It’s not always there when you need it, but rainfall typically results in more even soil moisture, greater turf health and improved growth rates.
After a significant rain event during a dry summer, many turfgrass practitioners will ponder the relative merits of rainfall versus irrigation. The benefits of rainfall are definitely greater. This is mainly due to lower plant water loss, greater uniformity and more consistent soil wetting during and immediately after the rainfall event. Other factors can be of benefit depending on the venue (i.e. water sources, irrigation systems and soil types), but these have a relatively minor impact in comparison with the 4 main advantages.
The Big Four Advantages
1. Minimal Water Loss at the Time of the Rainfall Event.
Water loss encompasses both the water from inside the plant (transpiration) and the soil surface (evaporation) i.e. evapotranspiration (ET). This is largely driven by:
- Increasing sunlight.
- Increasing sunlight.
- Increasing temperature.
- Increasing wind speed.
- Decreasing humidity.
When a significant rain event occurs in summer, sunlight is reduced, humidity rises and the temperature can often decrease. This combination significantly slows the rate of water loss.
The weather after rain can often be overcast, slowing any surface drying.
Turf showing signs of a long, hot, dry summer of irrigation.
Turf response to rainfall.
2. Excellent Water Uniformity
No irrigation system has perfect water evenness/distribution. The higher quality systems apply water well but they still cannot compete with typical rainfall.
Irrigation and rainfall uniformity can be measured by a “catch can” study and expressed by their Co-efficient of uniformity (Cu%), Distribution uniformity (DU%) and Scheduling co-efficient (SC). Thus, ideal values can be set and irrigation systems compared. The higher the Cu% and DU%, and the lower the SC, the better the irrigation system (see Table 1).
Rainfall is typically very uniform (practically 100% in localised areas), despite some wind. However, as the wind speed increases, irrigation evenness declines.
Table 1: A comparison of watering uniformity between typical irrigation and natural rainfall.
Data shown is indicative only.
3. Lower Application Rates
Most turf irrigation systems typically apply water at rates in the range of 8-16mm/hr. These are much higher than typical rainfall events (i.e. <5mm/hr). Where water lands on turf at higher rates, there is a much greater risk of ponding and runoff, with subsequent uneven infiltration into the surface and wetting up of the root-zone. As the slope of the ground increases, so too does the risk of water running down and away from its intended target, resulting in uneven penetration and soil moisture. Wet and dry areas can then develop.
Compared to normal rainfall, irrigation’s higher application rates over shorter timeframes also increase the potential for runoff and uneven soil wetting/rewetting (see Table 2).
Table 2: Typical irrigation application and soil infiltration rates.
Wind disrupts how evenly irrigation is applied to turf.
4. Better Soil Wetting
Soils, particularly those that are sandy or top-dressed with sand, are more susceptible to uneven wetting and localised dry patch
(hydrophobic soil). When the surface becomes very dry, both soil and sand root-zones can become crusty; meanwhile sand top-dressed areas can also become water repellent which makes rewetting even slower and generally uneven. With irrigation the dry areas get drier and the wet areas get wetter. Rainfall typically falls at a lower rate over a longer period of time (and is not affected by wind), so it is far better at wetting/rewetting soils than irrigation. As the bulk of the cool-season turfgrass root system is near the surface (0-40mm), a rapid and significant moisture response is likely to be seen in the turf growth and health. It “perks up”, looks better and we noticed more clippings when it’s mown.
Uneven soil wetting.
All Together Now!
When it rains, water falls on the turf very evenly and at a much slower rate than via normal irrigation cycles. At same time, turf water loss slows. As the surface stays wet for longer, it increases the potential for even re-wetting of the surface. This typically results in more even soil moisture, resulting in greater turf health and an increased growth rate.
Brendan Hannan
Agronomist
New Zealand Sports Turf Institute