Both badlands and gullies appear to have developed since European settlement and to be part of the same hydrological system with extensive areas of bare ground (badlands) feeding water to incising gullies. Experiments using simulated rainfall throw some light on current processes. Badland areas are active under high-frequency, low-magnitude rainfall events. Major gullies are likely to be the result of occasional, high-magnitude events, but these have not been observed. Overgrazing in the past is the most likely cause of the degradation.

Emergence and erosion : a model for rill initiation and development

Soil erosion by overland flow, resulting from infiltration-excess rainfall, generates rill networks on hillslope areas. The way in which these networks emerge and develop suggests that hillslope erosion functions as a self-organizing dynamic system. Based upon this argument, a model for soil erosion (RillGrow 1) has been developed: this operates at the spatial scale of raindrops and microtopography. In this paper the second generation of the model (RillGrow 2) is described and applied to four different soil surfaces. Results suggest that, even at this early stage in its development, RillGrow 2 is capable of replicating the success of the earlier model and in some cases of extending them. The success of both models suggests that this self-organizing view of rill generation may capture some fundamental aspects of the operation of real erosional systems. Copyright

Flooding of property by runoff from agricultural land in northwestern Europe

Abstract In the last twenty years there has been an increase in the incidence of flooding of property by runoff from agricultural land in many areas of northwestern Europe. These events take the form of inundations by soil-laden water associated with erision and the formation of ephemeral or talweg gullies developed in normally dry valley bottoms. Costs of such events may be considerable e.g. almost US

Nonlinear responses of soil erosion to climate change: a modelling study on the UK South Downs

2M at Rottingdean, southern England, in 1987. These costs are largely borne by individual house occupants, insurance companies and local councils. The distribution of flooding is widespread but areas of high risk can be identified: the hilly area of central Belgium, parts of northern France, the South Downs in southern England and South-Limburg (the Netherlands). All these areas have silty, more or less loessial soils. Two types of flooding may be distinguished: winter flooding associated with wet soils and the cultivation of winter cereals, and summer flooding due to thunderstorm activity and runoff particularly from sugar beet, maize and potato crops. The distribution of these types of erosion varies in relation to the interaction between physical characteristics (soils and topography), climatic conditions and land use across the region. The reason for the recent increase in flooding events appears to be changes in land use, in the area of arable cropping, and the continued intensification of farming such as the use of chemical fertilizers, the decline in aggregate stability, the increase in the size of fields and compaction by farm vehicles. In some regions the risk of flooding has also increased because of expansion of urban areas in valley bottom locations. Communities have responded to the flooding hazard with emergency or protective measures usually involving engineered structures rather than land use change. The policy response to the increased risk of flooding has been very limited especially at the national and provincial level, the exception being plans developed with farming organisations in South-Limburg and the Pays de Caux. In southern England initiatives have been few and largely consist of protective measures undertaken by urban municipalities.

Muddy floods on the South Downs, southern England: problem and responses

Abstract A modelling approach is used to estimate some effects of changed climate upon rates of soil erosion on agricultural land on the UK South Downs. Previous studies have concentrated only on estimating shifts in long-term mean erosion rate: these were found to be approximately linear. However such simple shifts mask changes in the underlying distributions of annual erosion. A first series of simulations indicated that, under a wetter climate, erosion rates in wet years will generally increase more than rates in dry years. Under a “best guess” rainfall scenario with a 10% increase in winter rainfall, annual erosion increased by up to 150%. Erosion rates for individual years were shown to change in more complex nonlinear ways however, with decreases as well as increases occurring. These could be explained by the interaction of timing of rainfall with changes in the rate of crop growth. Most earlier work also assumed an equilibrium climate for the simulations, with climatic parameters such as mean monthly rainfall having stabilised at some new value, usually for a 2 × CO2 atmosphere. This however leads to an “initial conditions” problem: how will soil characteristics have changed by the time of CO2 doubling? A decrease in erodibility of about 20% by the time of CO2 doubling was indicated, resulting from changed soil profile properties. However, a second series of runs employed “transient” weather sequences (i.e. with a trend imposed). For these, present-day soil profiles could legitimately be used.

Climate change and soil erosion in Britain

Abstract In the years 1976–2001, 138 incidents of damage to property by muddy runoff from farmers’ fields occurred on the eastern South Downs. At several sites flooding occurred in more than 1 year and at some there have been multiple cases of flooding in the same years. The great majority of cases occur in the wetter autumns and are the result of runoff from winter cereal fields which are bare at that time. The short-term, emergency, response to flooding has been to build earth dams and diversion trenches. Medium and long-term responses have also relied on engineering approaches but, notably at two sites, land use change in combination with small dams has been effective in eliminating the risk of flooding. A mapping and modelling approach is shown to be a useful tool for predicting sites at risk. Future climate change will greatly increase the risk of flooding and property damage unless targeted land use change based on risk prediction maps is used to reduce runoff at vulnerable sites.

Soil erosion and flooding on the eastern South Downs, southern England, 1976–2001

The recent increase in soil erosion in Britain is the result of continued intensification of farming and a major land-use change from spring-planted to autumn-planted cereals. Field studies of erosion and computer models are the basis of this attempt to predict changes of rates under future climate conditions. Increases in winter rainfall, summer storm frequency, the area of irrigated land, and the introduction of new erosion-susceptible crops such as maize, will increase erosion rates. Off-farm impacts of flooding and pollution will continue to pose a greater threat, at least in the short term, than soil loss or yield reduction as a result of soil thinning. We require more detailed information on future climate, land-use change and economic conditions to make prediction of erosion rates less speculative.