The water balance

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The water balance

This is the balance between inputs into a drainage basin and outputs. It is important for understanding the processes operating in a drainage basin and water balances throughout the year.

It is expressed as follows:

P = Q + E (+/- change in storage)

P = precipitation

Q = run-off

E = evapotranspiration

The diagram below illustrates the main features of the water balance:

The water balance

Here are some questions and answers that will help you to learn to read the graph accurately:

In which months is there a water surplus?

Jan, Feb, Mar, Apr, Nov, Dec.

Why is there soil moisture recharge in October?

Due to the excess of evapotranspiration over precipitation in May - Sept.

When is field capacity attained?


Why is a water deficit not shown on the graph?

Precipitation always far exceeds evapotranspiration.

Soil moisture

Surplus: If precipitation exceeds evapotranspiration and the excess is not been used by plants.

Deficiency: Evapotranspiration exceeds precipitation.

Recharge: Replacement of water lost during drier periods.

Field capacity

The maximum amount of water soil can hold.

A water surplus can result in wet soils, high river levels and run-off whereas a deficit leads to dry soil, falling river levels and possibly drought. Management is shown in the example at the end of this topic.

Water Deficit

Evapotranspiration is in excess of precipitation and any previously available moisture has been used, in soil moisture utilisation.

The regime of a river is expected to have a seasonal pattern of discharge during the year. This is due to factors such as climate, local geology and human interaction. Equatorial rivers have regular regimes but in the UK where seasons exist one or two peaks may be recognisable.

These show times of high water levels followed by lower levels. They exist as a result of a glacier melt, Snowmelt, or seasonal rainfalls such as monsoons.

If a river has more than one period of high water levels and/or low water levels, a more complex regime results. It is more common on large rivers that flow through a variety of relief and receive their water supply from large tributaries, for example, The Rhine.

The water balance

A river has two main functions: one, to transport water and two, to transport sediment. The type of flow that occurs depends on factors such as gradient, volume of water, channel shape, and friction.

There are two types of flow:

Laminar Flow: This rarely occurs, water flows smoothly in a straight channel. It is most common in the lower parts of a river. It is shown in the diagram below:

The water balance

Turbulent flow: This is far more common, it occurs where the shape of the rivers channel is varied with pools, meanders, and rapids. A great deal of turbulence results in sediment being disturbed. The greater the velocity the larger the quantity and size of particles that can be transported. Turbulent flow is illustrated in the diagram below:

The water balance