Planning farm drainage

Waterlogging can be a problem over winter and early spring in the high rainfall zones of Victoria. In wetter than average years, this period  can last several months.

Waterlogging occurs when rainfall exceeds the ability of some soils to drain surplus water away. It is often seen that waterlogging is a surface water problem that surface drains will overcome. But in many situations waterlogging is due to the soil profile below the ground surface being saturated. Some type of subsurface drainage may be necessary to overcome this problem. Unfortunately, some soils and areas, because of their location, cannot be economically or feasibly drained by any means.

Damage caused by waterlogging

Waterlogging reduces the strength of the soil, making it vulnerable to pugging, which leads to pasture damage that results in lost production and repair costs. Soil structure can be severely affected on some soil types. Unfavourable anaerobic conditions occur in waterlogged subsoils that leads to a buildup of toxic hydrogen sulphide and reduced iron and manganese. It can also cause reductions in potassium, chloride and magnesium in plants.

Weeds and pastures that are more suited to waterlogged situations take over and summer production is decreased because of reduced rooting depth of surviving pastures. Another costly outcome of waterlogging is that fodder conservation is delayed by several weeks resulting in poor quality fodder, less dense pastures and even more weed growth.

Planning a drainage system

The aim of farm drainage is to improve the soil environment to provide favourable growing conditions in the root zone for high-producing pastures and crops. To be successful, you must consider many factors when assessing which drainage system to use, its design and its implementation.

There are some key questions to ask that will help you plan farm drainage:

• What is causing the waterlogging problem?
• What is the frequency of waterlogging?
• Is there a sufficient outfall available?
• What are the likely benefits of draining this area?
• Which areas should be drained first?
• What type of drainage system is required?
• Do I need assistance to develop a plan?

Cause of the waterlogging problem

• Is the waterlogging due to surface water, subsurface water or both?
• Is this water flowing from upslope area from the ground surface or subsurface or both?

Installing a subsurface drainage system is very expensive, so excess surface water must firstly be drained with a surface drainage system.

Frequency of waterlogging

The frequency and duration of waterlogging will influence the type and cost of the drainage system.

Does waterlogging happen:

• every year
• only in a small area or short periods in most years
• only in very wet years

The answer will dictate the priority and expense needed in designing and implementation of the necessary system.

Outfall available

This a most important factor to consider. If water cannot be drained by gravity to an outfall then it must be delivered to a sump and then pumped to a main drain, adding substantially to the cost and maintenance to the system.

In some areas where the land is relatively flat, there are aren't enough main drains either through paddocks or on roadsides to take away excess water quickly.

A further complication is that often these drains are not deep enough, which limits the ability of the farmer to drain land on his own property. If this is the problem then a possible solution may be that a group of landholders may agree to install a drainage scheme. This may enable a sufficiently deep and large main drain system to be installed to everyone's satisfaction and needs at a shared cost. Agreement must ensure proper maintenance to the entire drained area and outfalls and that no harm is done to the environment.

But on many farms there is enough fall to have adequate outlets so that internal drainage systems can function properly.

Benefits of draining

Consider how productive this area will become once drained. If the soil type is poor, even if drained, improved pastures may not grow to their potential. However, maybe just draining a small area can allow faster or easier access to other sections of the farm. Would draining a spring line or preventing uphill water from gaining access to the lower areas increase productivity in the protected areas?

Areas to drain first

Which area should be drained first: the paddock on the flat that floods (which may potentially be the best paddock), or the ones higher in the landscape that are known to allow water to run off easily?

If the higher paddocks are drained first then smaller main drainage pipes (or drains) will give initial cost savings. Draining the paddocks closest to main drain or a creek outlet generally requires having to put in larger mains so that when the scheme is extended upslope, the existing pipes (or drains) can cope with additional drainage water.

Type of drainage system required

There are 2 basic types of drainage systems:

• surface
• subsurface

Surface drains

This is the first thing that should be looked at in any drainage scheme. Surplus water running over farmland will keep it saturated for longer, reducing pasture growth and increasing problems such as pugging and tractor mobility. An area should only have to deal with the rainfall falling on it, not also the rain that fell on the areas further upslope.

