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| ISBN-13: | 9781780407289 |
|---|---|
| Publisher: | IWA Publishing |
| Publication date: | 02/15/2017 |
| Pages: | 404 |
| Product dimensions: | 6.12(w) x 9.25(h) x 0.75(d) |
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CHAPTER 1
INTRODUCTION
"There is no life without water. It is a treasure indispensable to all human activity."
EUROPEAN WATER CHARTER, COUNCIL OF EUROPE, 6TH MAY 1968
1.0 INTRODUCTION
Water, the subject matter of hydrology, is both commonplace and unique. It is found everywhere in the Earth's ecosystem and it is essential to all known forms of life. The Earth is called the 'Blue Planet' because most of its surface (71%) is covered by water, although the total amount of water is under 0.5% of the Earth's total volume. About 97% (depending on the method of calculation) occurs as saline water in the seas and oceans. Of the 3% that is fresh water considerably more than half is locked up in ice sheets and glaciers, and another substantial volume occurs as virtually immobile deep groundwater that is not easily accessible. The really mobile fresh water, which contributes frequently and actively to rainfall, evaporation and stream-flow, represents only about 0.02% of the global total (see Figure 1.1).
A reliable source of water is the essential basis for human civilisation, and an integral part of the natural world. Without water every form of life on Earth would stop. Indeed, water makes up the bulk of most living things and it is a major medium for transporting energy, dissolved chemicals and sediments at scales from the molecular to the global. Moving water and ice are the agents of erosion and deposition. However, water creates risk to humans, in the form of floods and droughts, and polluted water harbours disease. Water resources for human exploitation must be allocated and used in a sustainable and equitable way to serve the competing and often conflicting requirements of agriculture, industry, households, power generation, navigation, flood protection and recreation, while maintaining a healthy environment. Taken for granted where plentiful, a prized possession where scarce, water is the only naturally occurring inorganic liquid, and is the only chemical compound that occurs in normal conditions as a solid, a liquid and a gas. The abundance of water is the feature that sets this planet apart from its fellows in the solar system.
Hydrology is defined formally as "the science which deals with the waters of the Earth, their occurrence, circulation and distribution on the planet, their physical and chemical properties and their interactions with the physical and biological environment, including their responses to human activity" (UNESCO, 1964). This book focuses on the principles and processes of hydrology which are concerned largely with physical or environmental hydrology. In this context, water is viewed in the same way as soil, vegetation, climate or rock, as an element of the landscape to be investigated and ultimately understood by means of rigorous, scientific examination and analysis. The science of hydrology overlaps with many other sciences. The study of precipitation and evaporation lie within the realm of the meteorologist, the storage of water in the soil and its percolation to groundwater are of direct relevance to agriculturalists, soil physicists and geologists. The flow of water in the river is the province of the hydraulic engineer. Only the hydrologist deals with these fragmented aspects of the hydrological cycle in its entirety (McCulloch, 1975). The aim of hydrology is to seek knowledge and understanding of the hydrological cycle (see Figure 1.3) in ways that lead to its safer exploitation, more reliable prediction and more effective control of water and water resources.
Hydrology is of fundamental importance in managing water, and seeks to understand the movement of water through the environment, and predict how water bodies will behave under different circumstances. At its broadest, hydrology encompasses all aspects of water as it moves through the water cycle, but is more usually taken to focus upon water on the land surface and in the soil profile, rather than in the air or the sea
Specifically, hydrology – and hydrologists help to provide a safer and better quality of life for people, and an enhanced environment for wildlife through:
• Securing water supplies for public use, including drinking water and sanitation,
• Ensuring the provision of water for food production, crops and livestock,
• Protecting against, and giving warning of, approaching floods and droughts to reduce their impact through economic and physical damage and loss,
• Protecting people and the environment from pollution and overabstraction,
• Maintaining and improving aquatic habitats for wildlife, navigation, and recreation.
1.1 WATER – FACTS AND FIGURES
Water is present on Earth in three phases. In its liquid form, precipitation meets the basic water needs of humans, animals, and plants. Its runoff into streams sustains ecosystems and, along with percolation into aquifers, ensures long term storage and supply for human uses. The oceans are the world's primary source of water vapour that feeds precipitation. Atmospheric water vapour is a greenhouse gas, allowing much of the sun's shortwave radiation to pass through but absorbing the long-wave radiation emitted by the Earth's surface, which results in the Earth's surface temperature being about 30°C warmer than it would be otherwise (Trenberth, 1992). In addition, water vapour may condense into clouds that reflect and absorb solar radiation, thus directly affecting the Earth's radiant energy balance. In water's frozen form, sea ice and snow cover tend to cool the planet by reflecting the incoming solar radiation. Glaciers, especially those at mid-latitudes, provide water storage and summer supply for both agriculture and urban areas around the world.
