This research was carried out at the Campbell Farm of The University of Arizona, from June 1994 to February 1995, with the aim of determining a growing-degree-days-based crop coefficient (Kc) equation for leaf lettuce (Lactuca sativa L.), using the Fourier sine series model. Ten randomized experimental plots were irrigated, in order to maintain a soil matric potential greater or equal to -20 kPa, as replications for the determination of crop maximum evapotranspiration (ETm). Each plot consisted of six raised beds, 7.3 m long and 1.0 m wide, with two rows of lettuce (Lactuca sativa L., cv. Waldmann's Green) thinned to a final population of 48 plants per bed. The thinning time occurred with a GDD of 228ºC and average Kc of 0.4. The harvest time occurred with GDD of 742ºC and average Kc of 1.2. The values of Kc as function of GDD, during the crop season, allowed the generation of an equation that can be inserted into irrigation scheduling software, considering an ending GDD (adjustment GDD) of 900ºC.
Lactuca sativa L.; irrigation management; evapotranspiration; AZSCHED
