Foreword 1. Introduction 2. Runoff, erosion and delivery to the coastal ocean 3. Temporal variations 4. Human impacts Appendices. Global River Database: Appendix A: North and Central America Appendix B: South America Appendix C: Europe Appendix D: Africa Appendix E: Eurasia Appendix F: Asia Appendix G: Oceania References Index.

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River Discharge to the Coastal Ocean: A Global Synthesis

The ‘pulse–reserve’ conceptual model—arguably one of the most-cited paradigms in aridland ecology—depicts a simple, direct relationship between rainfall, which triggers pulses of plant growth, and reserves of carbon and energy. While the heuristics of ‘pulses’, ‘triggers’ and ‘reserves’ are intuitive and thus appealing, the value of the paradigm is limited, both as a conceptual model of how pulsed water inputs are translated into primary production and as a framework for developing quantitative models. To overcome these limitations, we propose a revision of the pulse–reserve model that emphasizes the following: (1) what explicitly constitutes a biologically significant ‘rainfall pulse’, (2) how do rainfall pulses translate into usable ‘soil moisture pulses’, and (3) how are soil moisture pulses differentially utilized by various plant functional types (FTs) in terms of growth? We explore these questions using the patch arid lands simulation (PALS) model for sites in the Mojave, Sonoran, and Chihuahuan deserts of North America. Our analyses indicate that rainfall variability is best understood in terms of sequences of rainfall events that produce biologically-significant ‘pulses’ of soil moisture recharge, as opposed to individual rain events. In the desert regions investigated, biologically significant pulses of soil moisture occur in either winter (October–March) or summer (July–September), as determined by the period of activity of the plant FTs. Nevertheless, it is difficult to make generalizations regarding specific growth responses to moisture pulses, because of the strong effects of and interactions between precipitation, antecedent soil moisture, and plant FT responses, all of which vary among deserts and seasons. Our results further suggest that, in most soil types and in most seasons, there is little separation of soil water with depth. Thus, coexistence of plant FTs in a single patch as examined in this PALS study is likely to be fostered by factors that promote: (1) separation of water use over time (seasonal differences in growth), (2) relative differences in the utilization of water in the upper soil layers, or (3) separation in the responses of plant FTs as a function of preceding conditions, i.e., the physiological and morphological readiness of the plant for water-uptake and growth. Finally, the high seasonal and annual variability in soil water recharge and plant growth, which result from the complex interactions that occur as a result of rainfall variability, antecedent soil moisture conditions, nutrient availability, and plant FT composition and cover, call into question the use of simplified vegetation models in forecasting potential impacts of climate change in the arid zones in North America.

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Modifying the ‘pulse–reserve’ paradigm for deserts of North America: precipitation pulses, soil water, and plant responses

Agricultural activities and their complex effects on nature conservation, and the services that ecosystems deliver to humans are controversial. We present an overview of land abandonment, its driving forces and its consequences for landscape, biodiversity and humans. A descriptive metaanalysis of independently published studies highlighted the fact that the abandonment of agricultural land is a phenomenon mostly driven by socio-economic factors such as immigration into areas where new economic opportunities are offered to rural people. Ecological drivers such as elevation and land mismanagement leading to soil erosion are of secondary importance. We identified the major problems related to abandonment of agricultural land and quantified their relative importance. In order of decreasing importance, they were biodiversity loss, increase of fire frequency and intensity, soil erosion and desertification, loss of cultural and/or aesthetic values, reduction of landscape diversity and reduction of water provision. The impacts of these problems were not equally relevant in all regions of the world. The abandonment of agricultural land may also benefit humans. The benefits include passive revegetation and active reforestation, water regulation, soil recovery, nutrient cycling and increased biodiversity and wilderness. In a world that is becoming less natural and more intensively exploited by humans, we suggest that (1) farmland must be viewed in a context of multi-functionality to take advantage of ecosystem goods and services, (2) at the global scale, the abandonment of agricultural land is mostly positive for humans and (3) there is a need for the implementation of policies based on the payments for environmental services that encourage human societies to reconcile agricultural use, nature conservation and ecological restoration.

