3.1 Water, the medium of life The list of necessities for the provision of life includes various nutrients and water: water is one of the basic resources needed for the process of photosynthesis. Since it is an excellent solvent, water, even in its 'natural' state, is never pure H2O but contains a variety of soluble inorganic and organic compounds. Water can also carry large amounts of insoluble material in suspension. The amounts and types of impurities vary with location and time of year, and determine some o
2.12 Summary The hydrological cycle is a complex process involving evaporation, transpiration, condensation, air circulation, precipitation, infiltration, surface run-off and percolation. Aquifers are an important source of fresh water supplying about one-third of the UK's potable water demand. Most of the world's water is present as saline water in the oceans. 2.11 Storage In a given fixed space at any phase of the hydrological cycle, there is an inflow and an outflow of water, the rates of which vary with time. The total cumulative difference between inflow and outflow is the storage. So within that space there is a body of water whose mass is not directly controlled by instantaneous values of inflow and outflow. For example, in river flow the movement of the whole body of water in the channel is generally downstream, yet a given reach contains a volume whose 2.10 Aquifers Groundwater is water that, after infiltrating and percolating through surface soils, flows into an aquifer, an underground water-bearing layer of porous rock. About one-third of the UK's drinking water is drawn from aquifers. To permit economic development, an aquifer must be able to transmit large quantities of water from one point to another and therefore it must have a high permeability. The groundwater contained in aquifers is released from springs and 2.9 Percolation Movement of infiltrated water downwards through the zone of aeration (Figure 5) is known as percolation. The infiltrated water which does not remain held by capillary forces in the surface soils moves by the action of gravity through the unsaturated layers of soil or rock until it arrives at the water table. Here the percolated water joins the body of groundwater which seeps slowly to the sea. 2.8 Surface run-off In some inland drainage areas, all water is removed by evaporation and infiltration. However, precipitation not penetrating the land surface usually runs off the surface along defined channels which have been produced by geological processes, previous storms, or possibly by people. This accelerates the process. Its eventual destination is the ocean, except, of course, where it runs to inland seas such as the Dead Sea. It is in the runoff phase of the cycle that physical intervention by humans 2.7 Infiltration Entry of precipitation through the soil surface and on downwards, by gravity, is known as infiltration. The rate at which this process can take place is governed by the permeability (a measure of the ease with which water can flow through the subsurface layer) and by the existing degree of saturation of the soil. Infiltration can be impeded by outcropping impermeable rocks or by paved areas, and also by the presence of finegrained soils with a low permeability (such as clay). At certain times 2.6 Precipitation Precipitation is defined as the depth of rainfall, or the water equivalent of snow, sleet and hail falling during a given measurement period. It may be in the form of rain, snow, sleet or hail, or in minor forms such as dew and hoar frost, but existing theories do not yet satisfactorily account for all the observed characteristics. In tropical climates, precipitation occurs as a result of the gradual coalescence of the tiny condensed droplets as they collide within the cloud l 2.5 Air circulation At this stage, air circulation enters and plays a dual role. Firstly, winds transmit moisture horizontally from one location to another. In this way, moisture derived from oceanic evaporation can be transported many miles to a land mass. Secondly, convective or vertical currents arising from unequal heating or cooling can transmit moisture upwards. When it cools, some of the water vapour condenses. It is from these currents that most precipitation develops. 2.4 Condensation As air rises it expands, owing to the decrease in pressure with height, and as it expands, in theory it cools at an average rate of 1°C for every 100 m of altitude. As the air cools, it becomes saturated with water vapour which condenses around small particles in the air. These particles may occur naturally, such as soil particles or salt particles residual to evaporation of sea spray, or they may be produced artificially during combustion. A measure of the necessary cooling to produce conde 2.3 Transpiration If there were no vegetation, the rate of evaporation from land surfaces after rain would diminish rapidly to a very low value. Plants increase this rate by transpiration. In this process, water is transferred from the soil through the roots to the leaves by osmosis and capillary action. Water evaporates from the surface of the leaves and the resulting vapour diffuses into the atmosphere. For hydrological measurements, this phenomenon is frequently lumped with evaporation because the two proce 2.2 Evaporation At an interface with the atmosphere, water changes its state from a liquid to a vapour in response to an increase in temperature caused by an external heat source. This temperature change is normally the result of solar radiation. The transfer of moisture into the air is called evaporation. The process is also controlled by the relative humidity, or level of vapour saturation, of the air. The greater the relative humidity of the air, the less likely it is that evaporation will take place for 2.1 Introduction The hydrological cycle, the continuous cycling of water between land, open water surfaces and the sea, either directly or indirectly, is an extremely complex process which has been known for a long time (Figure 1). The identifiable mechanisms of the cycle are complicated not only by the characteristics of air-water-land interfaces across which the cycle operates, but also by climatic factors which vary in both time and space. The various operations and mechanisms within the cycle are illustra 1 Some facts about water Did you know? At any one time, half the hospital beds in the world are said to be occupied by people suffering from water-borne illnesses. This Unit looks at water treatment and gives an insight into the use of science and maths for the betterment of people's lives. Learning outcomes After studying this course, you should be able to: describe the operation and mechanisms of the hydrological cycle list and describe the major physical, chemical and biological characteristics of clean fresh water, and explain their effects on aquatic organisms explain the mode by which potable water is produced through the processes of screening, microstraining, aeration, coagulation and flocculation, sedimentation, flotation, filtration and disinf Acknowledgements Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence Cover image: Author(s): References Keep on learning   There are more than 800 courses on OpenLearn for you to Conclusion This free course provided an introduction to studying Engineering. It took you through a series of exercises designed to develop your approach to study and learning at a distance, and helped to improve your confidence as an independent learner. 4.2 New ways of thinking and acting: systems practice There are a wide variety of concepts and theories relating to management and managing. This course is centred on the ideas and techniques that we believe define systems thinking, but it also draws upon concepts and theories from other areas where these are deemed to be useful. On top of this we see systems practice as requiring a readiness to use the experiential model of learning set out by Kolb, bringing theory and practice together in a meaningful way. It may be helpful to set out wh
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