Extreme symptoms of failing or failed drainage have been common this year. Many fields suffered ponding, water sat in tramlines for weeks and plenty of combines have been stuck up to their axles this harvest.

Hidden damage caused by crops sitting in soaking soils for weeks on end is also likely to affect yields this season. And waterlogged soils have been as much to blame as unripe crops or wet weather for hindering harvest progress and delaying subsequent cultivations.

This year has certainly highlighted the need for good drainage, says Kirk Hill, soil and water engineer at ADAS. However, that does not mean vast swathes of the countryside need to be re-drained – far from it, he believes.

Almost all soils that would benefit from drainage have been drained at some point over the past couple of centuries, Mr Hill says. Much was re-drained in the 1960s and 1970s, thanks to 60% grant aid and free impartial advice from the Ministry of Agriculture.

Despite its age, he believes much of this infrastructure is still fit for purpose. “It has been 30-50 years since many of our modern systems were installed. Whether they are clay or plastic, these plus many more historic schemes would still function with better maintenance.

“I suspect only a relatively small percentage of the total drained land needs a brand new comprehensive scheme – the most likely being schemes that predate the 1960-70s, depending on land use requirements.”

Ongoing research suggests decreasing performance is usually due to a lack of simple maintenance, such as keeping outfalls clear and maintaining ditches, he says. And some mole drainage schemes have not been re-moled for 15-20 years, when 5-10 year intervals would be more appropriate.

“As the lifetime of a drainage system is so long, its value, purpose and, importantly, its maintenance is often forgotten.”

Drains can also become blocked without regular jetting, and tiles can sink and pipework can fail if compacted, for example by a bogged-down combine.

Good maintenance involves walking the farm, says Mr Hill. “Keep a record of the drainage system and outfalls. Inspect these outfalls and ditches regularly. Look at the condition of the soil surface. And look at conditions when working the fields – not so easy these days with bigger, more powerful machines.

“Be aware of ground conditions or crop health deteriorating over the years, localised slumping, or water rising to the surface. If you suspect a problem get a spade and dig holes to inspect soil conditions.

“It’s also worth remembering that drains won’t do anything if you don’t manage your soil structure well. Subsoiling and avoiding compaction will allow drains to do their job efficiently.”

Good drainage can increase the length of the growing season by up to six to eight weeks,” says Mr Hill. “This can help the farmer save money, increase his cropping options and reduce the risk of failure at harvest.”

Tim Sisson, managing director of Norfolk-based land drainage contractors William Morfoot, agrees that maintenance is the key to longevity with any drainage scheme. “Some schemes of 50-60 years are still working well. But they need management,” he says.

While some problems may be cured by jetting, unblocking outfalls or ditching, when it comes to more remedial work he questions how far it is worth going before opting to renew.

“You can spend a lot of money investigating older schemes and patching them up. And you can end up with a system of varying ages that’s a headache to manage.”

He believes some schemes put in during the height of the drainage boom may not be worth repairing. “They will be reaching the end of their 30-35 year life expectancy. And in the days of grant aid many were installed by contactors intent on putting as many metres into the ground as possible.

“Some were good, many weren’t. Some drains were put in with no stone, or not enough. Heavy clay subsoils must be filled to within subsoil or moler depth otherwise water won’t get down. And the equipment used is nothing like that available today, which can install pipe milimetre by milimetre along a gradient.”

Mr Sisson believes the best approach is a continual programme of improvement so the whole farm is re-drained over the life expectancy of one scheme. “Given all the downsides of poor drainage, when you work out the annual cost it pales into insignificance,” he says.

What’s involved?

The standard system uses plastic pipe at 20m spacing and 800mm deep installed by a trenching machine, backfilled with stone to just above subsoiler depth.

This will cost £2,000-2,500/ha, depending on scheme layout, field size and shape, soil type and quantity of backfill required. That costs £80-100/ha assuming a 25-year lifespan on Fen soils and about £60-70/ha for the 35 years of service expected on stable clay soils, Mr Sisson points out.

Where gradient and subsoil (uniform with more than 41% clay) allows, a cheaper option is to widen pipe spacing to 40m or even 60m and create mole drains at right angles in the subsoil. These should pass through the backfill so water can quickly enter the main pipes.

“However, you have to redraw the moles every five to seven years, more frequently in a run of wet winters, so their condition needs to be monitored,” says Mr Sisson.

Drainage revamp pays off

As a 10-year cleaning programme on an abandoned ditch network nears its end, Jon Parker needs no reminding of the value of maintenance in keeping drainage systems working efficiently.

About 1,200ha of the 1,500ha he manages at Ragley Home Farms, near Stratford-upon-Avon, Warwickshire is medium to heavy land growing wheat and oilseed rape.

“It all has a drainage system of some description under it,” he says. “About half was re-drained with plastic in the 1970s when grant schemes were available and the rest is tile drainage of varying vintages.”

When he arrived on the farm a decade ago the ditches had not been touched since the plastic pipe was installed. “I spent the first winter going round with a drainage map locating outfalls. I wondered at first why they were so hard to find. I soon realised they were all hidden behind a good foot or two of soil.”

Mr Parker implemented a ditch-cleaning programme immediately, and a 360° digger has been set to work cleaning 2,000-3,000m of ditches each winter for the past 10 years. The heaviest, Keuper marl soils have also been mole-drained. He has no doubt the mammoth task is paying off.

“The five-year wheat yield average on the farm was 2t/acre when I took over. It is now 3.4t/acre. A lot of that must be down to better drainage and the effect it has on soil structure. It is amazing how the land dried out. Crops are not sitting in soaking soils and are putting roots down, mining nutrients much more effectively.

“It has also done wonders for timeliness – for example, before we started ditching we couldn’t travel on many areas with a sprayer during the winter months. I doubt we would be as far on with this harvest either, and we probably would have had to abandon a few crops.”

The fact that the existing systems still function so effectively reflects their quality, he says. “They were installed well – the plastic system was put in properly at 20m centres with plenty of stone backfill. It was also documented and mapped accurately.”

A few tile-drained areas have suffered from neglect, with some drains blowing in the fields due to blocked outfalls. “The ditching was the most urgent job. Now we have nearly completed that, we will put the digger to good use revamping these problem areas.”

A further 80-120ha need renewing completely. “Some fields are showing their age and we’ll soon be experiencing new problems. We will have to employ contractors on an annual basis with a view to replacing all this area in the next few years.”

Ditching will also remain a regular job. “It’s time to redo where we started 10 years ago. It will be a lot quicker this time around and there’s no doubt it will be worth every penny,” says Mr Parker.

Good drainage – the benefits

• Better yields from healthier growing medium
• Aerobic conditions allow stronger root growth to maximise nutrient access
• Drier, warmer soils boost germination and emergence
• Lower input costs – crops less prone to weed competition, pests and disease
• Wider window for cultivations and other key operations
• More timely operations
• Reduces operating costs
• Less soil damage
• Reduces peak surface flows, silting and pollution
• Increases land value