When turf managers fully understand how much water the root-zone can store, they may develop the confidence to change from daily to less frequent irrigation.

A fundamental principle of irrigation taught to turf management students is to water deeply and infrequently. In reality, this rule is hardly ever abided by when automatic irrigation systems are present. The usual practice inevitably involves daily irrigation with the amount of water applied loosely based on the published evapotranspiration rate.

It is difficult to understand why this should be the default position because daily irrigation is only required where the root depth is very shallow (<10mm). Such a situation can occur, but is very unusual.

Useful Concepts Explained

When scheduling irrigation, it is helpful to understand these concepts:

Root-zone

For irrigation purposes, the root-zone is literally the effective rooting depth, i.e. the depth of the soil profile that the turf plant
is able to extract water from. This is often quite different from the depth of root-zone material (e.g. root-zone sand) that was used to construct the profile in the first instance. The only way to determine the root-zone depth for irrigation purposes is to take core samples from the profile, and then observe and measure the root depth.

Field capacity

When a root-zone is at field capacity, it is holding the ideal moisture content for plant growth. This means that all water-holding pore space is water-filled, and all air-holding pore space (the larger pores) is air-filled, allowing the plant roots to breathe.

Immediately following heavy rainfall or excess irrigation, the root-zone will be saturated, i.e. above field capacity, because most of the pore space will be water-filled. But then gravitational drainage will occur and water will drain from the large pores and be replaced by air. In practice, it takes about 24 hours for this to occur, following the end of rainfall or irrigation.

If irrigation is being applied daily, gravitational drainage can’t be completed. At least part of the root-zone (the upper layer) is being maintained above field capacity at all times. The corresponding lack of air in the root-zone places stress on the plant. The volumetric water content at field capacity is soil-type dependent. For a silt loam, it will be around 32% (this can be quickly and accurately measured with a hand-held moisture metre).

Permanent wilting point

This is a specific moisture content. It is reached when the plant is no longer able to extract any moisture from the soil and hence wilts. To the naked eye, the soil will be completely dry but some moisture will still be present. The volumetric water content at permanent wilting point for a silt loam will be around 8%.

Plant available water

This is the volume of water that is theoretically available to the plant, i.e. the difference between field capacity and permanent wilting point.

A soil at or close to permanent wilting point. While it appears dry to the naked eye, the volumetric moisture content measured by the moisture meter is 9.0%.

When to Irrigate?

The rule of thumb is that irrigation should be applied when about half of the plant available water has been used. For example, based on the typical, silt loam:

• Field capacity = 32%.
• Permanent wilting point = 8%.
• The difference between these two is 24% (= plant available water).
• Apply irrigation when half of the plant available water has been used (half of 24% = 12%).
• Therefore, irrigation should be applied when the volumetric water content has fallen to 20% (32% – 12%).
• At that point, sufficient water should be applied to restore the root-zone water content to field capacity (= 32%). The amount of water to apply (in mm) is 12% of the rooting depth (also in mm).

How Much Water

The amount of water required to return the root-zone moisture content to field capacity will depend on the root depth, i.e. the greater the root depth, the more water required. In the example above, the amount of water to apply (in mm) is 12% of the rooting depth
(also in mm), therefore:

If the root depth is 100mm:

• 12mm of water needs to be applied to return the root-zone to field capacity (12% of 100mm is 12mm).

If the root depth is 200mm:

• 24mm of water needs to be applied to return the root-zone to field capacity.

Note that these water requirements are much greater than those that would be applied by daily irrigation (which, if based on published evapotranspiration rates, would be around 5mm). Hence the principle of deep and infrequent irrigation is to allow a “safe” root-zone moisture deficit to accumulate, before applying the necessary amount of water to restore the root-zone to field capacity.

How Has the Daily Irrigation Become the Norm?

With automatic systems, daily irrigation is easier than the more correct, deep and infrequent irrigation. It is simpler to set a programme that will water everything daily (relying on a rain switch to skip irrigation if significant rainfall occurs), than it is to set (and reset) a programme based on irrigating once every few days once a significant root-zone water deficit has been allowed to develop.

The irrigation water supply is a major influence. Typically, turf irrigation systems and water supplies are designed to be able to replace the daily water loss to the entire irrigated area overnight (e.g. entire golf course or all of the sportsfields at a sports park). Therefore, it is not surprising that the default approach is to do just that.

The thinking behind the design is entirely correct – the system has to have the capacity to replace daily losses. But there is nothing that says that a little bit of irrigation has to be applied daily to each area. Instead, irrigation of various areas can be staggered over several days, so that greater amounts of water can be applied less frequently to the different areas.

For example, a school might heavily irrigate its 4 sportsfields sequentially over 4 nights (i.e. field 1 on the first day, field 2 on the second day, and so on).

If the root depth is good and the correct amount of water is applied to achieve field capacity, it may then be 10 days before the root-zone moisture content has fallen to the point at which the irrigation cycle needs to be repeated. In that case, 6 days after field 1 was irrigated, the irrigation cycle would start again. Clearly this is more complicated than irrigating everything a little every day, and this is probably the main reason why daily irrigation is standard practice.

When using a moisture meter to schedule irrigation, it is important that the probe length matches the root depth as closely as possible.

Irrigating as Required

The ideal approach uses a moisture metre and the following steps in determining when to irrigate:

• Take cores from the turf root-zone and measure how deep the roots are (the probe length on the moisture meter should match the root depth as closely as possible).
• Take cores from the turf root-zone and measure how deep the roots are (the probe length on the moisture meter should match the root depth as closely as possible).
• Use a moisture meter to determine the water content of the root-zone at field capacity (e.g. measure 24 hours following the end of heavy rainfall in mid-spring).
• Allow the root-zone to dry out during spring. As soon as any areas dry to the point where moisture stress is visible (wilt occurs) and the soil appears dry, measure the soil moisture content (this will be close to the permanent wilting point).
• Apply irrigation when the moisture content has fallen to halfway between field capacity and the permanent wilting point.
• Based on that and the root depth, work out how much water needs to be applied (in mm) and, hence, how long the irrigation needs to run for to restore field capacity.
• Apply the calculated amount of water and, 12 – 24 hours later, measure the root-zone water content. If it is less or more than your targeted moisture content (field capacity), adjust the amount of water applied next time.
• In addition, take and observe core samples to ensure that uniform rewetting of the root-zone is being achieved.

Developing Confidence

Daily irrigation tends to be standard practice. In all probability, it results in too much water being applied and root-zone moisture levels being maintained above field capacity. The consequent lack of air in the root-zone creates stress on the plant and increases susceptibility to disease. Conversely, if deep and infrequent irrigation is correctly implemented, less water will be used and plant health/turf quality will improve.

Once turf managers understand how much water the root-zone can store and supply to the plant, they can develop the confidence to move from light, daily applications to deeper, less frequent irrigation.

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