Most sports fields need some kind of supplemental irrigation to get them through dry spells or periods of intense use. Even fields located in parts of the U.S. that get up to 40 inches of rainfall each year can still have spells six to eight weeks long during the summer without rainfall.
A clogged sprinkler head results in uneven watering.
PHOTOS COURTESY OF PAM SHERRATT.
Supplemental irrigation is used to maintain turf growth, but at the same time can help to encourage new seedling or sod establishment; lower canopy temperatures during intense heat stress; water in fertilizers and pest control products; soften hard, compacted surface playing conditions on native soil fields; produce firm, fast playing surfaces on sand fields; and control dust on baseball and softball fields. Irrigation is oftentimes applied with an inground sprinkler system that’s installed by a professional irrigation company. An installed sprinkler system is obviously more expensive than a rain train or water wheel, but it provides precision application and doesn’t rely on manpower to physically move equipment.
The problem with sprinkler systems arises after it’s installed but is not monitored or maintained by people on-site. At the school and municipality level, it’s not uncommon for irrigation systems to turn on when no one is there to watch it and it doesn’t get serviced until something breaks. A common scenario I come across is that one person has the key for the irrigation shed, and the system is programmed to come on every day for 30 minutes, whether it needs it or not. This usually results in fields that are constantly wet and heavily compacted, with poor turf growth and large populations of Poa annua and Poa trivialis.
The irrigation audit
The irrigation system is an invaluable piece of equipment that requires monitoring and servicing like all other equipment. It is important to monitor how well the system is working because it ultimately relates to how much water is being applied. Reducing the amount of water applied to sports fields is in everybody’s interest, especially if city water is being used, since water bills could be lowered.
Performing an irrigation audit (especially if done in-house) can offer a good return on investment if the aim is to reduce water use and therefore costs. An irrigation audit is conducted in two stages:
1. Testing the performance of the existing system and rectifying problems.
2. Retesting the performance of the system and creating a watering schedule.
Basic performance testing of an irrigation system should be done by the sports field manager every one to three years and involves physically turning on the system and checking for uniform water distribution (referred to as distribution uniformity or DU). Distribution uniformity is assessed by laying out catch cans in a grid pattern throughout the area that the zone or station irrigates. You can buy catch cans from an irrigation distributor or make them from any household container as long as they are all the same size and depth. The irrigation zone is turned on for a specific length of time and after a measurable amount of water has been collected, the water in each can/container is measured. Carrying out this operation will help to identify zones or heads that are not working correctly.
Problems with water distribution could be attributed to such things as a broken sprinkler head; a head that is sunken, too high or tilted; incorrect nozzles; mixed heads; incorrect spacing; and broken or clogged parts. Since there is high-pressure water moving through the system, it’s easy to assume that over time there is wear and tear on the parts. In particular, if water distribution is low and the precipitation rate is high, it could be an indication that the nozzles need to be replaced. Fixing these problems should not be too daunting or too costly. Inadequate water pressure is harder and potentially more costly to correct, since it might involve a new pump, but many of the hardware problems can be dealt with quickly and at relatively low cost.
Once the irrigation system is deemed in “good working order,” with a good DU, a second catch can test is performed. The results from this test are then plugged into an online spreadsheet that will automatically do the math, or field managers can create their own spreadsheet to determine the actual precipitation rate (inches of water applied per hour) for each zone. In addition to the precipitation rate, the auditor should also record the depth of the rootzone and the type of soil, since sandy soils have less plant-available water than a loam or clay soil and will need to be irrigated more frequently.
Broken sprinkler heads will prevent uniform irrigation.
How much water?
The amount of water a field needs is based primarily on the evapotranspiration (ET) rate for that particular location. In addition to ET, water is lost from turf by other means (see Figure 1).
Compared to warm-season grasses like bermudagrass, cool-season grasses have high water use rates (WUR) during the summer, typically between .25 and .35 inches per day. Surprisingly, tall fescue has a higher WUR than perennial ryegrass, but tall fescue is much more drought tolerant because of its deep root system. The ET rate is commonly used as a guide for the amount of supplemental irrigation needed each week if there is no rainfall. A ballpark figure would be that turfgrasses require 1 inch of water per week, but there are now inexpensive field sensors ($300 to $350) that can more accurately predict the local ET rate. Lower amounts than the weekly ET rate are oftentimes applied if there is a severe drought and the irrigation water is scarce or expensive. It is not uncommon to see irrigation strategies of replenishing only 50 to 80 percent of the calculated ET rate in order to conserve moisture.
It is more difficult to predict water loss and replenish water on a sand-based field than a native soil field. Sand fields are constructed as such to achieve a water retaining porosity of around 15 to 25 percent of the soil volume. This poor water retentivity and the relatively shallow depth of turfgrasses mean that the effective soil depth for retention of useful water is around 5 inches. If the available water capacity is approximately 15 percent of the 5-inch depth, then the sand may only contain about .75 inch of water. Based on the ET rates above, this water reserve would be exhausted in three days. If the field was a newly constructed 100 percent sand field, the available water may be as low as 5 or 10 percent. This would significantly reduce the water content at a 5-inch depth to .25 to .5 inch, which would be exhausted in one to two days. On sand-based fields that drain very quickly and have a tendency to become hydrophobic, a wetting agent could be used to aid infiltration and moisture conservation.
In addition to differences in water use rate, some turfgrasses have unique morphological characteristics that make them more drought tolerant, like deep root systems, rolled leaf blades, thick cuticles and smaller leaf areas. For example, tall fescues provide excellent drought tolerance compared to Kentucky bluegrass (Table 1).
Resources and new technology
There are resources to help a field manager determine the irrigation requirements of a particular field or sports complex. In particular, the Irrigation Association website (www.irrigation.org) has irrigation audit tools and calculators, and local extension services also have useful guides (e.g., Texas A&M has a lot of irrigation resources for turf managers). Technology can also help, like the on-site ET sensors that can be hooked up to a controller so irrigation schedules are based on local ET rates. These sensors are not as precise as a weather station, but they are pretty accurate and will help to lower water usage. Another technology that is gaining momentum is remote access, whereby a field manager can view, make changes to, and turn an irrigation system on or off by using a smartphone or tablet.
Through the Irrigation Association website, local irrigation distributors can be found that will be able to demonstrate how these new technologies work. There is also a new perspective on designing irrigation systems for athletic fields that take into consideration the sport, drainage design of the field and usage, since each area of an athletic field can have different water needs. For example, the crown of a football field is typically drier than the sidelines, or a soccer goalmouth may need more light and frequent irrigation for seed establishment.
Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011.