THE floods which have devastated York with increasing frequency in recent years have traditionally been blamed on the rain - too much falling, too often.

But now scientists are suggesting that a key culprit may in fact be ... sheep and cows.

They believe too many livestock trampling upland pastures since the 1980s may reduce water infiltration into the ground and increase the speed of run-off.

This would increase the amount of water getting quickly into rivers after rain, and so increase the likelihood of flooding.

Researchers from Leeds University, led by physical geography professor Stuart Lane, are studying factors behind the growing number of flooding disasters in an effort to identify what action needs to be taken to reverse the trend.

They said the increase in flooding had not been matched by an increase in rainfall.

Flooding in York had risen from ten incidents per decade at the turn of the 20th century to nearly 50 between 1991 and 2000, but there had been no noticeable change in rainfall patterns during the century.

Prof Lane said: "We've taken dry summers out of the equation and looked only at only winter rainfall. We've looked at the frequency of extreme rainfall events, but there is still no major change in the rainfall patterns since 1900 which can adequately explain the dramatic increase in floods.

"Something other than rainfall is causing the flooding, something which affects how the water drains off the land, where it can go and at what speed. That 'something' is the land itself, and how that land is managed."

The research project has been using Upper Wharfedale as the case study, to look at land management in the uplands where the rivers begin their journey and where the rainfall is high.

The researchers say changes in agricultural practices have had a knock-on effect in lower-lying areas many miles downstream in the Vale of York.

The numbers of floods in York increased sharply first in the 1940s and then in the 1980s.

In the 1980s, the European Union started subsidising farmers per head of livestock, leading to a massive increase in stock density in the uplands.

The increase in floods in the 1940s came after the digging of drainage ditches, or grips, in the peat bogs, which increased run-off into rivers.

The project has involved the installation of hundreds of pieces of equipment in Upper Wharfedale, to measure water level, colour and sediment discharge. Some grips have been blocked to analyse the effect on where the water goes, and how quickly.

Updated: 08:59 Thursday, April 03, 2003