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Key Takeaways

  • When using ET-based irrigation scheduling, switching from a traditional, constant crop coefficient (Kc) to a variable one that is based on growing degree days or other data that reflects seasonal weather patterns can significantly reduce water use.

  • Models using variable Kc values instead of fixed coefficients reduced annual irrigation amounts by approximately 10% for cool-season grasses and 15% for warm-season grasses.

  • Cool-season grasses exhibited higher year-to-year variability, while warm-season grasses used less water overall, but both benefited from dynamic irrigation scheduling.

  • All warm-season grasses tested used significantly less water than any cool-season grass; however, there was no significant difference among the individual grass species and varieties tested within both the warm- and cool-season groups.

  • The data necessary to implement dynamic irrigation scheduling is readily available and modern irrigation systems can automatically incorporate weather data that will enable ET-based scheduling using a variable Kc.
     

No golf course benefits from using more water than necessary. For some courses, like many in the West, water conservation is a necessity because of limited supplies and rising costs. But even if water availability isn’t a concern, optimizing water use is a priority for all superintendents because it plays a key role in maintaining healthy turf and providing the best playing conditions possible. Overwatering leads to softer turf, more pest pressure and many other problems. Fortunately, many courses can save water and improve playing conditions at almost no cost by adjusting irrigation scheduling to better reflect actual turfgrass water needs. Using ET-based irrigation is one way to do this, but the models used to estimate daily water requirements are often not as responsive as they could be and are typically based on maximum water needs; hence, they are pretty accurate in summer but excessive in spring and fall. To help superintendents refine their irrigation scheduling methods, a recent study looked at ways to improve current ET-based irrigation models by better tailoring them to seasonal differences in actual turfgrass water use rather than generalized water use estimates.

Using Variable Crop Coefficients and Growing Degree Days To Improve Turfgrass Irrigation Efficiency

Traditional ET-based irrigation frequently relies on constant crop coefficient (Kc) values to estimate water needs of different grasses. ET is primarily a function of climate, weather and grass type – so measurements taken at one site are generally not good predictors of ET at a different site unless growing conditions are very similar – which they often are not on golf courses. Kc is simply correlating measured turfgrass ET (ETa) with a well-established turfgrass reference crop ET (ETos). The derived ratio (ETa/ETos) is the Kc value and may be used to calculate estimates of actual plant water requirements at different sites. Since Kc is the main way superintendents using ET-based irrigation try to further refine irrigation scheduling to account for differences at their golf course, it’s important that it be as good of a coefficient as possible. Fixed Kc values are convenient, but do not reflect well the seasonal fluctuations in turfgrass water use and often result in overwatering during spring and fall, and occasional underwatering during peak summer heat.

To address this issue, a study conducted at New Mexico State University’s Agricultural Science Center in Farmington, New Mexico, was initiated to directly measure ETa of several warm- and cool-season turfgrasses over the course of a year and sought to develop variable Kc models normalized by growing degree days (GDD) or week of the year (WOY). These models will enable superintendents to synchronize irrigation schedules more precisely with turf growth and weather conditions, resulting in measurable water savings without reducing turf quality.

"These models will enable superintendents to synchronize irrigation schedules more precisely with turf growth and weather conditions, resulting in measurable water savings without reducing turf quality."

Field Study: Determining Actual ET of Various Grasses

The study location has a high-desert climate characterized by hot, dry summers and cold winters, with average annual precipitation of about 8 inches. This is a transition zone where both cool- and warm-season turfgrasses can be grown. Cool-season grasses included in the study were Kentucky bluegrass (‘Adelphi’ and ‘Park’), turf-type tall fescue (‘Shenandoah’) and perennial ryegrass (‘Seville’). Warm-season grasses included seeded varieties of bermudagrass (‘Guymon’), buffalograss (‘Bison’) and blue grama (‘Lovington’). Nutrients were applied as needed to prevent stress and all grasses were mown at 2.75 inches except for blue grama, which was raised to 4 inches due to poor initial quality at the lower height.

