In the classification of climates Britain falls generally into the cool temperate, humid type. This suggests that it has a surplus of rain, and over the whole year this is true. In fact the rain is so abundant that if it could all be caught and saved there would be sufficient to supply every man, woman and child with 2,000 gallons per head per day.
The present average domestic consumption is about 60 gallons per day by all those who have piped supplies, though some rural districts are still without mains. Water engineers base their calculation of necessary storage on the dryest three consecutive years in the records, and do their best to match the supply with the demand. Occasionally there is a shortage; reservoirs run low and water has to be rationed. Such, in fact, is the increase in demand for water for industrial processes, for example, for washing and cooling, especially of atomic power-stations, that alarm is felt for the future. It is becoming more and more necessary to provide additional storage, or to consider, for example, ways in which the wetter west can be called in to redress the balance of the drier east, where drought is a growing danger, not to survival, of course, but to the maintenance of adequate supplies for industrial and domestic purposes and for agriculture. Water is needed, too, for flushing the streams to remove pollution which is an increasing danger to life in the rivers through lack of oxygen. The remedy here seems to be a much more vigorous control of effluents, for not all the available water is enough to deal with the foul state of many rivers, especially in industrialized areas.
Generally speaking, about half the annual rain falling in Britain finds its way to the sea. In lowland areas the percentage is less, but in the highlands the water runs off faster, and quickly reaches the sea, unless there is industry farther downstream to consume and foul it; but it is, of course, available for water power. In days gone by it worked the mills by the river-banks, now it drives the dynamos of hydro-electric power stations and, thanks to the grid system, contributes more to the nation's power. The combination of steep slopes and heavy rainfall, needed together, limit the power stations to the highland zone and especially to the glaciated highlands.
Some of the rainfall percolates into the ground and some of this re-emerges as springs, but some goes to replenish the underground supplies in water-bearing strata such as the Chalk, the Greensand, the Triassic 'waterstones', and a number of other geological formations from which water can be extracted from wells. Though this is regarded as geological water, stored through the ages, it is being pumped out at an alarming rate and the replenishment is a meteorological matter. The great underground reservoir that lies beneath London was once 'artesian' and spouted under hydrostatic pressure through the fountains in Trafalgar Square. It has long ceased to be so, and so much pumping of 'free' water has gone on that the level is falling, in some places as much as 15 ft a year. No further wells may be sunk without special permission and ways are even being sought to replenish the reservoir by pumping river water back into the Chalk.
But the main reason why rivers fail to carry all the rainfall of their catchment areas to the sea is, of course, evaporation. Not only is there great loss from the surface of rivers and reservoirs, especially in the summer months, but the vegetation transpires enormous quantities through its vascular system and stomata, which by itself often exceeds the summer rainfall income. It is for this reason that rivers are at their lowest in summer and autumn and for the opposite reason that the season of extensive floods is in the end of winter (flash floods due to 'cloudbursts' are quite another thing), and since many crops and grasses are shallow rooted they exhaust the moisture of the surface layers. The soil falls below 'field capacity', replenishment by capillary rise from below is very slow indeed, and a state of partial, though often concealed, drought exists. This is now well known and it is becoming increasingly appreciated that some supplementary irrigation will increase the yield of crops and the stockcarrying capacity of grass in all but the wettest years over at least the south lowland areas of England.