With environmental pressures and a growing population competing for water within a changing climate, farmers say a review of abstraction licences and incentives for reservoir building must be high on the new government's agenda.

Water has always been one of agriculture's most vital natural resources – but its availability is increasingly being governed by an unpredictable climate and demand from a growing population.

Nowhere is that more true than in East Anglia, an arable heartland in one of the driest areas of the country, where many food production businesses would fail without secure supplies for irrigation.

So industry leaders are calling for a radical review of the nation's outdated abstraction licensing regime to be among the first priorities for farming ministers in the next government.

Paul Hammett, water resources specialist for the National Farmers' Union (NFU), said he was expecting a governmental announcement after the General Election on how the share of water should be fairly re-distributed among those who abstract it from rivers and boreholes.

'There is cross-party recognition that we need to get on the front foot with this, because failure to do so would present a risk to the environment, as well as to farm businesses,' he said.

'The current system is 50 years old, and many of the original licences are quite generous in what they allow to be abstracted.

'As you can imagine, when any section of society is faced with change from one system to another, there will be winners and losers and we will be looking very carefully at where the allocations of water are for people who have had allocations in the past.

'What if the volume is less than they have been used to? What are the rules going to be? Defra and the EU say there is not enough to go round, so are we going to slice 10pc off everybody, or look at a formula of what they have used in the past?

'Given the importance of irrigation, particularly in East Anglia, it is going to be very interesting'

Mr Hammett said there should also be more government support for the building of reservoirs which are an obvious – if expensive – means of collecting water when it is plentiful as an insurance against drought.

'The fact remains that a farmer might invest £250,000 building a reservoir that they may only need to use every five or 10 years, so the economics don't always stack up,' he said.

'So there is a lot to be said for some kind of government support for farmers to build reservoirs and, in so doing, take pressure off river flows. For the new government, whoever that might be, to offer business tax allowances for investment is pretty common. Reservoirs fall outside that system, so farmers who spend all that money on reservoirs cannot set it against their tax planning.

'We desperately need these tax capital allowances for reservoir construction.

'Secondly, the Rural Development Programme for England (RDPE) which offers grants for farmers was re-launched recently but, the way the rules are currently constructed, it looks like it will be very difficult for farmers to obtain a grant to build a reservoir. The NFU is talking to Defra to encourage it to relax the rules a bit, so farmers can access that money for reservoirs.'

Case study: GS Shropshire and Sons

An East Anglian salad grower has invested in innovative means of bolstering the farm's water self-sufficiency, acting as an insurance policy against volatility in the climate – and to the forthcoming reforms of abstraction licensing.

GS Shropshire and Sons, part of the G's Fresh group, farms 3,500 acres of land including 2,500 acres of black Fenland soils near Downham Market, specialising in Romaine and iceberg lettuce, celery, radish and Chinese leaf salad crops.

The family firm operates a unique sub-irrigation system – only possible because of the depth of the peat topsoil – involving laser levelling of the land to create a uniform water table, which can be accurately raised by up to 30 inches by flooding ditches and forcing water back up through drainage pipes at the perfect height for the new roots.

By irrigating underground, the farm saves the 50pc of water usually wasted through evaporation on the hot and dry summer days when it is needed most. It also reduces the risk of fungal infection on damp leaves and ensures a consistent supply to each plant in the field, with conventional water-sprays from above only used to start the growth of young plants.

And this year's crops are the first which could benefit from a £600,000 new reservoir – the farm's third – which Peter Shropshire estimates has now taken his farm's water self-sufficiency to around 75pc, reducing its reliance on summer licences, and the possibility of an abstraction ban in the event of a drought.

Mr Shropshire said: 'For these crops, we have got to have water every day, not just when the Environment Agency allows us. If not, we would be finished.

'That is where it is so important to increase our water self-sufficiency. It is like being in a factory and having a generator on standby in case the power fails. It is like a back-up.

'We have spent millions on land levelling and reservoirs to create that possibility. What really opened my eyes was in 2012. There was a hosepipe ban in March and the drainage boards were warned that there could have been a total ban by the end of May. That would have been a total disaster for us, but then thankfully it started to rain.

'I don't want to be in that vulnerable position again. It would affect our whole business if we were to let the supermarkets down. I have had to invest more than £1m that we might never need to use – but how can we afford the risk of not having those reservoirs there?'

The 70 million gallon reservoir, part-funded by the RDPE, was completed in October and filled during the winter.

Mr Shropshire said although the firm has not been able to expand its abstraction licenses in recent years, its annual cropping has expanded from five million to 60 million plants in the last 30 years.