As the world population increases towards 10 billion by 2050, and climate change progresses, increasing stresses will be placed on water resources. The global distribution of fresh water over the globe is amazingly uneven in both space and time, and many regions of the globe currently suffer from water scarcity. Even is the case of Britain, a generally humid land, the spatial pattern of rainfall and runoff (greater in the North and West) is largely the reverse of the distribution of its population (higher in the South and East), creating problems of water supply. In fact by the World Bank criterion (<1,000 m3 per head per year of available water) much of South East England can be classed as suffering from serious water stress.
Water is the only naturally occurring liquid most people see, and it is usually thought of as being a common, ordinary substance. But this colourless, tasteless, odourless fluid is far from being ordinary; water is one of the most extraordinary substances and defies many of the normal laws of physics and chemistry. It is its unusual attributes that make water of unique importance to life across the globe and to humanity.
1.1.1 THE SPECIAL CHARACTERISTICS OF WATER
Water combines two of the most common elements, but it does not behave like any other substance. Hydrogen and oxygen atoms form a strong covalent bond with electrons shared between them. Due to the distribution of electrons the oxygen side of the water molecule has a negative charge and the side with the hydrogen atoms has a positive charge. This means that the positive end of one molecule will attract the negative end of another water molecule to form a hydrogen or polar bond (Figure 1.2). This weak electrostatic attraction between molecules has only about 1/10 of the strength of a covalent bond within a molecule, yet determines most of water's unique properties:
Hydrogen bonds between water molecules result in water's cohesiveness - literally "sticking" to itself, giving it a high surface tension. Hydrogen bonds also form between water and a solid such as a soil particle. This is especially important to the movement of water in soil, and plants make use of capillary rise to draw water up through their stems to their leaves.
• Most substances shrink as they cool, but when water falls below 4°C it starts to expand and become lighter. That is why a water body such as the lake freezes from the top down rather than the bottom up, and why icebergs float. This insulates the lower liquid water, which helps aquatic life to survive through the winter.
• When water freezes its volume increases by about 9%; this is an important mechanism in rock weathering where liquid water percolates into rock cracks.
• Water has an exceptionally high specific heat capacity – the energy required to raise its temperature. This is about 5 times higher than dry soil, and so a water body will warm (or cool) much more slowly than the surrounding land. Its high thermal inertia minimizes temperature fluctuations and helps to keep the temperature of the planet stable. The ability of water to absorb, and later release, heat strongly affects the spatial pattern of climate as warm ocean currents transfer enormous amounts of heat from the tropics to higher latitudes.
• Changes of phase between liquid, vapour and solid (melting and freezing, evaporation and condensation) absorb or release more latent heat than most other common substances. This gives water an immense ability to store and transport heat around the globe, and atmospheric transfers of water vapour play a key role in the Earth's energy budget. Evaporation stores heat which is then released elsewhere with the condensation of water vapour.
• Due to its special molecular structure water is an almost universal solvent that can dissolve almost anything. The hydrogen bonds between water molecules are continuously being broken down and remade several billion times a second (Hillel, 1991). Water dissolves a substance by forming hydrogen bonds with its molecules and once in solution water molecules surround individual ions of the substance (parts of molecules that become separate, electrically charged, entities) preventing them recombining. Water transports dissolved nutrients through the environment and is the medium in which life's key metabolic exchanges take place.
But it may also carry pollutants, so water quality must be taken into consideration as well as quantity of water for human use.
The special properties of water described above are reflected in individual chapters of this book and include, for example, surface tension in soils (so they do not drain instantly after rain ceases), its large heat capacity resulting in its crucial role as a global energy transporter, and its dissolving power essential for transporting nutrients.
1.2 THE CHANGING NATURE OF HYDROLOGY
Hydrology is both an old and a new subject; old in that humans have been attempting to control and manipulate water out of practical necessity for many thousands of years; new in the sense that hydrology has only been studied as a separate academic discipline in its own right for less than a century.
The origins of hydrology can be traced back to the control and management of water at the start of civilisation. Indeed, the fact that water is essential to life and that its distribution and availability are intimately associated with the development of human society meant that it was inevitable that some development of water resources preceded a real understanding of their origin and formation. Some of the oldest civil engineering structures still in existence were built for the storage and supply of water. Hydrology as a modern scientific discipline had its origins in hydraulics, the study of flows within well-defined boundaries such as river channels.
