Due to recurring droughts and increasing demands for limited water in the southwestern U.S., many municipalities have imposed, or may impose, water restrictions that limit volumes available for landscape irrigation. Through careful turf selection and irrigation scheduling, however, potential adverse effects of these limited water supplies on landscape quality can be mitigated. Efficient climate-based irrigation scheduling requires accurate estimates of the turf's water-use or evapotranspiration (ET) at various times during the growth cycle. These estimates are possible if weather data and crop coefficients (relationships between measured turf ET and climate-based, reference ET [ET_o]) that typify the locale are available. While some crop coefficients (K_cs) have been formulated for turfgrass growing in desert areas of the southwest and southern California, limited K_c data have been published for turfgrass in the transitional zone. The objective of this research was to formulate K_cs for both cool season and warm season turfgrasses commonly grown in the transition zone.

Sprinkler line-source experimental designs were used to impose irrigation treatments on several turfgrasses from 1998 to 2005 at New Mexico State University's Agricultural Science Center at Farmington, N.M. Evapotranspiration was measured at each irrigation treatment using a water-balance approach. Turfgrass quality was evaluated at each irrigation treatment. Crop coefficients, based on Penman-Monteith ET_o and a relative growing degree-day time scale, were formulated for each grass at the minimum irrigation treatment exhibiting acceptable turf quality using regression analyses. The cool season grasses, as a group, required about 35% more water than the warm season grasses to maintain an acceptable quality turf and, during the active growing seasons, cool season turf K_c averaged 0.85 while warm season turf K_c averaged 0.70.