University of Florida

Turfgrass Crop Coefficient (Kc)

A crop coefficient (Kc) is the ratio of the crop evapotranspiration (ETc) to the potential evapotranspiration (ETo) that varies in time based on growth and horticultural practices, and it is represented as:

Kc = ETc/ETo                       (Eq.2)

Using different ETo equations will generate different Kc values, which is one reason why the ASCE-EWRI Standardized Reference ET methodology was developed (Eq. 1; Allen et al., 2005), trying to standardize the estimation of Kc. Once such coefficients have been generated, only estimates of ETo are required to calculate the actual evapotranspiration of a crop.

Kc Studies in Florida

A study using bahiagrass ‘Flugge’ was presented by Jia et al. (2007). Daily Kc values were determined for July 2003 through December 2006 in central Florida, under well-watered conditions, where the eddy correlation method was used to estimate crop evapotranspiration rates. ETo was calculated using the standardized reference evapotranspiration equation. Monthly Kc values were low in the winter time (dormant grass status) although the Kc values also decreased in the summer time from peak values in May.  The multiannual average Kc value was minimum in January (0.35) and maximum in May (0.90). Jia et al. (2009) also calculated turfgrass Kc values for southern Florida using Stewart and Mills (1967) Ft. Lauderdale, FL water use data for two warm season grasses (St. Augustinegrass and Bermudagrass) growing under well-watered conditions. Reference ET values were calculated using climate data for Miami, FL (USDC, 2007) where the daily average solar radiation values were estimated using Hargreaves’ equation (Allen et al., 1998). The results showed that calculated Kc values for southern FL were higher than those in north Florida, especially in winter months. The reason of this difference is likely due to growing conditions persisting all year in the southern part of the state. The Kc value was maximum in May (0.99) and minimum in December and January (0.71). See Table 3 for more detailed information.

Another study in the southern area of Florida, the water budgets of a monoculture St. Augustinegrass ‘Floratam’ and an alternative ornamental landscape were compared (Park and Cisar, 2006). ETc was determined by a water balance equation and ETo was estimated using the McCloud method (McCloud, 1955). The average wet season crop coefficient for St. Augustinegrass was 0.30; however, for the dry season the crop coefficient increased to 0.51 (Table 3). The low Kc values obtained in this experiment are the results of using the McCloud method, which was calibrated for Gainesville, Florida. For other locations, like south Florida where mean air temperature exceeds 70 F, the estimated ETo are over-predicted , producing estimated low Kc values (see section Measurement and estimation of ETc).

Table 3: Summary chart showing turfgrass species Kc, methodology used to determine Kc, and respective references.

Turfgrass species
 Kc
 Study period length
 Methodology
 Reference
Bahiagrass












St. Augustinegrass +
Bermudagrass
 Jan (0.35)
Feb (0.35)
Mar (0.55)
Apr (0.80)
May (0.90)
Jun (0.75)
Jul (0.70)
Aug (0.70)
Sep (0.75)
Oct (0.65)
Nov (0.60)
Dec (0.45)

Jan (0.71)
Feb (0.79)
Mar (0.78)
Apr (0.86)
May (0.99)
Jun (0.86)
Jul (0.86)
Aug (0.90)
Sep (0.87)
Oct (0.86)
Nov (0.84)
Dec (0.71)
 July 2003 through
December 2006.









5-yr average monthly
data.
 ETc: Eddy correlation
method.
ETref: ASCE-EWRI
equation (Allen et al., 2005).
Kc: ETc/ETref







ETc: data from Stewart
and Mills, 1967.
ETref: Hargreaves eq.
(Allen et al., 1998) using
data from Miami.
Kc: ETc/ETref
 Jia et al., 2009.
Central Florida,
FL.








Jia et al., 2009.
(Using data from
Stewart and Mills,
1967,  South
Florida,
FL.
St. Augustinegrass
 Wet season:
(0.30)

Dry season:
(0.51)
 4 years
 ETc: water balance.
ETo: McCloud method.
Kc: ETc/ETo.
 Park and Cisar,
2006.
South Florida,
FL.


Southwest Florida Water Management District