The Water Balance (or Water Budget)



The Water Balance (or Water Budget)

Streamflow can only occur when the water stores in the drainage basin are capable of releasing water, and when there is direct surface runoff. In order to understand the streamflow pattern across a year (called the river regime) you have to understand the changing balance of three key variables – evapotranspiration, soil storage and groundwater storage. This dynamic relationship can be expressed as the water balance equation:

Precipitation (P) = streamflow (Q) + evapotranspiration (E) +/- changes in storage (S)

[Q = discharge in cumecs]

For streamflow we can transpose the formula to read

Q = P – E +/- S

Soil Water Stores

The presence of water in soil does not necessarily mean it is available for plant use. Plants growing in clays may still suffer from water stress even though it has a high water holding capacity. Soil water is classified according to the tension at which it is held. After heavy rain the soil may be saturated with all the pore spaces filled with water – if it kept on raining overland flow would occur. When infiltration ceases water with a low surface tension drains away rapidly because of gravity. This is called gravitational or free water which is available to plants when the soil is very wet. But unavailable after it has drained away.

Once the excess gravitational water has drained away the remaining water that the soil can hold is called its field capacity. Moisture at field capacity is either held as capillary water or as hygroscopic water. Capillary water is attracted to, and forms a film around the hygroscopic water which itself is held around the soil particles. The capillary water has a lower cohesive strength and is available to plant roots. However, this water can be lost from the soil by evaporation. When a plant loses more water by transpiration than it can take up through its roots it is said to suffer water stress and begins to wilt. At wilting point, photosynthesis is reduced but provided water can be obtained relatively soon or if the plant is adapted to drought conditions, this need not be fatal.

Hygroscopic water is always present, unless the soil dries out completely, but is unavailable for plant use. It is a thin film around soil particles and sticks there by the strength of surface tension.

The diagram above shows how field capacity in a soil can vary around the year. During the summer, high rates of evaporation and transpiration may reduce the soil moisture below its field capacity. This leads to soil moisture deficit and under such conditions plants begin to wilt and may eventually die. When rain falls again the soil is recharged as water infiltrates into the soil. In areas where a soil moisture deficit occurs regularly or more or less permanently farming is only possible with the use of irrigation.

Water Balance Graph

A useful way of investigating the water balance of a place is by plotting temperature, precipitation and evapotranspiration rates on a single graph to show the balance between them. The water balance graph looks complicated at first but it is just a series of line graphs plotted on the same frame.

Water budget graphs usually show potential evapotranspiration (PET) which is the amount of water which would evaporate if an adequate supply was continuously available to the vegetation. Actual evapotranspiration will be less than this.

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E Precipitation is greater than potential evapotranspiration. The soil water store will start to fill again (soil moisture recharge).

F The soil water store is full. Field capacity has been reached. Additional rainfall will percolate down to the water table, and groundwater stores will be recharged.

In the UK rainfall input nearly always exceeds evapotranspiration loss so there is a positive water balance. In the long dry summers of 1995 and 1996 these drought summers had a temporary negative water balance in the SE of England as. In other climatic regions (like Deserts) the water balance graphs will be very different.

Activities:

GeogOnline - Half a Minute Quiz

1 The annual stream flow pattern.

2 The main water loss to the atmosphere from grassland vegetation. (Wye)

3 The main water loss to the atmosphere from forest vegetation. (Severn)

4 The % of air spaces in a soil in relation to the total volume.

5 The ability of the soil to let water drain through it.

6 Water draining through cracks in a rock.

7 Large pores as in sand.

8 Small pores as in clay.

9 Water which will drain away quickly when the soil is very wet after rain.

10 The maximum amount of water left after ‘free water’ has drained.

11 Water held around soil particles but available to plant roots.

12 Water held tightly around soil particles not available to plants.

13 The point at which a plant is stressed when transpiration exceeds rainfall.

14 Type of evapotranspiration with unlimited supply of water.

15 Type of evapotranspiration that really occurs.

16 When rainfall exceeds evapotranspiration.

17 Line graphs on one frame showing rainfall, temperature and E/T.

18 When E/T exceeds rainfall soil moisture ??what?? occurs.

19 Occurs when (in Autumn in the UK) the rainfall exceeds the E/T again.

20 A place with about 850/900 mm of precipitation a year.

GeogOnline - Match-Up Quiz: South Wales Water Balance

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Researchers on Plynlimon in mid-Wales found that vegetation affects the water balance as it affects interception, infiltration and throughflow. They found the forested Severn catchment lost more water by evapotranspiration than did the grassland Wye catchment. Grassland returned about 16% of the precipitation input to the atmosphere by evapotranspiration – nearly all by transpiration. Whilst the forested catchment accounted for 30% (made up of 25% by evaporation and 5% by transpiration).

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The rate of infiltration of water into the soil or of percolation into rocks depends on the porosity and permeability of the material. Sand has large pores and this allows water to infiltrate quickly at a rate of about 200mm/hr whilst in clay the rate is only about 5mm/hour. Both materials are porous (the % of the material that is air spaces or pores) but the sand is much more permeable. [Perviousness is where water can drain through cracks in the rock].

Soil water is important because it affects upward and downward movement of water (and dissolved nutrients). Drainage depends on the balance between the water retention capacity (water storage In the soil) and the infiltration rate. This is controlled by the porosity and permeability of the soil. Clays have many micropores which can retain water for long periods – giving it a high water retention capacity but reducing its infiltration rate. Sands have fewer but much larger macropores which permit more rapid infiltration.

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Annual cycle of soil moisture [pic]

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A Precipitation is greater than potential evapotranspiration. The water store is full and there is a soil moisture surplus for plant use, runoff and groundwater recharge

B Potential evapotranspiration is greater than precipitation. The soil water store is being used up by plants or lost by evaporation (soil moisture utilisation).

C The soil moisture store is now used up. Any precipitation is likely to be absorbed by the soil rather than produce run-off. River levels will fall and could dry up completely.

D There is a deficiency of soil water as the store is used up and potential evapotranspiration is greater than precipitation. Plants must have adaptations to survive and crops must be irrigated.

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