Water is absolutely necessary, not only for the growth of grass on an athletic field, but also to condition the grass to handle traffic and recover from injury. Not enough water is bad, as is too much – but there is an amount of water that’s just right. Of course, this right amount is different for just about every athletic field. This is where irrigation agronomics come into play.
Getting the basics down
Evaporation is the loss of water as gaseous vapor to the atmosphere from the plant or soil surface.
Transpiration refers to the loss of water out of the plant’s stomata.
Evapotranspiration is a contracted word that describes the total water loss from the plants and surrounding soil. Water is used not only to circulate metabolites in the plant and to keep it cool though transpiration, but also for growth. When a plant cell is formed, solute is pumped into that cell through an energy-driven process that subsequently creates a concentration gradient, which causes water to enter the cell wall, thus expanding it.
The water use rate (often abbreviated WUR) is a term that defines the water requirements to account for both growth and evapotranspiration losses.
Water can also be lost from the turf system though either runoff or drainage. Of course, soil type has a major impact. Sandy soils have a high infiltration rate, rapid drainage and low water-holding capacity. Because of this, sandy soils need to be watered more often, but with less water in order to avoid excess drainage loss. Irrigation can be delivered to sandy soils more rapidly because of sand’s higher infiltration capacity.
The opposite is true of soils with high clay content. Clay soils can hold much more water, which means they don’t have to be irrigated as often. But the infiltration capacity of a clay soil can be very low. A potential issue to be aware of is that an irrigation system can deliver water at a rate much higher than what the infiltration capacity of a heavy clay soil is. This is something that happens quite often on home lawns, where a sprinkler runs for about an hour, which delivers about 1 inch of water to a rain gauge. But perhaps as little as 0.10 inches of that water might have infiltrated the soil during that hour, meaning the rest is being lost to runoff or evaporation. Thus, it’s important when using an irrigation system to match the delivery rate of the water to the infiltration capacity of the soil.
There are other factors that impact irrigation requirements, such as the turf species being utilized. For example, bermudagrass requires less water than cool-season turfgrasses, such as perennial ryegrass. Weather conditions (humidity, temperature, rainfall received, etc.) also have an impact. The microclimate of the field should also be considered – shade, for example, may reduce irrigation requirements. A slope, even a slight one, can affect runoff rates.
How turfgrass is managed also impacts irrigation requirements. Lowering the height of cut results in more frequent mowing, which usually raises the fertility and irrigation requirements. Ironically, plants also exhibit what’s called “luxury consumption” – increasing the irrigation can increase your irrigation requirements as the plants adapt to the increased amount of water. Finally, intensity of traffic has an impact. Greater traffic results in more injury, which leads to production of new leaves, which require water to grow. The total irrigation requirement for a particular field is measured by determining the amount of water that’s needed to meet the WUR, plus compensating for any loss due to drainage or runoff.
When irrigation is needed
There’s plenty of equipment that can be used to monitor soil water status, from quite simple to very sophisticated. When using a soil probe (core sampler), irrigate at a depth of 4 to 6 inches if the soil is dry. A tensiometer is a device that measures soil water content. If you have it in your budget, many irrigation systems now have automatic soil moisture sensors. If not, there are a couple of basic ways to tell if the turf needs water without fancy equipment. One is footprinting – moisture-stressed turf isn’t as turgid and won’t spring back to its original position as rapidly after foot or equipment traffic (Figure 1).
Turfgrass should be irrigated just as the plants are beginning to wilt. A slight change in the color (to blue or grayish green) or quality of light reflected from the leaf precedes wilting. The cause of this color change is that the leaf begins to fold and then roll up (as viewed in cross section) in an effort to conserve moisture (Figure 2). If the moisture level in the leaf drops near to or below the permanent wilting point before rain or irrigation, parts of or all of that leaf tissue dies. Some of these leaves may be rehydrated but may take on a mottled or injured appearance, which can mimic insect or disease activity (Figure 3).
Reducing irrigation requirements
How much water is required to irrigate turfgrass? For starters, 620 gallons are required to apply 1 inch of water to 1,000 square feet. Also, 27,154 gallons (3,630 cubic feet) of water are required to apply 1 inch of water to 1 acre. As water resources become scarcer and/or expensive, conservation has become more important.
That being the case, there are several ways irrigation requirements can be reduced:
- On lower-maintenance surfaces, it’s traditionally recommended that the turf be irrigated deeply and infrequently so as to apply between 1 and 2 inches of water per week. But, rather than just setting a program to deliver that amount of water each and every week, this can and should be adjusted based on weather conditions. In many states, there are websites that publish daily evapotranspiration (ET) rate estimates. This data can be used, along with precipitation data, in order to determine how much additional irrigation to apply in a given week.
