Turfgrass Crop Coefficients (Kc)

Lawns and other turfgrass plantings closely match the standard conditions used for estimating plant water use with reference evapotranspiration (ET_o) data, so true turfgrass K_c’s have been scientifically determined.

The K_c’s in Table 1 below adjust ET_o to estimate the water required by common turfgrass species to perform as good quality lawns or other general turf surfaces. Intensively used and high-performance sports turf surfaces may require more water than these K_c’s provide in order to meet recovery and performance expectations.

The annual average K_c’s are most commonly used, but monthly values more precisely match turfgrass needs through the year.

**Table 1. Crop coefficients (K_c) for cool-season and warm-season turfgrasses in California¹.**
Month	Cool-Season²	Warm-Season³
January	0.61	0.61
February	0.64	0.54
March	0.75	0.76
April	1.04	0.72
May	0.95	0.79
June	0.88	0.68
July	0.94	0.71
August	0.86	0.71
September	0.74	0.62
October	0.75	0.54
November	0.69	0.58
December	0.60	0.55
*Annual Average*	*0.80*	*0.60*

¹ Meyer et al. 1985. Irrigation of turfgrass below replacement of evapotranspiration as a means of water conservation: determining crop coefficient of turfgrasses, pp. 357-364 in: F. Lemaire (ed.) Proc. 5^th Intl. Turfgrass Research Conf., Avignon, France, July 1985. INRA Publications, Versailles, France.

² Species include tall fescue, ryegrass, bentgrass, and Kentucky bluegrass.

³ Species include bermudagrass, zoysiagrass, and St. Augustinegrass.

Note even though tall fescue is the reference crop for estimating ET_o, turfgrasses require an amount of water that is less than ET_o to provide acceptable performance during most of the year. However, the total amount of water needed to be applied to turfgrass through irrigation usually equals or exceeds ET_o because of inefficiency and non-uniformity of the irrigation system. Also, note that warm-season turfgrasses (bermudagrass, zoysiagrass, St. Augustinegrass) require about 20% less water than cool-season turfgrasses (tall fescue, rye grass, Kentucky bluegrass) during the growing season.

The turfgrass K_c’s are used with ET_o data to determine the approximate amount of water that a lawn or other turfgrass planting will need for good performance. For example, to perform acceptably in a lawn, bermudagrass needs about 60% of ET_o on an annual average. If ET_o for a day is 0.20 inches a day, then bermudagrass would use 0.12 inches (0.20 in. × 0.6) per day.

Tall fescue and other cool-season grasses will survive and provide soil cover with many brown areas, considerable thinning out, and little growth if they receive about 60% of ET_o. Bermuda and other warm-season grasses will provide similar minimal performance when irrigated at about 40% of ET_o.