An Introduction to MultiAgent Systems.

Transactions of the Institute of British Geographers

The Analysis of Time Series: An Introduction, 4th edn. By C. Chatfield. ISBN 0 412 31820 2. Chapman and Hall, London, 1989. 242 pp. £13.50.

The Analysis of Time Series: An Introduction

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

▪ Abstract The composition and structure of the semiarid or desert grasslands of southwestern North America have changed over the past 150 y. Brushy or woody species in these communities have increased in density and cover. This increase in density of woody species is called brush encroachment because most of these species have been present in these communities at lower densities for thousands of years. The brushy or woody species were not introduced from other continents or from great distances. They are indigenous species that have increased in density or cover because of changes in local abiotic or biotic conditions. The brushy and woody plants are not the cause of these changes, but their increase is the result of other factors. The causes of changes that have led to the present woody-brushy composition of these semiarid grasslands has been difficult to determine. Warming of the climate seems to be a background condition, but the driving force seems to be chronic, high levels of herbivory by domestic ...

Shrub Invasions of North American Semiarid Grasslands

The interest in the study of hydrological connectivity has developed in recent years and its potential to provide an integrated approach for the study of complex systems has been increasingly recognised. Research on hydrological connectivity can also help to understand better the effects of changes in the system and thus provide crucial knowledge to managers and decision makers. The practical study of the concept in context-specific field settings needs to consider both the structural and functional components of connectivity and to define appropriate system boundaries. Aspects that need to be taken into account for an effective and comprehensive practical application of hydrological connectivity are discussed, and a framework to obtain field-based knowledge, in which both structure and function are considered, is proposed. This framework is based on a nested approach, and requires an effective description of all relevant components of the system and an understanding of the processes and feedbacks taking place within and between different scales in relation to connectivity. This basic approach can lead to a more holistic understanding of real systems and contribute greatly to modelling and synthetic approaches. Such improved understanding can also be useful to respond to the increasing demands for Integrated Catchment Management based on scientific knowledge, including those of the EU Water Framework Directive.

Hydrological connectivity: Linking concepts with practical implications

Resistance to overland flow on semiarid grassland and shrubland hillslopes, Walnut Gulch, southern Arizona

Five different algorithms for calculating slope from digital elevation models (DEMs) have been compared from regional to global scales. Though different methods produce different results, the most significant outcome is that slope varies inversely with the DEM grid size. Thus, slopes estimated from coarse resolution data can be considered to produce significant underestimates of the true slope. A fractal theory is adapted to solve this problem. The variogram technique for the definition of fractal parameters is demonstrated to provide a relationship between slope and the spatial resolution of measurement. The variation of fractal parameters is discussed at various scales, and a model is developed to estimate the high resolution slope based on the coarse resolution DEM by using fractal parameters. The fractal parameters are estimated from the standard deviation of elevation in a 3 × 3 window of the DEM to account for local variability in the surface. Standard deviation of elevation is found to be the most invariant property of different scale DEMs of the same area. The model is validated using different resolution DEMs in southern Spain and it is used to estimate the high resolution slope values at global scales based on a coarse resolution DEM. The slopes estimated using the technique outlined are a significant improvement on those estimated directly from the coarse resolution data. Slopes estimated in this way allow the more effective use of available coarse resolution data in regional and global scale modelling studies. Copyright © 1999 John Wiley & Sons, Ltd.

John Wainwright

Papers

Hydrological connectivity: Linking concepts with practical implications

Resistance to overland flow on semiarid grassland and shrubland hillslopes, Walnut Gulch, southern Arizona

Comparison of slope estimates from low resolution DEMs: scaling issues and a fractal method for their solution

Modelling and model building

Is sediment delivery a fallacy