Introduction

Water is an essential component of all living things including humans. The poet W.H. Auden said 'many have lived without love, none without water'. Water is the most widely occurring substance on the surface of this planet, and living cells depend upon it. We not only contain large amounts of water, about 70% of our body weight, but it also provides a medium for the passage of materials around our bodies. Ripl (2003) described water as 'the bloodstream of the biosphere' because of its importance as a major transport route for essential chemicals. It is a naturally occurring solvent capable of dissolving, to a greater or lesser extent, a very wide range of chemicals, which is perhaps its most important biological role (Sharp 2001). This includes both small molecules, such as nitrates, phosphates and sugars, and very large molecules, such as proteins and nucleic acids. Virtually, all biologically important chemical reactions need to be in solution to work. Water is also essential for the provision of food, as a drink, and is also needed for hygiene as well as providing a large amount of energy that we use. Water moves over the surface of this planet as well as through the atmosphere in a continuous cycle that is partly driven by gravity and partly by energy from the sun. This movement is called the hydrological cycle (see Chapter 4). This cycle involves water as a liquid, a solid and a gas. It is needed to sustain the myriad of ecosystems and provide the ecosystem services used by human societies (Jimenez-Cisneros 2015). Most of the water on this planet is saline and occurs in the oceans. Only a small amount is present as freshwater.

Water does not stay in any one state or location indefinitely. It does, however, remain in each location, or reservoir, for different lengths of time before moving to another reservoir either in the same state or a different one. For example, changing from liquid in a lake to water vapour in the atmosphere by evaporation. The amount of time that the water stays in any one reservoir before moving to another is called its residence time, typical examples of which are given in Chapter 4. From these examples, it is clear that water can exist in different forms and that availability for human and ecosystem use will vary greatly.

Water has many key properties that affect its behaviour and are also exploited by organisms. These properties both cause and allow water bodies, especially standing waters, to behave in certain ways, and these properties have been capitalised by many aquatic organisms to their advantage and moulded their behaviour (see Chapter 6). As land plants including trees rely on soil as a main medium in which they grow, although they require soil moisture for mineral transport, fish and aquatic plants rely on these water properties. Many aquatic organisms are equally dense or less dense than water that is then also used for physical support. Pure water is very transparent so the upper layers of a water body have reasonable levels of light intensity. The fact planktonic cells are continuously bathed in a medium with dissolved chemicals as well as being supplied with solar energy is only good for growth if they are all in the correct quantities. If some are present in excessive amounts, e.g. the major nutrients (see Chapter 6), it can lead to major imbalances in the biological response such as excessive growths of harmful algal blooms causing severe water quality problems and be dangerous for human health. Atmospheric moisture and its circulation play an important role in the movement of heat energy across the planet. Water movement is also important in shaping our landscapes by erosion, weathering of rocks and transporting minerals. Its interaction with the whole biosphere results, if the patterns of change are regular, in a self-supporting system (Ripl 2003). Human societies have traditionally developed close to readily available supplies of water such as rivers, lakes and springs. In the past, and even today, water has provided food in the form of fish and has been harnessed for that purpose. It is now commonly used for recreational activities as well as being significant in a number of religious beliefs and activities. Unfortunately, human interventions in the system can lead to destabilisation of these processes so that whole sectors of the system can be disrupted. It must always be remembered that this planet can support life as we know it because of the biosphere that is composed of many different ecosystems maintaining an environment suitable for human life. Green plants capture solar energy and absorb carbon dioxide from the atmosphere and with the aid of water produce more complex organic molecules, and as a by-product, pass out oxygen and water vapour into the atmosphere through the process of photosynthesis.

There are potential problems concerning the uneven distribution of freshwater over the surface of earth especially as human population growth rates are in regions of the world where water is, or will soon become, scarce and also suffer from low incomes making them less able to cope with future shortages. Some areas are naturally water abundant whilst others are water scarce or arid. With human populations rising globally, it is not just a question of sufficient water for basic human needs but also enough for food production. Many countries have the aspiration of being self-sufficient for food which means continuous increases in water demand. Water is also needed for economic activities. Although the problems of global climate change have come to prominence in the past few years, the problem of providing safe, adequate freshwater and proper sanitation to many people has been with us, and still is, for some time, and with some organisations it is still at or near the top of their list of problems of human quality of life. Water and climate change are inextricably linked together.

For many thousands of years, human settlements have used water and have developed technologies and strategies, but this has had only local impacts and did not have a wider effect on the hydrological cycle (Chapter 1). Since those times of small local communities when the view developed, at least in northern European countries and the northwest United States, water supplies were inexhaustible and, at least in the case of large rivers and the sea, they could be used for waste disposal because there would be infinite dilution that would render the waste harmless. In the past centuries, however, the global human population has grown exponentially and with it so has demand for more water per individual. With larger populations and greater industrial activities over the past hundred or more years, this has ceased to be true and anyway infinite dilution does not occur.

Warnings about water quantity and availability have been around for many years (Falkenmark 1997 and Chapter 4). Unfortunately, our attitude started to change with the onset of the industrial revolution. This new post-Holocene era has been termed the Anthropocene (Lewis and Maslin 2015). This was marked by both a rapid increase in population and an ever-increasing exploitation of natural water and other resources. Earth support systems are now being threatened. The IPPC (2007) has clearly pointed out the impacts of human activity on global climate systems. It is reasonable to assume that, in pre-Anthropocene times, as long as resource use was kept within sustainable limits, the stability experienced in the Holocene would continue for many thousands of years (Berger and Loutre 1991). If these limits are exceeded, then major changes could occur threatening human existence. It is therefore important to understand what these limits are.

Rockstrom et al. (2009) introduced the concept of 'planetary boundaries'. They describe these boundaries as 'human determined values of the control variables set at a safe distance from a dangerous level'. In other words, where possible we should determine the maximum limit for human exploitation of any particular resource and limit our use of it to be safely within that limit. The problem arises that our scientific knowledge of these boundaries and our ability to properly quantify them is often imperfect. Rockstrom et al. (2009) acknowledged the difficulty with some of these complex systems of assessing the effects of mechanisms, e.g. feedback mechanisms and self-regulation, in natural systems, and the timescales involved. Rockstrom et al. identified nine planetary boundaries.

Although because of the inter-relationships of all planetary systems consideration should be given to all of these when considering the hydrosphere, climate change, biogeochemical flows, including the nitrogen and phosphorus cycles, freshwater use, land system changes, chemical population in general and biodiversity loss will be examined in more detail in some of the following chapters. The main process of interest in this volume is global freshwater and factors that have a direct impact upon it. Shiklomanov and Rodda (2003) point out that human activities are the main driving force for change in global flows. They also have an important influence on the seasonal timing of cloud formation and precipitation. Molden et al. (2007) have estimated that 25% of the world's river basins run dry before they reach the sea because of over abstraction. Human alterations to the hydrological cycle affect many other areas of activity including food production, human health, climate, and ecosystem functioning. These adverse effects...