Eco-Hydrology

Andrew J. Baird, Robert L. Wilby, Andrew J. Baird, Robert L. Wilby

• Publisher: Routledge
• Year: 2005
• ISBN:
  9781134715435
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geography
• Index: Geography

^iEco-Hydrology is the first book to offer an overview of the complex relationships between plants and water across a wide range of terrestrial and aquatic environments. Leading ecologists and hydrologists present reviews of the eco-hydrology of drylands, wetlands, temperate and tropical rain forests, streams, and rivers and lakes. Contents include:
* background information on the water relations of plants, from individual cells to strands of plants
* the role of mathematical models in eco-hydrology
* explanations of how plants affect patterns and rates of water movement and storage in a range of terrestrial and aquatic ecosystems.

1

INTRODUCTION

Andrew J. Baird

WHAT IS ECO-HYDROLOGY?

Eco-hydrology, as its names implies, involves the study of both hydrology and ecology. However, before attempting a definition of eco-hydrology, and therefore a detailed description of what this book is about, it is useful to consider briefly how hydrology and ecology have been studied separately and together in the past. As a discipline, hydrology has a long history, which has been described in detail by Biswas (1972) and Wilby (1997a). As a science, its origins are more recent. Bras (1990: p. 1) defines modern hydrology as ‘the study of water in all its forms and from all its origins to all its destinations on the earth.’ Implied in this definition is the need to understand how water cycles and cascades through the physical and biological environment. Also implied in this definition is the principle of continuity or the balance equation; that is, in order to understand the hydrology of a system we must be able to account for all inputs and outputs of water to and from the system as well as all stores of water within the system. As noted by Baird (1997) after Dooge (1988), the principle of continuity can be considered as the fundamental theorem or equation in hydrology. The dominance of the principle is relatively recent, and it is surprising to discover that the earliest hydrologists were unable to perceive that rainfall was the only ultimate source of stream and river flow. Remarkably, it was not until the studies of Palissy (1510–1590) that it was realised that rain falling on a catchment was sufficient to sustain stream discharge (Biswas, 1972: p. 152). Much of the history of hydrology is dominated by the necessity to secure and distribute potable water supplies (Wilby, 1997a). The discipline, therefore, has an engineering background.

Today, hydrology is still in large part an engineering discipline concerned with water supply, waste water disposal and flood prediction and, as studied by engineers, has close links with pipe and channel hydraulics. This link between hydrology and hydraulics together with the practical application of hydrological knowledge often forms the focus of standard hydrology and hydraulics textbooks written for engineering students, such as Chow (1959), Linsley et al. (1988) and Shaw (1994). Notwithstanding the seminal work of Palissy and others, hydrology has emerged as a science only in the last few decades. The publication of Ward’s (1967) Principles of Hydrology represents one of the first attempts to set down the fundamental principles of hydrology as a science for a non-specialist audience. There are now many texts and journals that deal with scientific hydrology; examples of the former include Parsons and Abrahams (1992), Hughes and Heathwaite (1995) and Wilby (1997b), and examples of the latter include Journal of Hydrology, Water Resources Research and Hydrological Processes. However, since about the early 1980s hydrologists have paid increasing attention to the relationship between water in the landscape and ecological processes. For example, in the past a hydrologist might have thought of plants in a river channel merely as representing a particular roughness coefficient for use in Manning’s 1889 Universal Discharge Formula. Now an increasing number of hydrologists are concerned with how flow velocities affect plant growth in channels and the relationship between river flow regimes and ecological processes in riparian habitats (see, for example, Petts and Bradley, 1997; Petts et al., 1995). This has spawned the so-called sub-discipline of hydro-ecology, which has as its focus the study of hydrological and ecological processes in rivers and floodplains and the development of models to simulate these interactions (for example, the Physical HABitat SIMulation model or PHABSIM – see Bovee, 1982). Plants and ecological processes are no longer regarded by hydrologists as static parts of the hydrological landscape. Another striking example of hydrologists studying the role of plants in the hydrological landscape has been in studies of evapotranspiration and rainfall interception (for a recent example, see Davie and Durocher, 1997a and b). In a similar fashion, ecologists have become much more sophisticated in their appreciation of water storage and transfer processes in ecosystems (see below). Finally, the 1990s, in particular, have seen a blurring of the distinction between engineering hydrology and scientific hydrology. Engineers are increasingly concerned with the impact of engineering works on ecological processes and ‘naturalisation’ of previously engineered rivers (Kondolf and Downs, 1996). Equally, engineers are centrally involved in scientific advances in the discipline. For example, engineers as well as scientists are studying complex flow structures and sediment movement in natural and semi-natural channels using sophisticated field and laboratory equipment and computer models (for a collection of relevant papers see Ashworth et al., 1996; see also Hodskinson and Ferguson, 1998; Niño and García, 1998; Tchamen and Kahawita, 1998).

