Water Resources

An Integrated Approach

Joseph Holden, Joseph Holden

• Verlag: Routledge
• Jahr: 2019
• ISBN:
  9780429827204
• Auflage: 2
• Thema: Biowissenschaften
• Thema: Ökologie

Now in its second edition, Water Resources: An Integrated Approach provides students with a comprehensive overview of natural processes associated with water and the modifications of these processes by humans through climate change and land management, water-related health issues, engineering approaches to water and socio-economic processes of huge importance to water resources. The book contains chapters written by 24 specialist contributors, providing expert depth of coverage to topics.

The text introduces the basic properties of water and its importance to society and the nature of the different regional imbalances between water resource availability and demand. It guides the reader through the changing water cycle impacted by climate and land management, water flows in river basins, surface water quality, groundwater and aquatic ecosystems, and covers the role of water in human health and associated hazards before turning to engineering solutions to water and wastewater treatment and reuse. The book deals with physical and social management strategies required for water resource planning, the economics of water and treatment of issues associated with conflict over water. The concept of virtual water is covered before the text concludes with a chapter considering the challenges of predicting future water issues in a rapidly changing world and where environmental systems can behave in a non-linear way. The need to work across disciplines to address challenges that are connected at both local and global scales is highlighted. Water Resources also includes global examples from both the developing and developed world. There are 58 case study boxes. Each chapter is supplemented with these case studies and with reflective questions, project ideas and further reading, as well as links to a glossary of terms. The book is richly illustrated throughout with over 160 full-colour diagrams and photographs.

The text provides a novel interdisciplinary approach to water in a changing world, from an environmental change perspective and interrelated social, political and economic dimensions. It will be an indispensable guide to undergraduates studying water resources and management, geography of water, and water in the environment.

CHAPTER ONE

Water basics

Joseph Holden

Learning outcomes

After reading this chapter you should be able to:

• describe the nature of water;
• describe some of the roles of water in historic and contemporary society;
• outline some of the major global water resource issues.

1.1 Introduction

Water is a very common substance on Earth, covering 73% of the Earth’s surface in oceans, rivers and lakes. However, water is also a special substance. It is one of only a handful of substances that expands when it freezes. Water is one of the few substances found in solid (ice), liquid and gas (water vapour) form within our natural environment. It is the best naturally occurring solvent, meaning that it acts to dissolve and carry more different types of material within it than anything else, being good for cleaning and helping to sustain life in plants by providing them with nutrients. Indeed, water in solid, liquid and gaseous forms has been very important in shaping the landscapes on Earth through its role in weathering, erosion and transport of materials. Water is also fundamental to life on Earth. Humans, for example, are around 70% water by mass and we quickly dehydrate if we do not drink; people will die within a few days without water. Water can also be harnessed for other purposes, such as being a cooling agent for industry and power generation, and it thus serves a wide range of economic functions. Water also takes on a cultural and religious significance for many societies in terms of rituals, blessings, emotions formed around scenic views of waterscapes, leisure activities and physical and mental well-being. Water was fundamental to the development of agriculture and, hence, civilisations.

While global freshwater resources are many times more than required by humans today, because these resources, and human populations, are unevenly distributed there are many regions which do not have adequate freshwater supplies to meet today’s demand. Projecting into the future, there will also be increasing demands for freshwater and it is important to assess these needs to understand the scale of the water challenge that lies ahead.

Accordingly, this chapter sets the scene for the rest of the book. The chapter outlines some of the fundamental properties of water, introduces the role of water management in societal development and culture, and then outlines some of the major issues around global freshwater resource availability. With these things in mind, the reader should be able to see why an interdisciplinary approach is required when considering water resources and how important it is to gain a broad understanding across the rest of the chapters in this book.

