Over 100 farmers gathered near the small village of Ballinagree about 15 minutes North East of Macroom last week to hear how Con Lehane manages draining land.

Con Lehane is a farmer involved in the Teagasc Heavy soils program and he carried out drainage to the value of €5,500 on a five-acre field. The field in question is a south-facing field located half way up the side of a hill, within walking distance of his milking parlour. There were already open drains on three sides and at one stage there were drains on all four sides of the field (see Figure 1).

Like all drainage jobs, the first step is to dig a test hole or two. The hole allows you to see the different soil layers and to develop a plan around the depth drains need to be.

Pat Tuohy reminded us that drainage can be broken down into two main methods – (1) where the water lies on the surface and can’t get down through the topsoil, and (2) where water can get down through the topsoil but the layers underneath prevent it from going anywhere else or at least slow the passage of the water down.

In Lehane’s case, the rainwater was getting down through the topsoil but groundwater moving downslope was maintaining a shallow water table and slowing down the surface water getting away or down through the soil layers.

When the researchers dug the test hole at the top of the field, they were almost 1.7 metres down before water started to seep into the hole. When they dug a test hole at the bottom of the field (lower down the mountain) the water started to come into the test hole when they were only a metre down.

So the plan for draining this field was to deepen the water table so that whatever rain fell on the field would percolate more quickly through the soil and move away with the ground water.

The focus of this drainage project is not mole plough-type drainage to take surface water away – it’s improving groundwater drainage to lower the depth of the groundwater table which would indirectly help water get down through soil layers.

Researcher Owen Fenton explained the information gathered from the test holes showed the field had a consistent layer of permeable soil about 1.5 metres below the surface. The plan was to exploit this as much as possible by installing drains across the slope, not down the slope, and let the water exit at an outlet at the lowest point (Figure 2).

The first step was to deepen the open drains at the top and side of the field. They were only 0.3 to 0.9 metres in depth and this wasn’t deep enough to tap into the layer of highly permeable soil that they saw in the test holes. If the plan was to install drains across the slope at 1.7m depth then the open drains collecting at the side needed to be around 2m deep.

For many farmers it takes a leap of faith to bury drain pipes almost 2m down in the field and expect it to improve drainage. Many farmers would prefer to put in the drains a metre deep and fill them up to the surface with stone, but according to Pat Tuohy this is where many farmers go wrong.

He said: “You need to assess each field separately and make the right decision to help drainage or it can be a complete waste of time and money. From the test holes in this field we can see that there is a nice layer of soil that allows water to pass through easily about 1.5 to 2 metres deep, so if we can improve the flow of water in this layer by putting in a pipe and some nice washed stone it will take the ground water from the field allowing the upper layers to drain out and stay in relatively good shape.”

Soil type

At first glance the Lehane soil doesn’t look too heavy (see video). It’s not your typical heavy gley soil that won’t allow water down. OK, it’s not your brown, free-draining clay, but it’s fair to say the soil type is moderately permeable with some stones and loose material that would allow water to get down into ground water streams.

Drains around the perimeter were cleaned and deepened to two metres and then internal drains across the field were installed every 20m to a depth of 1.7m. On this job, 80mm corrugated yellow drainage pipes were installed on a bed of stone and covered by 0.3m of stone.

Field drains were installed in two stages in June 2013. Firstly a 1m deep trench was dug using a wide moulding bucket and then a narrow tile drainage bucket was used to complete the job. Each drain was installed and backfilled immediately. Only 0.4m of stone was filled in over the soil and then the drain was backfilled.

Last week, farmers asked why not put more stone in over the pipe. Owen explained this would drive costs sky high and there was no need. He said: “All you want to do is have enough stone to make the connection with the permeable soil layer to improve the flow out from the field.”

Con’s farm is located between 500 and 1,000 feet above sea level in an area of heavy rainfall (about 1600mm annually).

Con and his father Neilie milk 100 cows at the moment and they plan to grow the herd to 150 cows and feed them by growing more grass and ensuring fields are drained so they can utilise grass better.

The big cost of the drainage project was opening the drains and installing new drains at €45 per hour for 73 hours or €3,285. Drainage pipe cost about €525 and stone was €13.87/t (118t in total) or €1,640.