THE WATER BALANCE:What? The relationship between precipitation, evapotranspiration and storage (in the form of soil moisture and groundwater) can be expressed as the water balance equation:

Precipitation (P) = streamflow (Q) + evapotranspiration (E) + change in storage (S)

* (Balance between inputs (precipitation) and outputs (streamflow/runoff, evapotranspiration and storage)Uses/importance:

1. It is used by Hydrologists to plan and manage water supply within a drainage basin, for example; when there is water shortages hosepipe bans are implemented to preserve stocks.

2. For irrigation, pollution control and control flooding.

A water budget graph is a useful way to look at the water balance of a location over a year. Precipitation and evapotranspiration rates are plotted on to a single graph.

Important terms:

WATER BUDGET GRAPH (Soil moisture budget graph):Ref: http://www.heinemann.co.uk/shared/Resources/NonSecure/00000001/AS_Level_Geog_Edexcel.pdf

*Draw the diagram of soil moisture budget graph/water budget graph)

*WATER BUDGET GRAPH:

1. Actual evapotranspiration (AET): the loss of moisture to the atmosphere by the processes of evapotration and transpiration which actually takes place.

2. Potential evapotranspiration (PET): the maximum amount of evapotranspiration which occur if an adequate supply of water were continuously available. (In Desert PET >AET amount of water is limited)

3. Soil moisture surplus: occurs when the soil water store is full and thus there is surplus of water for plants, runoff and groundwater recharge. (PP>PET)

4. Soil moisture utilisation: Plants (and people) use moisture stored in the soil, leaving it depleted. (AET >PP)

5. Soil moisture deficiency: Equivalent to the extra water which would be needed to maintain maximum plant growth. There is little or no water available for plant growth (irrigation could make good this deficit) (PET > AET)

6. Soil moisture recharge: The soil water store starts to fill again after a period of deficiency)

7. Field capacity: The moisture a freely drained soil can hold after all free or gravity water has drained away. Such moisture is held by tension around soil particles, mainly as capillary water.

1. What is a water budget?

A water budget reflects the relationship between input and output of water through a region.

The water balance graph shows precipitation and potential evapotranspiration both as line graphs. Thus we have a direct comparison of supply of water and the natural demand for water.

It is possible to identify the periods when there is plenty of precipitation and when there is not enough.

2. Definition of terms:

The following terms will be used in the questions that follow:

Potential Evapotranspiration (PE): All the water that could enter the air from plants and evaporation if present.

Precipitation (P): All moisture from the atmosphere, rain, snow, hail and sleet.

Surplus: Water above what is lost naturally from the soil (when P is greater than PE)

Deficit: Water that would be lost above what is in the soil if it were present (when P is less than PE)