1.2 What is water?

Water is not an element in itself. It is a compound made from two other elements: hydrogen and oxygen. Each water molecule contains two hydrogen atoms and one oxygen atom (H₂O), which are strongly bonded together. Each hydrogen atom shares a pair of electrons with the oxygen atom. Such pairing is called a covalent bond. All of the electrons position themselves as far apart from each other as they can, as they are all negatively charged and hence repel one another. However, the electrons that are not bonded to the hydrogen atom are closer to the oxygen atom than the ones being shared with the hydrogen atom and therefore the repulsion forces are slightly stronger for the unpaired electrons. This stronger force pushes the hydrogen atoms slightly closer together so that the two hydrogen atoms rest apart at 104.5° (Figure 1.1). While the overall charge of the water molecule is neutral, the electron arrangement shown in Figure 1.1 indicates that more of the negative (electron) charge is concentrated towards the oxygen side of the molecule. This means that the slightly positive hydrogen side in one water molecule is attracted to the slightly negative concentration on the oxygen side of another water molecule, forming a ‘hydrogen bond’. Hydrogen bonds are between 10 and 50 times weaker than covalent bonds.

Figure 1.1 A schematic diagram showing water molecules and the strong covalent bonds and weak hydrogen bonds.

The strong covalent bonds and the weak hydrogen bonds are both important properties of water, and the latter, in particular, makes water unusual in its nature. The covalent bonds result in it being very difficult to force apart the hydrogen and oxygen atoms within water molecules, which is why water is so prevalent on the planet and has not disappeared. The hydrogen bonds result in water molecules themselves being able to move between one another relatively easily, meaning that water is highly mobile, enabling it to flow rapidly and allowing materials (e.g. fish, people, ships, sediment) to pass through it easily. The hydrogen bonds also enable water to act as an excellent solvent, as the molecules can attach to other compounds, enabling them to be taken up into solution (see below). Finally, the hydrogen bonds mean that water is present as a liquid over most parts of the planet’s surface. If it were not for these bonds, water would boil at −80°C rather than 100°C.

1.2.1 Physical properties of water

1.2.1.1 Thermal properties

Water is at its most dense at 4°C. This temperature–density property is highly unusual. At this temperature water is seen to be at a standard volume of 1 g per cm³ or 1 kg per litre. Water at 4°C will tend to sink towards the bottom of water bodies and there will be thermal stratification. Upon freezing, water expands by about 9% (~0.91 g cm⁻³) and the hydrogen bonds form a strong structure. Thus, ice, which is less dense than water, floats upon the surface of water. This is important because, if ice were denser than water, it would sink, thereby crushing the creatures living in water bodies or leaving them stranded at the top of the water body. Indeed, lakes would freeze solid quite quickly during winter if it were not for the special properties of water. The ice at the surface insulates the lower liquid water and also reflects more of the Sun’s energy, thereby reducing the chance of the lake freezing solid.

Normally, as liquids are heated, the molecules gain energy and bounce around more, thereby taking up more space. However, this does not happen until >4°C for water. As water warms between 0 and 4°C, the hydrogen bonds bend or break frequently, causing the water molecules to pack more tightly. The expansion of water at temperatures greater than 4°C is very small in comparison to that caused by freezing. For example, in warming from 4°C to 10°C, the expansion of water is less than 0.03%. Nevertheless, this thermal expansion is still important for sea-level rise under climate change. Between 1970 and 2010 the IPCC estimates that thermal expansion of water in the oceans contributed 3.2 cm of global mean sea-level rise out of a total of 8 cm rise (Church et al., 2013). It is estimated that the sea level between 2081 and 2100 will be 40 to 65 cm higher than 1986–2005 levels, with about 40% of this increase being due to thermal expansion. Most of the rest of the contribution to sea-level rise will come from melting of ice on the land surface (see Box 13.2 in Chapter 13). Note that melting of sea ice does not directly affect global sea levels, since the ice is already part of the water body and sea levels are already displaced by the mass of ice sitting in the water. It should also be noted that seawater does not behave like freshwater, due to its high salt content, so that the density is not at a maximum at 4°C and instead it behaves more like a normal liquid with maximum density at its freezing point (Berner and Berner, 1987).

To raise the temperature of 1 kg of water by 1°C requires 4200 joules of energy (the specific heat of water). This is the highest of any liquid except ammonia. Thus heating water for cooking is energy intensive. Cooling water by 1°C would release the same amount of energy. This is why water takes a long time to heat up and cool down and why coastal areas have a moderate climate compared to inland areas (McClatchey, 2017). Coastal areas often have seawater temperatures warmer than air temperature during winter, as the sea cools slowly from the previous summer’s heat. During summer, water temperatures can be a lot lower than air temperature, as the water warms only very slowly.

Energy is also used or released when water c...