How does the water cycle work?

• Energy from the Sun heats the surface of the Earth.

• Water is evaporated from oceans, rivers, lakes, etc.

• The warm, moist air rises because it is less dense.

• Condensation occurs when water vapour is turned back into water droplets as it cools down. Clouds are formed.

• Precipitation occurs as water droplets get bigger and heavier they begin to fall as rain, snow and sleet, etc.

When the precipitation reaches the surface, some falls directly into the sea but other water falls on land:

• Some water is intercepted by vegetation. Some water may then slowly reach the ground. Some will evaporate from the surface of leaves or be taken up by the plant roots, and some of this water will eventually return to the air as vapour through the process of transpiration. This slows down or prevents some water flowing back to the river.

• Some water flows across the surface of the ground - surface run-off. This happens when the surface doesn't allow water to penetrate. Surface run-off is more likely to occur if the ground is saturated with water or when the rock is impermeable. This water moves quickly to the river.

• Some water infiltrates into the soil. This through flow moves more slowly back to the river than surface run-off.

• Some water percolates deeper into the ground and is slowly transferred back to the river or sea.

Condensation - A change of state in which gas becomes liquid by cooling.

Evaporation - The process in which a liquid changes state and turns into a gas.

Flood - An overflow of water onto normally dry land. The inundation of a normally dry area caused by rising water in an existing waterway, such as a river, stream, or drainage ditch. Ponding of water at or near the point where the rain fell.

Precipitation - Moisture that falls from the air to the ground. Includes rain, snow, hail, sleet, drizzle, fog and mist.

The source of a river is often, but not always, in an upland area. Near the source, a river flows over steep slopes with uneven surfaces.

It often flows over a series of waterfalls and rapids.

As a river flows down steep slopes, the water performs vertical erosion. This form of erosion cuts down towards the river bed and carves out steep-sided V-shaped valleys.

As the river flows towards the mouth, the gradient of the slope becomes less steep.

Eventually the river will flow over flat land as it approaches the sea.

As the river moves from the source to the mouth – both the depth of the river and the width of the river will both increase.

The load of a river will also change as it is transported and eroded along the river's profile.

As a result, the size and shape of stones will become smaller and more rounded as they journey through the river profile.

The discharge will increase as the river approaches the sea.

The discharge is usually calculated as the cross sectional area (depth x width) multiplied by the velocity or speed of the water.

Erosion

As a river travels downstream, it erodes the landscape. Erosion is when land is worn away by another material. It's a natural process that's been going on for millions of years and it continues to shape our planet's landscape today. 

Water is a very powerful force when you come to think of it. Although seemingly harmless, it wears away the landscape and sculpts some of the most beautiful spots around us.

There are four main types of water erosion:

• Hydraulic action refers to the force of water that hits against the river bed and removes material. It tends to be quick, high-energy water.

• Abrasion occurs when sediment carried by the water hits the river channel and removes material in a sandpapering effect.

• Attrition happens when stones carried by the water hit into each other, gradually making the rocks smaller and smoother.

• The final main type of water erosion is solution, when soluble materials dissolve in the water and are carried along.

The rate of erosion depends upon the force of the water and the durability of the material that’s being worn away. For example, limestone dissolves quicker in a fast-running river than a light drizzle in someone’s backyard and it dissolves at a faster rate than basalt because it is less durable. In rivers, faster currents will erode the riverbank and riverbed at a faster rate as the water moves along compared to small streams. However, that’s not to say that the gentle, slow flow of a stream will not have an impact over time.

River erosion caused by fast currents often creates a V-shaped valley. The river channel is where a river flows. Other streams and rivers will join it and it will become deeper and wider. Inevitably, a river will reach a large body of water, such as a sea, ocean or lake, that point of entry is called the mouth of the river.

Similar to erosion, weathering can also cause the breaking down of materials in the atmosphere. There are three main types of weathering: chemical, physical and biological.

• Chemical weathering occurs when chemicals in the rain dissolve the rock.

• Physical weathering is when a temperature change affects the rock and causes it to break open.

• Biological weathering happens when plants and animals weaken the structure of the rock until it breaks away.

Bangladesh is a developing country in Asia and it is frequently affected by flooding. For example, in 2007 flooding made 9 million people homeless and approximately 1,000 people died from drowning and from waterborne diseases

Causes of flooding in Bangladesh

• Cyclones cause coastal flooding.

• Lots of low-lying land.

• Melt water from the Himalayas.

• Heavy deforestation.

• Heavy monsoon rains.

• Increasing urban areas.

What has Bangladesh done to cope with flooding?

Bangladesh is a very poor country and so has less money to spend on flood defences than richer countries. Most people in Bangladesh do not earn enough to pay for insurance against flooding, so when there are floods they risk losing everything.

Short-term responses to flooding

• Food aid from the Government and other countries.

• Water purification tablets.

• People repaired embankments and helped to rescue people.

• Free seed given to farmers.

Long-term responses

• Building embankments.

• Building raised flood shelters.

• Introducing flood warning systems.

• Emergency planning.

• Dams planned.

• Reducing deforestation.

Unfortunately, many of these long-term responses are difficult to pay for and maintain. They are not always successful and don't always help enough people.