0 Drainage - the least understood of all the turf system operations

NZSTI SoilSampleFrom my experience drainage is one of the least understood of all the turf system operations. It never ceases to amaze me how even seemingly experienced turf industry personnel struggle to understand the fundamentals of sports turf drainage. The following reviews a few of the more common drainage myths.

"The drainage problem is due to a high water table (the myth of the underground 'lake')"

Reference to the mythical underground lake is commonplace. When things are wet, the water table is said to be high, but what is really meant is that the soil pores are completely saturated. True, there are genuine water tables that rise up to the surface to create drainage problems. However, these generally occur in extremely low lying areas and on soils that are permeable to depth (e.g. deep sand profiles), and are not that common on sports fields. So, unless you are on a low lying site, it is unlikely there is a rising water table.

"The drainage problem is due to collapsed or blocked pipe drains"

When a sports turf drainage system fails, the underlying pipe drains become the number one suspect. However, this is seldom the case. It is probable that the drainage capacity of the pipe drains may have declined with time, but it is almost certain that sufficient drainage capacity remains to cope with whatever water actually gets to them. Instead, drainage failure is invariably due to the very poor rate of water transmission through the soil to the installed drains. In the case of a sand carpet, the problem is generally due to organic matter accumulation stopping water movement through the sand carpet. Knowing the cause of the problem is essential - it is no use wasting money on a solution that won't actually work.

"Installing pipe drains is the only way to solve drainage problems"

In some situations, pipe drains may not be necessary to solve a drainage problem. A much less expensive option (for example, controlling earthworm activity or deep cultivation) may be all that is required. Don't assume pipe drains are required. The cause of the problem and the profile characteristics must be understood to ensure the correct "solution" is applied.

"Pipe drains exert suction that pulls water out of the profile. Therefore, drainage makes a soil more droughty"

This assumption is wrong - drainage of excess soil water is simply due to gravity, nothing else. Removing excess water from the larger (macro-) pores provides soil aeration and improved soil and plant health which, in turn, can actually improve the resilience and drought tolerance of the turf. Of course, there are exceptions to this, such as drainage in peat land areas, where over-drainage can lead to rapid oxidation of the organic matter and surface subsidence.

Optimising the investment in sports turf drainage

First, respect the fact that every site is different and, as such, there is no such thing as a standard recipe for drainage improvement. It is important to seek professional assessment of the site at the outset in order to identify the potential sources of excess soil water (e.g. runoff, rising water table, perching, over-irrigation), to identify site features that will impact on any drainage improvement (such as available outlets, depth to irrigation piping, landfill capping depth, etc), and to recommend an appropriate solution.

Second, we need to recognise that the most common limitation to drainage performance in sportsturf is perching of water over a barrier layer. In other words, excess water cannot freely drain through the soil profile. This barrier layer could be at or near the surface (such as in a silt loam soil that has become capped due to traffic in wet conditions), within the rootzone (such as a compacted layer), or could be below the rootzone (such as a sportsfield built over a clay or clay loam sub-base). Most commonly in sports turf the cause of the problem is close to the surface, usually within the top 200mm of the profile.

Where a barrier exists, it is possible to estimate how much rainfall can be stored before the soil becomes waterlogged. The table below provides an indication of what could happen in an undrained soil with a barrier layer.

As expected, data shows that the greater the depth to the barrier layer, the more water can be held before waterlogging occurs. However, if rain continues, the reservoir above the barrier layer will eventually fill and waterlogging will prevail.

Once the reservoir is full (i.e. the soil is saturated), ideally no play should occur until the soil dries. If water can't drain from the profile because of the barrier layer, the only way for water to leave the profile is via surface evapotranspiration. In the winter, this is minimal - as little as 1mm per day.

ReturnToPlayA third point to note with sports turf drainage is that the key to successful improvement is closeness of drain spacing, and not the size of the pipes used. In other words, the limiting factor is invariably the passage of excess water through the soil to the drainage system. Given that the rate of excess water movement through a soil is generally very slow (there are exceptions such as soil profiles that have layers of sand or stone), we cannot expect excess water in the soil pores to gravitate to a pipe drain that is many metres away - at least not quickly. It is the often painfully slow rate of water movement through soils and seasonal fluctuations that give rise to some of the drainage myths. In sports turf, the problem is almost always that the water can't get to the drains quickly enough. It is, therefore, not surprising that we have seen the progressive development of machines that provide close-spaced, narrow width drains, such as sand slit drainage, sand banding etc. The objective with these systems is to provide flow paths for excess soil water through to an underlying free-draining layer or pipe drainage system. The drains are close together so the water doesn't have to travel far to reach a drain.

The final point I wish to make is the need for ongoing maintenance of the entire system. This isn't just the pipe drains - it is everything between the surface and the pipe drains. A well-constructed pipe drainage system should last indefinitely. However, the overlying slit drains, sand bands, sand carpet or topsoil layer, can deteriorate if not looked after. The most likely cause of failure is surface capping (organic matter or silt) in the sand carpet and/or over the slits/bands, thereby restricting water entry. Earthworm activity and capping is a common cause of slit drain system failure. Preventative maintenance in this instance would include periodic sand topdressing to maintain infiltration and, where required, earthworm control.

Given the climate, coupled with the fact that a high percentage of sports fields were built out of inappropriate materials, it is inevitable that drainage problems will arise from time to time. But, no soil is undrainable. Having a sound understanding of the cause of the excess soil water problem is the starting point to deriving a cost-effective solution, and providing a sports field that offers, effectively, all-weather performance. Forget the myths of rising water tables and blocked drains - these are unlikely to be the cause of drainage problems. Instead, the problem is much more likely to be due simply to an inability of water to move quickly enough away from the surface. That is the problem that has to be solved.

Article by kind permission of the NewZealand Sports Turf Institute. www.nzsti.org.nz

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