Drainage work does not come cheap. It is a significant investment on all farms that must pay for itself in a very meaningful sense.
In the opinion of Teagasc soil hydrology specialist, Pat Tuohy, this means that an investment in improved drainage must generate a worthwhile return for a substantial number of years.
If it is simple a case of going in and fixing a problem that might only arise one year in 10, the question then arises – is a new drainage scheme worth the investment?
He makes this assertion during his contribution to the current ‘Tillage Edge’ podcast.
According to Tuohy, there isn’t a field in Ireland that cannot be drained, if there is enough money spent on it.
“But that doesn’t justify doing it unless there is going to be a payback,” he added.
Good drainage on tillage ground is essential to enhance crop output. This is a simple reflection of our wet climate.
Farmers having knowledge of the topsoils and sub-soils on their land is essential when it comes to the determination of drain maintenance and, if needed, the installation of a new drainage system.
The objective is to ensure that land will drain water in a normal year but also at those times when excessive rainfall can become a challenge.
Ponding in the middle of fields has become an issue over recent weeks. So how does this reflect on the drainage systems already in place at these specific locations?
“So the fundamental issue becomes – is the current system appropriate?” Tuohy continued.
“It may well be perfectly adequate in years like 2022 or in most other seasons. But how do we determine its effectiveness in years like 2023? As a rule of thumb, a well designed drainage system can cope with 12-15mm of rain per day.
“This is well above the average rainfall level that we would ever expect to get.”
In contrast, a less effective drainage system might only be dealing with 4-5mm of rain on a daily basis.
“This is fine in a relatively dry year. But it’s well under the capacity required for a year, the likes of which we have just come through,” Tuohy added.
Soil type and profile
The Teagasc soil specialist stressed the need for drainage systems to suit the specific soil type within a field.
“Understanding the deeper soil profile is the forerunner of designing a drainage system,” he explained.
“The aim of a drainage system is to remove ground water. Or if there is permeability issues throughout a soil profile, then it’s case of putting in place a shallower system, using mole drains or sub-soiling.
“There are different approaches that can be taken, depending on location and soil type.”
Most Irish tillage fields have a topsoil that is of adequate quality. However, this may not be the case where sub-soils are concerned.
The make-up of sub-soils has a fundamental impact on the natural drainage capacity of fields.
“Drainage profile pits should be dug to a depth of at least 2m,” Tuohy continued.
“Getting to this depth is important to see what the issues are. Only in this way is it possible to see where water is being held up.
“In cases where heavy clays of grey marls predominate, there may well be difficulties in getting water down through a soil profile.
“In cases like this, a form of shallow drainage system will be required. This means establishing field drains at a metre depth and criss-crossing these with a mix of mole drains and gravel tunnels.”
Deeper drainage work
According to Tuohy, deeper drainage systems work when soil profiles allow the relatively free movement of water down to the required depth. Having a suitable drainage outlet is also required in these instances.
“Different drainage methods are suited to different soil types,” Tuohy commented.
“But getting the depth of the installed drains correct is very important. Teagasc updated its drainage and soil management manual in 2022.
“It is an obvious starting point for farmers seeking information seeking to establish a new drainage system at the present time.”
Cost benefit analysis
Deciding to commit to new a new drainage scheme can be likened to a process rather than a ‘here and now’ decision on the part of farmers.
“If the issue of drainage capacity arises in a year like 2023, only then can a case be made for letting matters sit,” Tuohy said.
“However, if problems continue to arise at the same location over a period of time, then an investment in improved drainage system should be considered.
“In terms of springs appearing in fields, then this too can be related to the weather over a specific period of time.
“Springs can also appear during the winter months but then disappear as the water table drops once we get into the spring and summer period.
Springs are a very site specific issue. It’s all about getting depth into the drain and getting under the spring, according to the Teagasc specialist.
“It’s a case of providing the spring with an easier exit from the site,” he said.