The Hydrologic Cycle

Hyrdologic Cycle - Diagram
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Hyrdologic Cycle - Diagram

When the Earth formed approximately 4600 million years ago it contained all its necessary materials which remain in the same quantities today. The exception is the addition of material by meteorite. We have on the Earth today the same amount of water and water vapour as at the time of origin.

This water has been in perpetual motion, the same water being conveyed from the surface to the atmosphere and back with little ever lost or gained. Solar energy and the relentless tug of the earth's gravity fuel the cycle and keep the water moving.

Water vapour enters the atmosphere by evaporation from bodies of water (the higher the water temperature, the greater the ease) and by transpiration from plants and trees. It has been calculated that 10 million billion gallons of water per year is carried aloft. 86% comes from the oceans, which contain about 97% of the earth's total water resources; 12% form rivers, lakes and streams; 2% from transpiration.

Once aloft the moisture cools and collects into clouds as it rises higher into the atmosphere. As the air cools, its ability to carry water vapour decreases until it reaches a critical point when the vapour condenses and falls as rain or snow. The movement of a saturated air mass is forced upwards when it meets a land mass causing cooling and explains why the windward side of land masses have a high rainfall figure.

Once returned to the surface the water may evaporate again; it may form surface run-off; it may infiltrate the soil to form soil-water flow; it may percolate into the underlying rock to form groundwater flow and may remain in aquifers for thousand of years until it finds its way to an outlet. Eventually it is all recycled.

At any moment only about .005% of the earth's estimated 326 million cubic miles of water is actively involved in the hydrologic cycle. Variable amounts are available in different regions accounting for the variation from drought to flood.

It should be borne in mind that earth's water is becoming increasingly contaminated. Rivers are used to transport effluent discharge including heavy metals. The River Rhine (Europe's largest sewer) provides drinking water for 30 million people. It is probably "cleaner" than that available in less developed countries where 17000 children per day die of diarrhoeal diseases. Developed nations consume vast amounts of clean water for activities which do not require that level of cleanliness while over 1 billion people do not have clean water to drink.

Chemicals discharged into the atmosphere are picked up by the water vapour and returned as polluted precipitation in the form of acid rain.

Rivers at Work

The main function of a river is as a drain carrying excess water from the land to the sea. A river also has the power to transform a landscape with its ability to erode, transport sediment and then deposit it. All rivers produce valleys and it is within these valleys that the array of erosional and depositional features are etched.

Each river is part of a much larger network of inter-connected streams and tributaries organised in a drainage system. Each system is dominated by a main channel, joined by tributaries which flow from all parts of the drained area, starting near the watershed. Close to the sea the river may divide into a network of distributaries to form a delta.

The majority of streams begin life at or near the watershed where the rainwater sinks into the ground until it becomes saturated. Once the ground is saturated any hollows fill up until threadlike channels, or rills carry the water away. This point where there is a noticeable flow of water is called the trickle point. Eventually these small channels join up to become streamlets and then streams.

The amount of water in a river is determined by the amount of precipitation in its catchment area and this is seasonally variable. In Britain the flow is greatest during the winter because of high rainfall. Rivers are capable of moving huge volumes of water, sometimes at high speed. It is the volume and velocity of the river which provides the energy to erode and transport vast quantities of material.

Over its whole course the action of a river varies between erosion, transport and deposition. This process does not operate over the whole course all the time but can be seen to operate over short sections and for short periods of duration. Although a river is able to erode throughout its entire length its greatest effect is where its flow is fastest. This generally occurs in the upland regions where the slope is steepest and water inputs are highest.

3 erosion mechanisms are at work namely solution, transportation and abrasion. Carbonaceous rocks can be dissolved in water and removed in solution. The force of the water can sweep away loose sediment by traction at the base of the flow. Armed with sediment the river then scrapes at the material of its channel and the contact between particles of sediment reduce them in size.

Material too heavy to be carried in the flow (suspension) is transported as bedload which rolls and slides (traction), or bounces (saltation) along the river bed. Some boulders may only move during high velocity floods but finer silts and clays can remain in suspension at much lower energy levels.

The volume and velocity of a river does not remain constant. Given the same volume, if a river reduces in velocity, owing to a change of gradient, it will deposit the heavier particles first. Should the river increase its velocity it will entrain more material from that resting in its channel. As a river channel moves from riffle to pool to riffle much material moves in short stages dependent on whether the energy of the river is increasing or decreasing.

Deposition is greatest in the lower, slower moving reaches of the river. The coarser the material the higher upstream it will be found. Rocks are found in the bed of an upland ravine whereas fine silt lines a flood plain or estuary.

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