Hydrology

Hydrology, scientific study of the waters of the Earth, including their occurrence, distribution in space and time, and their relation to people and the natural environment. Water is constantly circulating through the water cycle, where evaporation from the oceans and land produces clouds, leading to the precipitation of rain and snow. This replenishes soil moisture and underground rock stores, and feeds streams and rivers which flow back into the sea. Hydrology studies all aspects of the movement of water on the surface of the Earth and within the underlying soil and rocks.

Water is essential for all forms of life, but for human beings it is particularly important. We cannot exist without drinking water, particularly needing clean water to drink; it is vital for growing vegetables and other crops; as well as for cooking, bathing, washing, and for sanitation. Industry also relies on water as a raw material for manufacturing, and for generating hydroelectricity through hydro-power. Furthermore, rivers are used to dilute industrial and domestic wastes. As societies become more industrialized, demand for water continues to grow, yet water is a natural resource, and its supply is fixed. Hydrologists work towards planning the sustainable development and management of these finite water resources, including planning for extreme events such as floods and droughts.

As a branch of geophysics, hydrology has the objective of measuring and understanding the physical and chemical processes which control the movement of water through the water cycle. It includes the study of water's interaction with both the natural environment and with human activities. Hydrologists are concerned primarily with water movement within the land phase of the water cycle, yet this represents only 0.6 per cent of the world's water; 2.1 per cent of it is locked up in glaciers and ice sheets, and 97.3 per cent is held in the oceans. Hydrology overlaps with the scientific study of water in the oceans and atmosphere, but these are primarily the responsibility of oceanography and meteorology. However, considerable overlap also exists with other sciences, including geology, physics, chemistry, ecology, and biology, because hydrology is concerned not just with the quantity of water in rivers, but also its quality.

Rainfall and river flow are both very unevenly distributed in space and time, and this affects the availability of water resources. In Britain, the heaviest rainfalls occur in the hilly regions of Wales, the Lake District, and the Highlands of Scotland, yet the greatest population density is in the central and south-eastern counties, where rainfall, and hence river flows, are lowest. On a worldwide scale, imbalances in spatial run-off are even greater. Temporal differences also cause significant problems, with availability of water in rivers being least in the drier summer months, when demand for water supply is greatest for watering gardens and for irrigation. Hydrologists work with engineers to counteract these spatial and temporal imbalances through design and construction of reservoirs and water transfers, such as the aqueducts transferring water from the Lake District to Manchester, or from the Welsh mountains to Birmingham.

Hydrology developed originally as a branch of civil engineering, with most efforts going into provision of reliable, clean water supplies to rapidly growing urban centres. Further developments in hydrology helped engineers to design the major irrigation schemes of India, Africa, and South East Asia, and water-power schemes throughout the world. However, hydrology has now developed into a major science in its own right, concentrating on the study of all aspects of the distribution and behaviour of water on the land, with increasing emphasis on water quality and ecological aspects of water.

Hydrology is used to predict the likely effects of natural or artificial changes to river and groundwater flows. Thus converting farmland to roads or factories reduces infiltration, and hence produces more rapid run-off of rainfall into rivers and increased flooding. When forests are cleared to grow crops, there is again more rapid run-off, and hence less recharge of soil and groundwater stores, which sustain river flows during dry periods. Similarly, global warming, a consequence of the release of greenhouse gases into the atmosphere, is producing climatic changes which have an effect on rainfall, and hence run-off patterns. Hydrologists work with climatologists to assess the impact of these anticipated changes by developing mathematical models of river basins, by using their knowledge of the individual processes within the natural water cycle. These numerical models are used to predict the impact of changes of land use, or climate, on river and aquifer flows, and hence on water resources.

Although water has such a great impact on all aspects of life on Earth, it is too often taken for granted in developed countries, where people tend to be most aware of it only during extreme events such as droughts and floods, and where a lot of water is often wasted—even in drier areas. In developing countries, however, many thousands of people die each year from events such as floods and famines induced by droughts. Hydrology has a vital role to play in helping to alleviate problems associated with these extreme conditions, and in planning the development and management of the world's limited freshwater resources.