With the increasing world population, industrialisation, climate variations and climate change, and the growing awareness of the fragility of the natural environment, the importance and security of water supply and wastewater treatment assume an ever higher profile in national and international strategies. This renders water prone to disagreements and disputes from headwater streams to the largest river basins (see Section 9.2.3). The risk of water wars rises with scarcity, and many have predicted that the wars of the 21st Century will be fought over water. Indeed, the word 'rival' derives from a Latin legal term rivalis referring to a person who shares with others the water of the same stream. Where basins and aquifers are shared by nations, equitable use of water resources is often the aim of the protocols and treaties that have been negotiated.
Archaeological discoveries and later documentary evidence emphasise the significant part played by the location and magnitude of water supplies in the lives of ancient people. The Epic of Gilgamesh written at least 1,000 years before the Bible's first books describes a Great Flood that is very similar to the Genesis story of Noah. There are similar accounts of a Great Flood found in central Turkey, in the Royal library of the Hittites, as well as in Hindu, Greek and Roman mythology (Barnett, 2015).
Some have attempted to attribute the origins of hydrology to a particular location or country, such as Greece, Egypt, Mesopotamia, China or South America. From the evidence available, however, it is much more likely that, from very early times, understanding, engineering skills and large-scale resource development progressed interdependently in multiple areas, especially those where water was a 'problem' - either because of its shortage or its over-abundance. Evidence of early structures to control water can be found in areas as wide apart as the Middle East and South America. There is evidence of large scale control of agricultural societies and a high degree of social organisation for food production and security in other warm and arid or semi-arid areas including the Yellow and Yangtze Rivers in China; and in South America irrigation canals were dug nearly 7,000 years ago in the Peruvian Andes (Ortloff, 2016).
About 10,000 years ago humans began mastering the skills and tools necessary for the beginnings of agriculture. Due to the critical importance of water many of the great early civilisations developed in the valleys of important river systems, including ancient Egyptians in the Nile Valley, the Sumerians in the Tigris-Euphrates plain of Mesopotamia (Iraq) and the Harappa in the Indus Valley of India/Pakistan. All three rivers rise in areas of high precipitation, and then flow through arid regions where rainfall is scanty and the inhabitants rely on streamflow for their water needs. Their experiences can provide valuable lessons for the present and the future.
In Egypt, rainfall is almost non-existent and the annual Nile floods derived from monsoon rainfall on the highlands of Ethiopia provide the only source of moisture to sustain crops. The Nile has a strong annual cycle of flows and this formed the basis of successful, large-scale, agricultural irrigation for more than 5,000 years. According to the Greek historian Herodotus, "Egypt is the gift of the River Nile". The level of the River Nile regularly began to rise in late July, reaching a peak in late September when the floodplain was inundated to its maximum extent. Then, as the waters receded they left a covering of nutrient-rich silt providing very fertile agricultural land. It was recognised that the higher the annual river peak level, the larger the area that could be irrigated on a particular reach of river and the bigger the expected harvest for an agriculture completely dependent on the river flood level watering the farmland. Accordingly the early Egyptian officials recorded the maximum water levels at many points along the river and compared them to the peaks in previous years to determine the amount of tax to levy from the farmers (see Photo 1.1 ). The earliest records can be traced back over 4,000 years to about 2,500 years BC when gauges were cut into the rocks in the Nile valley. But high floods could result in much destruction, a paradox the Egyptians understood well. If the river level was too high, it would damage the banks of the irrigation dykes, destroy villages on the plain and ruin the crops. It has been claimed that in flood conditions extremely good rowers were despatched, and rowing downstream with the current were able outpace the flood peak and provide a warning to the townspeople downstream (Biswas, 1967a). If correct, this was probably the world's first flood warning system. Although the Egyptians did not understand that the annual flooding was predominantly due to rainfall in the upper headwaters, they were able to use engineering skills to build simple canals dikes and reservoirs to manage the water and increase crop production. The remains of what is believed to be one of the oldest dams in the world, built between 2950 and 2750 BC, lies about 30 km south of Cairo (Biswas, 1970). The High Dam at Aswan was constructed in the 1970s to control the Nile floods, by impounding waters in Lake Nasser, which stretches 500 km upstream and has a storage capacity of about 170 km3. This ended the annual flood risk and provided a more secure supply of water in Egypt when neighbouring countries suffered in severe droughts, but finished the natural deposition of fertile silt each year, so farmers must now use fertilizers for growing crops such as barley and wheat (see also Section 9.2.3).
(Continues…)
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Table of Contents
Preface, 6,
Chapter 1 Introduction, 8,
Chapter 2 Precipitation, 36,
Chapter 3 Interception, 80,
Chapter 4 Evaporation, 110,
Chapter 5 Groundwater, 156,
Chapter 6 Soil Water, 196,
Chapter 7 Streamflow, 232,
Chapter 8 Water Quality, 278,
Chapter 9 Hydrology in a changing world, 324,
References, 363,
Index, 392,
ACKNOWLEDGEMENTS, 400,