Yuma Sunset
Increasing Irrigation Efficiency
Water Use and Management
Updated Crop Evapotranspiration (ET) & Salinity Management

Iceberg and Romaine Lettuce

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.

Image

Table 1. Lettuce heat units, NDVI, and Kc values classified by FAO-56 growth stages.

Stage GDD (Co) GDD (F) NDVI Kc
INI     0.124 0.9
INI/DEV 399 750 0.157  
DEV/MID 783 1441 0.755 1.19
MID/END 1133 2071 0.755  
END     0.456  

GDD is based on minimum and maximum thresholds of 4.5 C (40.1 F) and 28.0 C (82.4 F), respectively.

Salinity management for lettuce is controlled by the need to maintain low salt concentrations at shallow soil depths. Lettuce is the most salt sensitive crop produced in the lower Colorado River region. Because seasonal irrigation application efficiencies at Yuma exceed 80%, the 20% required leaching fraction for salinity management is often not achieved during the growing period of lettuce or the growing period of the rotational crop (Figures 2, 3, and 4). Thus, a summer pre-irrigation is typically required to restore salt balance.

Figure 1. Eddy covariance system in watermelon field in the North Gila Irrigation and Drainage District.

Figure 1

Eddy Covariance System in an iceberg lettuce field near Yuma, Arizona.  

Figure 2. Field-wide salinity (dS/m) to 30 cm depth before and after iceberg lettuce for iceberg lettuce produced in the Yuma Irrigation district in 2018.

Figure 2

Field-wide salinity (dS/m) to 30 cm depth before and after iceberg lettuce for iceberg lettuce produced in the Yuma Irrigation district in 2018. 

Figure 3. Salinity changes as measured by ECe in Yuma Irrigation District for a lettuce/wheat cropping rotation in 2016-2017.

Figure 3

Salinity changes as measured by ECe in Yuma Irrigation District for a lettuce/wheat cropping rotation in 2016-2017. 

Figure 4. Projected lettuce yields following several lettuce rotations based on measured average ECe across sites after rotations. We project significant production losses without summer leaching.

Figure 4

Projected lettuce yields following several lettuce rotations based on measured average ECe across sites after rotations. We project significant production losses without summer leaching.