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Abandonment of agricultural land: an overview of drivers and consequences

Soil erosion is a key factor in Mediterranean environments, and is not only closely related to geoecological factors (lithology, topography, and climatology) but also to land-use and plant cover changes. The long history of human activity in Spain explains the development of erosion landscapes and sedimentary structures (recent alluvial plains, alluvial fans, deltas and flat valleys infilled of sediment). For example, the expansion of cereal agriculture and transhumant livestock between the 16th and 19th centuries resulted in episodes of extensive soil erosion. During the 20th century farmland abandonment prevailed in mountain areas, resulting in a reduction of soil erosion due to vegetation recolonization whereas sheet-wash erosion, piping and gullying affected abandoned fields in semi-arid environments. The EU Agrarian Policy and the strengthening of national and international markets encouraged the expansion of almond and olive orchards into marginal lands, including steep, stony hill slopes. Vineyards also expanded to steep slopes, sometimes on new unstable bench terraces, thus leading to increased soil erosion particularly during intense rainstorms. The expansion of irrigated areas, partially on salty and poorly structured soils, resulted in piping development and salinization of effluents and the fluvial network. The trend towards larger fields and farms in both dry farming and irrigated systems has resulted in a relaxation of soil conservation practices.

The effects of land uses on soil erosion in Spain: A review

Geomorphology, natural hazards, vulnerability and prevention of natural disasters in developing countries

What is overgrazing? Does it cause soil erosion? The recent debate from the ecological literature is reviewed as background to the debate on overgrazing and soil erosion. This debate stresses the need to view dryland grazing systems as dynamic ecosystems driven more by rainfall events than by livestock numbers. The case for soil erosion is then examined. Two case studies from communal rangelands in the Eastern Cape, South Africa, have cast doubts on the conventional wisdom that overgrazing leads to soil erosion. The first, a study of historical land-use change and erosion in a communal area, showed that the most intense erosion, taking the form of steeply dissected badlands, was associated with cultivated land that had been abandoned and reverted to grazing from the 1960s onwards. Such severe erosion was generally absent from land that had been under grazing since the 1930s. The second study demonstrated that erosion rates from communal grazing lands (‘overgrazed’) were only slightly higher than those from land under ‘optimal’ grazing, that is grazing at a level considered not to exceed the carrying capacity of the land. These results support the ecologist's contention that communal grazing systems do not necessarily degrade the range condition relative to management systems based on a notional carrying capacity. Copyright © 2004 John Wiley & Sons, Ltd.

Debunking the myth of overgrazing and soil erosion

MEDALUS soil erosion models for global change

The Mediterranean has been subject to changing human settlement and land use patterns for millennia, and has a history of human exploitation in an inherently unstable landscape. Environmental Issues in the Mediterranean reviews both physical and social aspects of this region, in relation to its environment.Ideal for students who are studying a range of environmental issues, but want to see them linked within one regional context. The book begins with an introduction to the Mediterranean region, its history, physical and human geography and its environmental problems. It then goes on to examine:* The Dynamic Environment - climate variables and fluctuations, vegetation, the hydrological cycle of the basin and its watershed, processes of erosion, fire and the Mediterranean Sea*The Human Impact on the Environment - prehistoric and historic land use, traditional agriculture, rural and urban settlement and use of mineral resources* The Mediterranean Environment Under Increasing Pressure - the present human landscape, changes in agriculture in the 20th century, the impact of depopulation, pollution, water resources, desertification and potential climatic change.It then concludes with a discussion of the region's on-going environmental issues of water resources, land degradation, agricultural intensification and tourism, and considers how these can be approached using management techniques and national and regional policies.

Environmental Issues in the Mediterranean : Processes and Perspectives from the Past and Present

Grazing is still widely held responsible for land degradation. The interaction between grazing and erosion is still rather poorly understood. As a consequence, counter-measures and associated management techniques have been slow to develop in southern Europe compared with Australia. Developments in ecological and economic-ecological modelling have improved our understanding of the processes and enhanced management capacity. Some of these developments are reviewed and two applications are described. One is a model for semi-extensive grazing in dry Mediterranean mountain conditions in which the shepherds, on random paths, seek to optimise resources use for economic benefit. The other is a spatial optimisation of vegetation canopy to minimise erosion rates.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1745-5871.2007.00426.x

Modelling Soil Erosion by Grazing: Recent Developments and New Approaches

Solar radiation, wind, and water are the driving agents of desert landscapes. Water has four major roles to play. First, in sustaining any life forms that exist; secondly, as a chemical substance which interacts with other chemical substances, notably salts; third, as a medium of transport of mass; and, fourth, as a direct source of energy. The last role, though small by comparison with the roles of solar and wind energy, may none the less be critical in determining the threshold of operation of runoff, through its impact on infiltration. Since infiltration is one of the major thresholds in dryland morphological development, the factors which control it have a role out of all proportion to the energy involved.

John B. Thornes

Papers

Debunking the myth of overgrazing and soil erosion

MEDALUS soil erosion models for global change

Environmental Issues in the Mediterranean : Processes and Perspectives from the Past and Present

Modelling Soil Erosion by Grazing: Recent Developments and New Approaches

Catchment and Channel Hydrology