Lettuce remains the most important economic crop in the lower Colorado River region near Yuma, Arizona. The acreage of lettuce produced during the fall-winter-spring period each year is approximately 120,000 acres having a value of approximately 600 million dollars. Types of lettuce include iceberg, romaine, leaf, and Boston. This document combines assessments of iceberg and romaine since they respond similarly to water and salinity management.

Water management for iceberg and romaine lettuce at Yuma is governed by sprinkler and furrow irrigations. Typically, lettuce is germinated with solid set sprinklers. After the stand is established, all subsequent water requirements are met by furrow irrigation. Lettuce is seeded from mid-September through late December. The growing period can range from 70 days for early September plantings, up to 120 days for mid-November plantings. Using eddy covariance observations (Figure 1) shows that seasonal crop evapotranspiration (ETc) ranges from 242 to 313 mm (9.5 to 12.3 acre-inches). Because days from first irrigation to harvest vary from field to field, growth and water use can be more accurately tracked using growing degree days (GDD) and satellite imagery. Average GDD and single-crop coefficients (Kc) classified by growing period are shown in Table 1.