Therefore, adequate surface drains need to be installed along fence lines, laneways and depressions, so that surface water can be controlled more effectively. Take care to ensure that potential problems such as scouring and erosion are not created by concentrating flows.

In some cases surface drains will need fencing off so they can be protected from stock and tractor damage.

Subsurface drains

Once the extent of the contribution of surface water and its potential for drainage is determined, the next step is to determine which subsurface drainage system is best suited. The basic subsurface systems are subsurface pipes, mole drains or variations or compilations of the 2 systems.

The drainage system should, ideally, remove excess soil water to a depth of about 30cm within 24 hours of the rainfall event. Ideally a drainage expert should be used to help design any drainage system that involves pipes. The final design of the drainage system greatly influences both efficiency and final cost of an installation.

To do this requires knowing:

• land gradients
• soil types
• soil profiles
• clay content and types
• permeability of the soil to be drained
• cost and amount of backfill required for some systems
• accessibility to drainage contractors
• the typical annual rainfall amounts and likely rainfall events (in millimetres)
• area to be drained
• location and depth of outfall(s)
• pipe diameters and lengths

Assistance developing a plan

An experienced drainage contractor can be most helpful in helping to develop a plan of works (the drainage plan). If large areas need draining then, ideally, the plan should cover 3 to 5 years of works. Joint discussions will produce possible layouts and the best sequence for carrying out works.

Plan ahead

Don't expect the drainage contractor to sort out for you a properly designed system in the middle of summer. Get in touch with the contractor when there is a problem (winter) so they can inspect the problem and discuss the options with you more thoroughly. This will give you plenty of time to organise finance and set priorities, and fit into the contractor's work schedule.

Other actions needed as part of a drainage plan

Other changes might also be needed to support the drainage plan, such as:

• altering paddock layouts to allow more efficient mole drainage installation
• re-laying water supply lines and altering trough locations to fit in with drain placements. As mole drains are likely to be redone about every 5 years, it is best to make sure the other pipes are located to minimise potential damage and save time and money.
• fencing drains off to protect outlets and minimise erosion, while allowing sufficient space for maintenance
• bigger bridges or culverts on main drains to allow for the increased rate of water removal

These changes will all have associated costs that need to be included in the budget.

Other capital investment into farm infrastructure may also be needed to capture the benefits of drainage. For example, if drainage improves pasture growth, then more cows might be needed to harvest this extra growth. It is unlikely to be profitable to drain paddocks and not increase cow numbers even if supplementary feeding is reduced. So, additional questions are:

• Can more cows be milked comfortably in the shed?
• Will more labour be needed?
• Is a bigger milk vat needed?
• Do the laneways need to be widened?
• Will the ability to harvest silage at the right time make it more attractive to invest in silage gear?

Even after putting a physical drainage system in place farmers (especially those with clay soils) will need to consider loafing-pads or feedpads to keep cows off drained paddocks during and immediately after rain events. This will protect the soil from pugging damage and compaction, which will reduce the effectiveness of the drainage system.

What are the non-drainage options?

If prolonged wet periods are not common simply adopting an on-off grazing strategy to reduce pasture damage caused by pugging and compaction is a relatively cheap option.

The more severe the waterlogging, the better the stand-off facilities must be to cope with animals being off pasture for longer periods. This may require building a feedpad or loafing-pad. Extra machinery such as feed out carts may be needed as well.

If these are used on a regular basis then attention to effluent management is vital to:

• use nutrients the cows would normally have put on the paddocks for you
• keep the effluent on the farm and out of waterways

The effluent system may need to be enlarged or redesigned.

Other strategies to reduce pasture damage include:

• grazing several shifts in a day
• standing on laneways or cow yards
• having separate day and night paddocks

Water Act

The Water Act 1989 provides guidance for the management of waterways and swamps. Before considering draining a wet area, contact your local catchment management authority for advice and to check if you need a permit.