For better or for worse, and quite unlike its continental counterpart, our rain falls frequently and gently, providing an annual quota equivalent to a countrywide layer of water nearly 4 feet deep.
About half of this replenishes any deficit there may be from time to time in the reservoir of groundwater; it then runs off through streams and rivers to the sea. The remainder is returned to the atmosphere, either directly by evaporation, or indirectly by plants through a related process called evapotranspiration.
Temporarily, however, some of the water resides within the soil and lends to it an enviable fertility. The amount of moisture that a soil can hold depends upon its type - on the size of the particles comprising it and on the extent to which they are loose or well-compacted.
When it contains enough moisture for it to resemble a saturated but not dripping sponge, is it said to be at field capacity. If more water is added to an area of ground at field capacity, the excess liquid occupies the larger air spaces in between the individual particles of soil and the ground is waterlogged.
If, on the other hand, the ground falls below field capacity, there is said to be a moisture deficit.
In areas where official injunctions to conserve supplies do not hold sway, a moisture deficit can be repaired by irrigation. Ideally, efficient irrigation requires that water be applied before the plants begin to show the signs of shortage, while at the same time avoiding the situation where water is added so frequently and generously that the soil becomes waterlogged and the crops' roots suffers from a lack of oxygen.
The questions as to when and how much water to apply can be answered by reference to data on evapotranspiration. Evapotranspiration, as the name implies, is the process whereby moisture exuded by vegetation from its pores is surrendered to the atmosphere by evaporation.
Assuming that there is no run-off, the required amount of water that must be provided artificially to a crop to keep it healthy over a period is equal to the amount of water "transpired" by the plant, less any rainfall that may occur.
The rainfall is easily measured. Moreover, the average loss by evapotranspiration can be calculated with reasonable accuracy from standard weather observations of temperature, humidity, wind and sunshine. A few simple calculations, therefore, allow the experts to estimate the amount of water that must be added at chosen intervals to combine a healthy growth with conservation of a valuable resource.