Researchers used a line-source sprinkler (LSS) system that delivered a gradient of water across test plots, ranging from full replacement of ETos (near 100%) close to the sprinkler line to less than 10% replacement farther away. Catch cans recorded irrigation amounts and soil moisture was monitored with neutron probes down to 54 inches. Then, ETa was calculated using a “water-balance” approach that considered irrigation, rainfall, changes in soil water, and gravitational drainage. Turf quality was rated visually on a 1-9 scale, where 6 represented the minimum acceptable level for golf course play. Importantly, the study determined ETa at the point along the irrigation gradient where turf maintained its minimum acceptable quality – providing a realistic basis for irrigation scheduling in water-limited conditions for each of the different grasses in the study. It’s important to note that this amount varies by grass type and is something superintendents managing different species should consider. Irrigation began in March or April and was performed two or three times per week to maintain 100% replacement of ETos in the plot nearest the water source. A weather station recorded data to calculate ET. The full details of the study materials and methods, as well as the statistical analysis and Kc modeling, can be found here in the peer-reviewed scientific article.

Developing Variable Kc Models Using Actual ET Rates and GDD

Rather than estimating Kc based on the calendar (which likely would not provide year-to-year consistency), the researchers normalized Kc values using GDD, a measure of accumulated heat units. This approach accounts for differences in growing season onset, duration and year-to-year temperature variability. The researchers also validated their models with independent data collected during a second field study in New Mexico from 2003 to 2005, which included different grasses, ensuring the findings were robust and applicable across seasons and for different grass varieties.

This is the first study to develop crop coefficient models adjusted for GDD specifically for turfgrass, and unlike prior research, it emphasizes minimum water requirements for acceptable quality rather than optimal quality for near perfect conditions. Its validation with independent data collected several years later demonstrates the reliability of the models across varying conditions.

Key Results: Turfgrass Water Use Patterns and Variable Kc Models

Measured ETa varied by grass type and year. At peak water use, cool-season grasses used between 0.21 and 0.26 inch of water per day, while warm-season grasses required between 0.18 and 0.20 inch per day. For cool-season grasses, year-to-year variability was more pronounced than differences among cultivars, indicating that weather conditions had a stronger influence on water use than genetic variation. Warm-season grasses, on the other hand, exhibited greater within-year variability, reflecting their sensitivity to seasonal heat patterns. When data were averaged across years and cultivars, daily cool-season predicted water use ranged from 0.04 to 0.24 inch and warm-season predicted water use ranged from 0.04 to 0.19 inch.

Kc values also displayed seasonal patterns. When averaged across years and cultivars, peak cool-season grass Kc values ranged from about 0.76 to 0.95, while warm-season grass peak Kc values ranged from about 0.66 to 0.76. But when Kc values were looked at over the course of a growing season, predicted cool-season Kc ranged from 0.44 to 0.90 and predicted warm-season Kc ranged from 0.36 to 0.69. These lower values indicate that at certain times of the year observed ET was much lower than constant reference ET, and irrigation can be reduced during these periods (Figure 1).

Basing irrigation scheduling on GDD (heat accumulation) means that GDD-based Kc models align more closely with the turf’s physiological processes (water needs) throughout the year. WOY is simply a calendar-based way to track GDD impact on the variable Kc, since when plotted, both GDD and WOY exhibited a nearly linear relationship throughout the growing season. This approach improves accuracy in both normal and abnormal weather years. Validation using independent data collected in later years showed a high correlation between predicted and measured ETa, with R² values exceeding 0.95. This level of accuracy provides confidence that golf course managers can adopt these models with minimal risk of turfgrass quality loss.

Modeling indicated that using variable Kc values instead of fixed coefficients could reduce annual irrigation amounts by approximately 10% for cool-season grasses and 15% for warm-season grasses at the research location. For a golf course in the high desert of New Mexico irrigating 100 acres, using a variable Kc model could translate to annual savings of roughly 3 to 5 million gallons of water.

"For a golf course in the high desert of New Mexico irrigating 100 acres, using a variable Kc model could translate to annual savings of roughly 3 to 5 million gallons of water."