- Raising the mowing height to the maximum recommended for the species used, or that’s permitted for the sport for which the field is used, reduces irrigation requirements. Taller grass has a higher water use rate but also has a deeper root system and better shades the soil surface, reducing evaporation loss. Raising the height of cut also tends to reduce mowing frequency. Some water loss occurs through mower wounds (a sharp blade reduces this) but it’s the regeneration of leaf tissue caused by the mowing that also contributes to increased water use.
- Reducing nitrogen rates decreases growth and thus water requirements. Increasing potassium rates had for a long time been thought to improve the drought tolerance of turfgrass, but this has fallen out of favor as potassium prices have increased. Potassium and other nutrients should be applied based on the results of a soil test.
- Remove excess thatch to avoid shallow rooting and decreased water penetration to the soil. Compacted soils should be cultivated in order to increase water infiltration, which reduces loss due to runoff and evaporation.
- Drought-tolerant species or cultivars can be used. It’s important to remember that water use rate doesn’t necessarily determine the drought tolerance of a species. For example, Kentucky bluegrass has a lower water use rate but is shallow rooted. It wilts more readily than tall fescue, which has a higher water use rate but also a deeper root system. In areas where warm-season grasses perform adequately (in the transition zone and the South), the water use rate is about half that of a cool-season grass.
- When determining irrigation requirements, adjustments also need to be made based on the efficiency of the irrigation system. This is important because water has to be applied for just as long as is necessary to put the adequate amount on the driest part of the field. If your irrigation system doesn’t distribute water uniformly and there are areas that are drier than others, water is wasted when overapplied to the wetter parts of the field. Many things can cause this, such as worn or misaligned irrigation heads.
- Localized dry spots are patches of turf where the thatch and soil turn hydrophobic. The most frequent occurrence is on sandy soils where bentgrass is grown. But this can be an issue on finer-textured soils with other grasses and occasionally can be an issue on athletic fields. The cause of localized dry spot isn’t entirely understood. Microorganisms secrete complex polysaccharides that coat the sand particles, making them hydrophobic. The symptoms of localized dry spot are pretty easy to recognize: areas of localized drought stress that watering doesn’t eliminate. Also, water tends to run off, rather than infiltrate into the affected area. Strategies for dry spot control include cultivation and the use of commercial wetting agents.
- Irrigate in the early morning hours if possible. Watering during the day is inefficient due to increased evaporation. Watering in the evening increases the incidence of certain pathogens. For practice fields or areas where this is an option, you should commit to regular irrigation or allow the turf to go dormant. Sporadic watering reduces the carbohydrate reserves of the plant and decreases survival. After moisture-stressed or dormant turf is irrigated, new leaf tissue must be regenerated. During periods of drought, the leaf tissue dies and the lawn goes dormant. The crowns of the turf plants must also remain sufficiently hydrated in order to avoid death of the turfgrass stand. For Kentucky bluegrass, this is thought to be about 0.25 inches per week.
Syringing refers to the practice of applying a light amount of water to the shoots of the plant. (Not enough water is applied to contribute to soil moisture.) Syringing is also employed to remove dew, frost, exudates and foreign matter from turf leaves. In some sports, such as soccer, syringing is practiced because it’s thought to have a positive impact on ball skip.
Syringing is mainly employed when the turf wilts despite adequate soil moisture. This is caused when the transpiration rate exceeds the plant’s ability to absorb moisture. Syringing basically mimics sweating, in that the evaporative cooling from the leaf surface helps to decrease the transpiration loss from the leaf. A potential benefit of syringing prior to playing on a hot surface is that there’s a reduction in leaf injury due to game play. The benefit of syringing is influenced by many factors, including air temperature, canopy temperature, relative humidity, irradiance, wind and the amount of water applied. In areas where cool-season turf is grown, syringing, in general, causes about a 1 to 4 degree Fahrenheit decrease in canopy temperature for about two hours.
When there’s too much
Not enough water is a problem that can be dealt with pretty easily if you have access to irrigation. Too much water is also an issue and a bit more complicated to deal with. Of course, this is more of an issue on soil-based fields, particularly those with heavy clay.
Whenever possible, a field that’s too wet should be closed. Many of the techniques attempted by sports turf managers to accelerate drying only exacerbate the issues that caused the problem. For example, when dealing with standing water, it may be tempting to try to sweep or push it onto an adjacent surface. But this can be very damaging to the soil structure, which makes it drain even more slowly. A pump can be used to remove standing water, but this only accelerates drying. The field will remain saturated for a period of time, and use under these conditions degrades the soil structure.
Another practice is to remove muddy material or add foreign material, such as sand, peat moss or drying agents. This only makes existing drainage problems worse. Unfortunately, the only viable fixes when a field is consistently too wet is to either take it out of play or address the drainage issue by attempting to modify the soil structure.