Seasonal Kc Curves for Turfgrass Using FAO-56 Dual Crop Coefficient Method

Abstract. Seasonal crop coefficient or Kc curves were developed for ten turfgrasses at Berthoud, Colorado utilizing the FAO-56 dual crop coefficient method. Nine grasses were cool-season, one was warm season. Actual turf evapotranspiration or ET was measured by small weighing lysimeters, with four replicates of each grass (40 lysimeters total). All were seeded in 2010. Daily lysimeter measurements of ET during three seasons (2011 2013) were compared to daily ETo calculated using the ASCE standardized reference evapotranspiration equation.

Each lysimeter was centered in a 122-cm square plot of the same turfgrass. Every lysimeter consisted of a PVC shell containing a free-draining sandy loam soil, 30.5-cm diameter, and 50.8-cm rooting depth. The lysimeters were continuously weighed in-place via electronic load cells. All grasses were irrigated on the same schedule and received the same base watering via sprinklers. Supplemental hand watering brought each individual lysimeter back to field capacity. Mowing height was 7.6-cm for all grasses.

The dual Kc method partitions ET into evaporation from soil and transpiration from vegetation. This provides a substantially improved fit of simulated ET (using calculated ETo) to measured ET (from weighing lysimeters). The increased evaporation from wetted surfaces following a rain or irrigation event and the reduced transpiration resulting from soil moisture deficits are calculated on a day-to-day basis, not simply imbedded 7 to 10 day averages as with the single Kc method. Consequently developed Kc curves are not skewed by local rainfall frequency or irrigation management practices.

ET = Kc ETo = ( Ks Kcb + Ke ) ETo

Ks stress factor based on available soil moisture,

Kcb basal Kc factor, visually dry surface soil, no stress, and

Ke evaporation factor based on percent soil surface wetted.

The transition from initial to active crop development was based on the 30-day running average air temperature. The time lengths for the crop development, mid-season, and late season segments were based on accumulated growing degree days and late season frost events.

The resultant seasonal Kc curves are expected to readily transfer to other similar locations and prove more accurate day-to-day than single Kc curves, particularly when rooting depths, frequency of rainfall, and/or irrigation management practices vary. The length of the four time periods within the seasonal curves can be readily adjusted to match localized plant growth and development.