Practical Recommendations for Golf Course Superintendents

Superintendents using ET-based irrigation should replace constant crop coefficients with variable coefficients linked to GDD (or corresponding WOY). If the golf course does not have an on-site weather station that can be used to measure hyperlocal ET values, local and regional weather networks such as AgriMet, AZMET or CIMIS provide daily ET and temperature data that can be combined with this study’s models to calculate actual turfgrass water needs more precisely. The main steps involve obtaining daily ET, calculating GDD, applying the variable Kc curve, and scheduling irrigation based on these adjusted values.

Here’s how you’d put variable Kc to work as a superintendent with a standard golf course irrigation system. Starting with your own data or a reliable nearby ET source like those listed above, you adjust that ET by multiplying it with a Kc value that matches your turf type and the time of year. For example, if your ET today is 0.20 inches and the variable Kc for bermudagrass this WOY is 0.70, then the grass used about 0.14 inches. You’d program your central irrigation computer to replace that amount, adjusting for your system’s precipitation rate and efficiency (often 70%-80%). The “variable” part means your Kc ought to be lower in spring and fall and peak in summer. Superintendents with cool-season turf should know that those grasses reach their peak water use at a slightly earlier WOY (about 1.5 weeks earlier) than warm-season grasses.

Superintendents already intuitively understand that irrigation should be reduced during the early spring and late fall when turf water needs are lower and, likewise, should increase during midsummer at the period of highest ET demand. However, using a variable Kc model that incorporates hyperlocal ET data and precisely measured water needs of different grasses further refines the coefficient value and offers additional savings. While variable Kc adjustments can deliver substantial water savings, even modest gains are critical in the western U.S., where superintendents must make every drop count.

Warm-season grasses such as bermudagrass, buffalograss and blue grama inherently use less water than cool-season grasses. When planning renovations or new course construction, incorporating warm-season species into roughs, fairways, or out-of-play areas can significantly reduce overall irrigation requirements.

Even when using advanced models, superintendents should verify performance through visual turf quality assessments and periodic soil moisture monitoring. Variable Kc models can be easily integrated with weather-based irrigation controllers or smart systems that adjust water applications in real time. Many modern controllers already accept ET and temperature inputs, making implementation straightforward.

While adjusting crop coefficients for GDD accounts for much of the seasonal and annual variability, it cannot fully capture all weather extremes or anomalies. This finding highlights the need for ongoing refinement of irrigation models as climate patterns evolve. Looking forward, incorporating data from additional turfgrass species, diverse locations, and longer time frames could improve model accuracy. Emerging tools such as machine learning and artificial intelligence also hold promise for detecting complex patterns in turf water use and could potentially lead to even greater water savings on golf courses.

"While variable Kc adjustments can deliver substantial water savings, even modest gains are critical in the western U.S., where superintendents must make every drop count."

Environmental and Economic Impacts

Adopting variable Kc models can save up to 15% of irrigation water without compromising turf quality. The savings can be better illustrated by looking at how much water a golf course uses with traditional Kc values. A standard approach for cool-season grasses would be a Kc of 0.8 throughout the year (growing season). ET from your weather station or other source is multiplied by 0.8 to give you the irrigation requirement. This study shows that for our location in New Mexico, during spring and fall you get away with a significantly lower Kc, typically around 0.4 or 0.5 in March, April and May and similar values in October and November. Using these lower Kc values during these early and late months accounts for the majority of the 10% to 15% water savings. The dynamic model can be used to find out the exact Kc for each day of the irrigation season. Water consumption during the summer will not change drastically, but since Kc is not constant, water savings are also extended beyond spring and fall to anytime the weather and/or the plant’s physiological needs offer an opportunity to reduce water use.

For a typical golf course, this variable Kc approach may equate to millions of gallons saved annually, reduced pumping and energy costs, and improved playing conditions. Beyond economics and playing conditions, implementing such practices enhances the course’s environmental stewardship profile – an increasingly important factor for community relations and certification programs. For courses looking to optimize their water use, the USGA Water Conservation Playbook is a valuable resource and offers a broad range of strategies as well as details on how to implement them.