Deficit Irrigation of Kentucky Bluegrass for Intermountain West Urban Landscapes

Due to end users irrigating with excess water, water conservation of turfgrass can make a large impact in urban water conservation by reducing water applied while still maintaining visual appearance. This study was conducted to determine if Kentucky bluegrass (Poapratensis L.) can be deficit irrigated to maintain minimum acceptable appearance while conserving water. The study investigated water stress in terms of stomatal conductance, chlorophyll index, leaf temperature and predawn leaf water potential at the point of water stress, or where visual quality no longer meets expectations during dry down conditions. Water use was measured over well established Kentucky bluegrass with an eddy covariance system that was validated with soil water measurements. Turfgrass was irrigated at 80% of reference evapotranspiration based on allowable depletion of 12 mm of soil water during growing season that was considered to be well-watered. Two dry downs were conducted over a two-year period (early and late summer). Turfgrass was allowed to dry down without irrigation until visual quality reached the minimum acceptable points (score ≤ 6). During drying periods, visual rating, chlorophyll index, predawn leaf water potential, and leaf temperature with stomatal conductance rapidly decreased once stomatal conductance fell to approximately half of well-watered levels. Both soil water content and evapotranspiration had weak correlation with stomatal conductance; however, stomatal conductance tended to have higher correlation with the change in soil moisture than with the change in crop evapotranspiration. Soil water use and eddy covariance data in terms of crop evapotranspiration had high correlation. The plant water use factor ranged from around 0.8 to 1.1 under well-watered condition corresponding to visual rating from 7 to 9. At the minimum acceptable point of visual rating, which is 5.5 to 6, the plant factor ranged from 0.65 to 0.87. This value of plant factor is quite high at this point. Even when Kentucky bluegrass went below acceptable visual quality, the grass still used significant amounts of water with the plant factor value ranging from 0.6 to 0.8. The data suggested that deficit irrigation cannot be applied with Kentucky bluegrass in the Intermountain West area