In dry weather, streams can keep flowing only if there is some natural store to maintain flow. In south-east England that store is ground water in permeable rocks such as 'the Chalk', the belt that stretches from the white cliffs at Dover as far as Wiltshire and up to Lincolnshire and Yorkshire.
Rainfall soaks into these permeable rocks, and 'fills up' the storage space within them until the water table reaches the ground surface in one or more places. This generally happens where the ground is low, usually in a valley. Water flowing naturally from the Chalk sustains the flow of many of the streams that do so much to enhance the landscape (and the property values) of the South-east. The flow from the ground continues as long as the water table remains above ground level. If it is always above ground, the stream is said to be perennial.
Until the advent of piped supplies many settlements grew up around sources of water such as wells and streams. Different areas of the country have their own names for streams; in southern England, they are bourns or bournes. These names, like the Scottish burn, derive from the Old English word burna meaning a brook or spring.
It was natural that these sources should figure in place names. So we find many place names, some ancient and some less so, that contain the word bourn or bourne. Examples are Pangbourne, where the Pang flows into the Thames; and Lambourn (of racehorse fame) on the Berkshire Downs.
Rainfall in England is spread throughout the year, but in spring and summer the demands of growing plants allow little of that rain through to the rocks beneath. The soil dries out and absorbs any rain that falls on it, until autumn, when plant growth slows down. This means water recharges the rocks for only part of the year, but is draining from them to the streams all year round. As a result the water table rises and falls seasonally; typically it reaches a maximum in late winter, and is at its lowest around October or November.
In some valleys it is above the ground surface in the lower part of the valley throughout the year, so this part of the valley will have a perennial stream; but in the upper part of the valley it is above ground only during the winter and spring. The stream in the upper part of the valley is said to be intermittent, and the highest point at which it flows throughout the year is the perennial head. However, streams are only perennial in the way that volcanoes are extinct - until the time when something changes.
The water table in the Chalk rises and falls more than in most rocks. Intermittent streams are therefore a particular feature of the Chalk country. Because they are bournes that flow mainly in winter they are called, not unreasonably, winterbournes.
Many 'bournes' are actually winterbournes for at least part of their length, and many place names with the bourne element lie on or near winterbournes, although only a few use the full epithet: there is a clutch in Dorset - Winterbourne Kingston, Winterbourne Abbas, and so on.
A source of water as abundant as that in the Chalk is attractive not just to fish, ducks and residents. For decades many of the water companies that serve the area have taken water from the Chalk to supply their consumers. When you take water out of a store the level will fall. When that store is as large as the Chalk, the fall will be small, but nevertheless the water table will be lower in the vicinity of the borehole. The fall may be enough to reduce the flow of a winterbourne, dry it up completely, or make it flow for a shorter period each year. The perennial head may migrate down the valley; this has happened, for example, to the Candover Stream in Hampshire, where the perennial head used to be a few hundred metres above Totford. Since the opening of a pumping station at Totford in 1952, it is a few hundred metres below it.
Equally, if less water enters the ground, there will be less to flow out. Between spring 1988 and spring 1992 south-east England experienced a remarkable drop in rainfall - only about 80 per cent of the long-term average, in an area where rainfall is low anyway. Much of the shortfall occurred in winter, when it would normally provide recharge to ground water.
To many residents these 'winter droughts' were not as noticeable as the disappearance of their local streams. Hearing that some rivers were being affected by pumping, villagers whose streams were disappearing looked around for a water company to blame.
The newly formed National Rivers Authority (NRA) did not always help matters. Some of its media pronouncements seemed to exaggerate the case of over-pumping, culminating in a news item in which the NRA apologised for switching on the Candover scheme - where water is pumped from the ground to augment the flow of the Candover Stream and the River Itchen in dry weather - on the basis that ground-water abstraction was bad for the environment.
The generally wet summer, autumn and early winter led to Chalk water tables rising dramatically this year, and the winterbournes are flowing again. But residents of winterbourne villages would do well to note one other feature common on the Chalk downs. Dry valleys show where rivers once flowed, testifying to changes in climate over which we have had no control. The bournes have come back this spring; but if they are not always with us, it may not necessarily be the water companies that are to blame.
The writer is senior lecturer in hydrogeology at the University of Reading.