The Water Cycle

The Water Cycle (or hydrological cycle) is the continuous transfer of water between the sea, the land and the atmosphere. It is a continuous cycle with no beginning or end.

A basic description of the Water Cycle:

Precipitation (rain, snow, sleet or hail) falls to the ground. This is either:

INTERCEPTED by vegetation or buildings

INFILTRATES into the ground

RUNS-OFF the surface of the ground (as a river or stream).

Energy from the sun evaporates the water. If the air cools it causes condensation (clouds), then precipitation.

River Basins

A river basin is an area of land drained by a river and its tributaries. River basins have typical features, these include:

Tributaries - smaller rivers flowing into a larger river.

A Watershed - an area of highland surrounding the river basin.

A confluence - where a river joins another river.

Source - The start of a river.

Mouth - Where a river meets the sea or an ocean.

Changes from source to mouth

A river flows from an upland source. Here the velocity of water is faster than downstream because the river's gradient is steep. Near a river's source the valley has a narrow floor and steep sides (v- shaped) This is evident in the photograph above).

The middle course of the river has a wider floor and the sides of the valley are more gently sloping. The velocity is slower than the upper stage. However, the channel is wider as the amount of water flowing in it increases as other streams and rivers join it.

The lower course of the river is very gentle sloping, almost flat. The channel is usually at its widest and deepest here because the amount of water flowing within the river is at its greatest.

Erosion

Rivers erode in four ways:

Abrasion or corrasion - This is when large pieces of bedload material wear away the river banks and bed.

Attrition - This is when the bed load itself is eroded when sediment particles knock against the bed or each other and break, becoming more rounded and smaller.

Hydraulic Action - This is when the force of water erodes softer rock.

Solution or corrosion - This is when acidic water erodes rock.

Flooding

Floods can bring both advantages and disadvantages to an area. Floods can deposit rich, fertile alluvium on agricultural areas. Also, flood water can replenish irrigation channels. On the other hand floods can destroy food supplies, homes and transport infrastructures.

Causes of flooding

Human causes:
Deforestation - Cutting down trees causes increased run-off (water flowing over the surface of the earth). Rain water reaches rivers faster. Flooding becomes more likely.
Urbanisation - Man-made surfaces such as concrete result in greater run-off. Rain water reaches rivers faster and can cause flooding.

Natural causes:
Heavy rainfall
Melting snow

Solutions to flooding

Afforestation - Planting more trees reduces run-off and increases interception.
Dams - Although very expensive, dams can significantly reduce the risk of flooding downstream

Case Study - Ganges/Brahmaputra River Basin

Flooding is a significant problem in the Ganges/Brahmaputra river basin. They cause large scale problems in the low lying country of Bangladesh. There are both human and natural causes of flooding in this area.

Human Causes
Deforestation - Population increase in Nepal means there is a greater demand for food, fuel and building materials. As a result deforestation has increased significantly. This reduces interception and increases run-off. This leads to soil erosion. River channels fill with soil, the capacity of the River Ganges and Brahmaputra is reduced and flooding occurs.

Natural Causes
Monsoon Rain
Melting Snow
Tectonic Activity - The Indian Plate is moving towards the Eurasian Plate. The land where they meet (Himalayas) is getting higher and steeper every year (fold mountains). As a result soil is becoming loose and is susceptible to erosion. This causes more soil and silt in rivers. This leads to flooding in Bangladesh.