Walk across the South Downs in not too many years time and you might not see the carpet of grass that has capped the chalk hilltops for centuries.

Because if soil continues to be washed into the valleys and bottoms all you are likely to find is a whitewashed moonscape.

First to go will be the thin layer of top soil, taking with it the compost and organisms that are the stuff of life.

And just like in Bevendean, where earlier this month dozens of homes and shops were flooded as a slick of liquid mud swept off the surrounding hills, it is people living in the valleys who will be the first victims.

What struck Bevendean two weeks ago was not exceptional.

Soil erosion on the downs has increased over the last 20 years, according to a study by Dr John Boardman, of the Environmental Change Institute at Oxford University.

Frequent, sometimes serious, erosion became a feature of the Eighties and Nineties, often accompanied by flooding of houses and roads.

Walmer Crescent, in Bevendean, was flooded eight times between 1983 and 1993. At Highdown, above Lewes, there were several floods in 1982.

Herbert Road, in Sompting, was flooded as soil washed off surrounding downland in 1980, 1987, 1991 and 1993.

Breaky Bottom, near Rodmell, was flooded eight times in October 1987 alone.

The same month a dam built to hold back surges of liquid mud above Rottingdean broke, sending a giant mudslide across the Falmer Road and into nearby houses.

The kind of rainfall that leads events like these is likely to become more frequent if predictions that global warming will result in more severe winter weather in south east England come true.

The culprit, according to Dr Boardman, is modern agriculture, and farmers need to change their ways.

The former University of Brighton geography teacher said: "You would think after so many incidents they would be aware of this problem but they continue to take a risk, it is a gamble they are involved in."

During the Second World War large tracts of the downs were ploughed to grow crops, a trend that continued in the post-war years.

Steep hillsides, which horse-drawn ploughs were unable to work, ceased to be the preserve of sheep and were turned instead into prairies.

But the most profound change did not come until the Seventies with the arrival of autumn-sown rather than spring-sown cereals.

Sowing in the autumn, when the weather is at its wettest, coupled with mechanisation, which allows ploughing on slopes that were once out of bounds, increases the risk of run-off during heavy rain.

Because of the steep slopes tractors can often only plough up and down, rather than across the hillside, increasing the downhill cascade of water.

The loss of organic matter on heavily fertilised one-crop fields is also thought to add to the risk of instability.

Dr Boardman said: "The big issue here is land use, it is growing winter wheat near houses rather than climate change.

"As long as you grow winter wheat next to houses we will continue to have intermittent flooding every few years."

Conservationists, notably Brighton and Hove Council's own Wildlife Advisory Group, have called for policies to combat soil erosion to be included in the authority's new local plan - the document that will guide planning decisions for the next ten years.

What the council does is crucial because some two thirds of farmland surrounding Brighton and Hove is part of its huge 10,000 acre downland estate.

Sir Herbert Carden, remembered as the maker of modern Brighton, persuaded the old borough council to buy large tracts of downland in the Twenties, both to preserve the landscape and safeguard the town's water supply.

It is the council's ownership of land above Rottingdean that has so far prevented a repetition of the 1987 floods, which were more severe than those that hit Bevendean this year.

The council reached a deal with the farmer, who agreed to halve the amount of winter wheat he planted and five new dams were built - preventing run-off into homes if not all soil erosion.

If the council can force a change of heart among farmers in general so the argument goes, other authorities might follow suit.

Kemp Town MP Des Turner said the council, as landlord of so much land around Brighton and Hove, should worry about erosion.

Mr Turner said: "For a long time I have been very conscious of the erosion problem on the Downs.

"Arable farming on the Downs is not good practice, most of the top soil is lost and a lot of the problems we have seen are scarcely surprising."

John Carden, an equally green descendant of Sir Herbert, said the scars of this month's heavy rain were clear to see in many places, and the speed that water was able to run-off the downs contributed to the deluge that overcame Lewes.

He said: "You walk up Juggs Lane and it is horrendous. It is not going to be a one off, the more and more we have global warming the more and more we are going to have heavy rain."

And he added his own voice to the growing chorus of people who want the council to step in and start preserving the soil on the downland it owns, a resource that can only be lost once.

He said: "What they should be doing is talking to the farmers, taking a lot of this land out of crop production, wheat and barley and things like that.

"In the next ten years with rainfall and intensive farming there won't be any soil up there and we won't get it back.

"It will be a chalk desert."