Baking potatoes are even more demanding, drinking 10 inches (25cm) of water over the summer to deliver the right size tuber free from blemishes.
To quench such thirst farmers are having to build their own lakes to guarantee supply and satisfy the specifications of the retail chains.
Erratic weather, with long dry spells and summer droughts, can spell disaster. Since 1991, when the National Rivers Authority turned the taps off completely, huge constructions have been sprouting across the countryside.
The average farm reservoir covers several acres and is best identified by a 30ft (10m) embankment and a scattering of saplings planted to hide it.
Nick Darby, whose 1,200-acre Norfolk farm supplies iceberg lettuces to McDonald's and other varieties to almost every supermarket chain in the country, said crops came close to dying in 1991 and yields fell dramatically.
"More reservoirs have been built in the past five years than ever before," he said. "The operation is geared to supply supermarkets, with the contracts specifying both volume and quality. Any variation on that costs us dearly."
Mr Darby's pounds 120,000, 20 million gallon reservoir is filled over the winter months from the River Gadder and the water is drawn down to give the lettuces their summer showers.
Officials from the Ministry of Agriculture and the National Rivers Authority are encouraging farmers to built reservoirs to ensure supply - although they do not provide capital grants for them - but Mr Darby blames water companies for over-exploiting the aquifers to provide domestic consumers in the expanding East Anglian population.
"We have had to spend vast amounts of money on a problem that wasn't of our making," he said. "The water authorities are robbing this area of ground water."
Richard Streeter, water resources manager for the NRA, said farm reservoirs helped both farmers and the environment. "They are beneficial to the environment as farmers no longer have to abstract water from rivers during low-flow periods," he said. Besides certainty of supply, farmers also benefit from a 90 per cent reduction in the cost of water if they take it in winter, rather than summer.
The light soil on Hill Farm, Tuddenham, Suffolk, is ideal for potatoes and parsnips. When the crop is lifted, the soil is easily washed off. But its lightness also means that it holds water badly and Christopher Wilson, managing director of the Frederick Hiam group of farms, said pounds 95,000 was spent on building a 34 million gallon reservoir in 1992. "If it had been in place the previous summer, it would have paid for itself in a year," he said.
Hill Farm specialises in growing baking potatoes, which attract the highest supermarket price. Baking varieties demand large quantities of water both to attain the correct size and to keep the tubers from developing common scab.
Water is sprayed on the crops by "guns" or 230ft (70m) booms, which distribute fine droplets six feet (2m) above the plants.
"Less than 1 per cent of water is used for farmers' spray irrigation but we are using it at a time when there is greatest pressure on supplies," Mr Wilson said. "Townies notice it when there is a hosepipe ban. Farmers get equally irritated when we get cut off and we go down the road and they are still using water for car washes."