The history of ecology is described in reasonable detail in a number of standard ecology textbooks, such as Brewer (1994), Stirling (1992) and Colinvaux (1993). According to Brewer (1994), the term ‘ecology’ (German Ökologie) was first coined in 1866 by the German zoologist Ernst Haeckel, who based it on the Greek oikos, meaning ‘house’. Although Haeckel used the term to describe the relations of an animal with its physical and biological environment, the term has almost always been used to describe the study of the relationship of any organism or group of organisms with their environment and one another. As noted by Brewer (1994, pp. 1–2), ‘Ecology is a study of interactions. A list of plants and animals of a forest is only a first step in ecology. Ecology is knowing who eats whom, or what plants fail to grow in the forest because they can’t stand the shade or because, when they do grow there, they get (sic) eaten.’ Ecology developed from natural history and until the middle of the century was concerned primarily with describing communities and the ‘evolution’ of communities through successional processes. Ecological succession sensu Clements (1916) refers to ordered change in a community to a final stable state called the climax. The essential ideas of succession are well illustrated by two systems: the coastal sand dune system and the lakeside system. In the former, the successional system is called the psammosere, in the latter the hydrosere. Interestingly, succession in both was seen to be the product of a close linkage between hydrological processes and ecological processes. In the hydrosere, for example, it was assumed that plants further from the water’s edge were less tolerant of waterlogging. Thus, both successions are early examples of eco-hydrological models in which there was explicit recognition of the role of water in plant growth and survival. It is now known that the simple progressions assumed in the classic descriptions of the psammosere and hydrosere rarely, if ever, occur and that the links between plants and their physical environment are more complicated than was once thought. For example, in wetlands research, Wheeler and Shaw (1995: p. 63) note that:

Conservationists would generally welcome a clear understanding of the inter-relationships between mire vegetation and hydrology, to help them predict the likely effects of hydrological change upon vegetation…. However, despite quite a large number of studies … the hydrology of fens and the composition of their vegetation is not at all well understood, except in gross terms.

This theme is developed in more detail in Chapter 5 of this volume (see also below).

The two-way linkage between plant growth and survival and hydrological processes has been studied in a range of environments, not just wetlands. For example, in recent studies Veenendaal et al. (1996) looked at seedling survival in relation to moisture stress under forest canopies and in forest gaps in tropical rain forest in West Africa, while Pigott and Pigott (1993) investigated the role of water availability as a determinant of the distribution of trees at the boundary of the mediterranean climatic zone in southern France. There are many other examples of non-wetland eco-hydrological study in the ecological literature, and these include Berninger (1997), Jonasson et al. (1997), Stocker et al. (1997), Bruijnzeel and Veneklaas (1998) and Hall and Harcombe (1998). However, the term ‘eco-hydrology’, which is used to describe the study of these links, seems to have been coined originally to describe only research in wetlands (see, for example, Ingram, 1987) and appears to have been in use by wetland ecologists for at least two decades (G. van Wirdum and H.A.P. Ingram, personal communication). In an editorial of a collection of papers on eco-hydrological processes in wetlands in a special issue of the journal Vegetatio, Wassen and Grootjans (1996: p. 1) define eco-hydrology purely in terms of processes occurring in wetlands:

Ecohydrology is an application driven interdisciplin [sic] and aims at a better understanding of hydrological factors determining the natural development of wet ecosystems, especially in regard of their functional value for nature protection and restoration.

