Farmers in the Wensum valley have collected data which they claim proves their contribution to water pollution is much lower than official figures suggest.

The Wensum Farmers Group (WFG) is a partnership of 29 landowners managing more than 10,000ha within this nationally-important river valley.

But despite its raft of ecological designations, the river is failing to meet water quality targets.

Modelling used in the diffuse water pollution plan (DWPP) for the Wensum suggests agriculture is responsible for 30pc of phosphate pollution - 17pc from livestock and 13pc from arable - while sewage treatment works account for 54pc.

WFG advisor Lizzie Emmett said the group has collected more than a year of water samples from tributaries and ditches at 15 sites across the catchment - data which contradicts the model and suggests the arable figure could be as much as three times too high.

That means growers could be disproportionately penalised with "drastic" mitigation measures and regulations, she said.

Meanwhile, data taken at a farm level shows the immediate impact of measures such as growing cover crops and reducing tillage and chemical use, with the majority of samples meeting standards set by the EU Water Framework Directive for phosphates and nitrates.

Miss Emmett said this proved that farmers were reducing pollution from the land under their control - and that other sectors needed to take their "fair share" of the overall responsibility.

"The specific on-farm data gives us a really clear overview of what our farmers' impact is before the water joins bigger water courses and the main river," she said.

"And the catchment-wide data is telling us that the current modelling on who is responsible is incorrect. That matters because this model is feeding directly into policy.

"We think the proportion from arable farming is considerably lower than is suggested in the models.

"The reason that is important is because stated within the DWPP they say if things don't improve we will look at landscape-scale measures, either grassing over fields, or even capping our fertiliser usage.

"These are big changes and massive threats to the system, so this data is absolutely ground-breaking."

Dr Richard Cooper from the UEA's school of environmental sciences, has been working with the group to assess the data.

He said it demonstrates the value of farmers' efforts to reduce pollution - but could not definitely prove that the modelling was flawed.

"The Environment Agency (EA) uses this SAGIS modelling, which it is a tool they have been using for 10 years to work out how much of the pollution in the river is coming from agriculture, and how much is from sewage treatment works or other industries.

"The farmers are doing lots of great things on the farm and they feel they are doing a lot to improve water quality, yet the water in the river is not getting any better - and this model is apportioning quite a lot of the blame to them.

"But I don't think we have enough information to say for definite that the model is wrong, and that the data the EA is using is inaccurate.

"A lot of that comes from the fact that the water you see in the river is not always washed off the land. 70pc of it has come up from the chalk bedrock, and has been polluted many decades ago.

"The farmers Lizzie is working with are doing great stuff but we wouldn't expect to see improvements in the river yet because of the delay in the water coming up from the ground.

"We need to better understand this model and test that it is working."

The EA declined to comment on the effectiveness of its modelling, but a spokesperson said: “We all have a part to play in protecting our rivers, as climate change and population growth are putting pressure on waterways.

“The Environment Agency works with government, business and others to meet the challenge brought by extreme weather, which can have an effect of water quality."

An Anglian Water spokesperson added: “It will take joined up action from the water industry, farmers, government, regulators and others to genuinely transform our rivers.

“Water companies have a proven track record investing in environmental protection and improvements, ever since privatisation, and Anglian Water is no different.

"However, improving river quality is about much more than tackling individual sector issues. Agriculture, highways teams, customers as well as ourselves, all have a role to play in the health of our rivers."

What are farmers doing to stop pollution?

One of the Wensum farmers seeking to improve water quality is Colin Palmer of Manor Farm in Horningtoft, near Fakenham.

He said he has reduced cultivations in his "min-till" system for almost 30 years, and he uses cover crops to ensure there is no bare land over the winter, as well as grass margins to prevent run-off, hedges which can stop wind erosion.

Similar measures to improve soil structure and stop nutrients running off into watercourses have been employed at the Sennowe Estate.

Farm manager Adrian Howes said: "What we have done is reducing cultivations, only cultivating if we need it, and we've put in 6m buffer strips on every field, which are funded by stewardship payments.

"Over the years we have bunded field corners to trap water and reduce pollution and run-off into the ditch. We have put six sediment traps in which have been funded through Norfolk Rivers Trust.

"For me, at the end of the day it is all about being a custodian of the land, water and rivers and leaving them in a far better position than when I got here - to be proud of what I've done."