Paul Brown is right to imply that "building houses across the south without any reference to available water supply" is not a good idea, especially at a time of climatic uncertainty (Weatherwatch, 10 May). He is also right that in southern England "the last three years [have seen] heavier than normal summer rainfall".
I would however take issue with him that it is only this unusual summer rainfall that has "maintained" the flows of the south's rivers for the last three years, and I disagree strongly that "a single dry summer, maybe this one, will find us out".
As I am sure Brown knows, aquifers (layers of permeable rock) receive most of their recharge during winter and early spring, when plants are not growing and the soil is not grabbing most of the rainfall; it is therefore dry winters that pose the problem for ground-water resources, not dry summers.
Because water enters the aquifers for part of the year but drains from them continuously to support the flows of many rivers, water tables rise and fall in an annual cycle; as they fall, stream networks shrink and flows decline naturally. This is particularly apparent in chalk areas, where the annual fluctuation of the water table may be more than 20 metres. Given the shallow slope of chalk valleys, the head of the stream may move several kilometres up and down the valley, giving rise to the familiar winterbournes.
The last three summers have been exceptional in that for some periods, especially in 2007, the rainfall was so heavy that soil-moisture deficits were reduced or eliminated in some areas and there was, unusually, a rise in water tables and an increase in the flow of some ground-water-fed rivers. However, far from being just sufficient to keep these streams flowing, as Brown implies, this pushed some of them to record flows for summer months.
Of course Brown is right that removing water for household supply also lowers water tables and decreases river flows, as does obstructing a river by building a dam; but the issue is often confused. In 1988-92 and 1995-97 a series of dry winters lowered water tables, leading to complaints from those living in winterbourne valleys about the disappearance of their streams. The winter of 2000-01 was so wet that some of the same householders were complaining of flooding and asking water companies to remove more water.
The predictions of climate change are that rainfall will become less predictable, so stream networks will expand and shrink more markedly than we are used to; maybe the past two or three decades have seen the first signs of that. But ground-water levels last month were at or above average.
In simple terms, we don't buy water for public supply, we just rent it – so most of it should be returned to the river with little net loss of downstream flow. This happens to a degree in the Thames catchment, and at Winchester where sewage is recharged to the chalk aquifer, but too rarely in other areas of Britain. This is where more effort might pay dividends.