It is instructive to read the papers in the special issue of Vegetatio, because they give an insight into how users of the term ‘eco-hydrology’ conduct their research. A recurrent theme of the papers is that hydrological, hydrochemical and vegetation patterns in the studied ecosystems are measured separately and then related to each other. This is particularly evident in the papers of Wassen et al. (1996), who compare fens in natural and artificial landscapes in terms of their hydrological behaviour, water quality and vegetation composition, and Grootjans et al. (1996a), who investigate vegetation change in dune slacks in relation to ground water quality and quantity. Somewhat surprisingly, manipulative experiments, whether in the laboratory or in the field, on factors affecting growth of plants do not figure highly in the research programmes presented. A second theme in the papers is that of the practical application of scientific knowledge to ecosystem management, especially for nature conservation. The compass of eco-hydrology as defined by these papers would, therefore, appear to be somewhat narrow. First, the term is used to describe wetlands research. Second, it describes predominantly field-based research where links between hydrological and ecological variables are sought; it does not appear to include manipulative experiments. Third, the term is associated with practical application of scientific ideas, especially in nature conservation. Interestingly, and somewhat confusingly, Grootjans et al. (1996b; cited in Wassen and Grootjans, 1996) appear to recognise that the term can be applied more widely. To them, ‘ecohydrology is the science of the hydrological aspects of ecology; the overlap between ecology, studied in view of ecological problems.’ Although broader, this definition still includes an emphasis on solving ecological ‘problems’, where presumably these are similar to the conservation problems mentioned above.

Very recently, Hatton et al. (1997) have used the term to describe plant–water interactions in general and suggest that Eagleson’s (1978a–g) theory of an ecohydrological equilibrium should form the focus of eco-hydrological research. The ideas of Hatton et al. on eco-hydrological modelling are discussed briefly later in this volume (Baird, Chapter 9; see also below).

THE SCOPE OF THIS TEXT

In this book, a wider definition than either (Wassen and Grootjans, 1996; Grootjans et al., 1996b) given above is used. In recognising that eco is a modifier of hydrology it could be argued that eco-hydrology should be more about hydrology than ecology. However, it is undesirable and probably impossible to consider the links between plants and water solely in terms of how one affects the other. Thus, while this book tends to focus on hydrological processes, it also considers how these processes affect plant growth. Additionally, as noted above, there is no intrinsic reason why eco-hydrology should be solely concerned with processes in wetlands. Eco-hydrological relations are important in many, indeed probably all, ecosystems. Although such linkages are very important in wetlands, they are arguably of equal importance in forest and dryland ecosystems, for example. Therefore, an attempt is made in this volume to review eco-hydrological processes in a range of environments, thus following the broader definition of eco-hydrology implied by Hatton et al. (1997). Eco-hydrological processes are considered in drylands, wetlands, forests, streams and rivers, and lakes. However, it is probably impossible to compile a volume which looks at every aspect of eco-hydrology. In recognition of this, the book focuses on plant–water relations in terrestrial and aquatic ecosystems. Thus the role of marine ecosystems in the global hydrosystem, although extremely important and acknowledged in the conclusion, is not considered in any detail. Full consideration of the topic would require a volume in its own right. For similar reasons, the role of water as an environmental factor controlling animal populations is not dealt with. Even with the focus on terrestrial and aquatic ecosystems it has been impossible to provide a comprehensive overview of all ecosystems. Thus, eco-hydrological processes in tundra and mid-latitude grasslands, for example, are not discussed. Despite this selective focus, it is hoped that the material presented herein will still reveal the key research themes and perspectives within eco-hydrology. Finally, in this volume a range of eco-hydrological research methods, including manipulative experiments, are reviewed.

Each of the five chosen environments or ecosystem types mentioned above are dealt with in Chapters 4 to 8. In Chapter 4, John Wainwright, Mark Mulligan and John Thornes consider eco-hydrological processes in drylands. The authors look at how dryland plants cope with generally...

Table of contents

• Cover
• Halftitle
• Title
• Copyright
• Contents
• List of figures
• List of tables
• List of contributors
• Preface
• 1. Introduction
• 2. Water relations of plants
• 3. Scales of interaction in eco-hydrological relations
• 4. Plants and water in drylands
• 5. Water and plants in freshwater wetlands
• 6. Plants and water in forests and woodlands
• 7. Plants and water in streams and rivers
• 8. Plants and water in and adjacent to lakes
• 9. Modelling
• 10. The future of eco-hydrology
• Index