Lettuce evapotranspiration and crop coefficients using eddy covariance and remote sensing observations

Abstract

Abstract Lettuce (Lactuca sativa L.) is a high-value crop for irrigation districts in the low deserts of the USA Southwest. To ensure maximal crop quality, negligible soil salinity stress, minimal nutrient loss and reduced pathogen susceptibility, lettuce irrigation must meet, but not exceed, crop water use requirements. Lettuce crop water use information is outdated in this region: prior studies were conducted at least four decades ago (1960–1980) and do not represent current varieties, management practices, and climate. To address this shortcoming, we studied 12 commercial sites in Yuma, Arizona, USA between 2016 and 2020 to update lettuce water use requirements and crop coefficients. The study measured crop evapotranspiration (ETc) using eddy covariance observations at 8 iceberg and 4 romaine sites, where planting dates varied throughout the fall. Observed ETc and remote sensing data were used to model the daily soil water balance and derive crop coefficients: single (Kc), basal (Kcb), and soil evaporation (Ke). The analysis was supported by lettuce crop height estimates and fractional vegetative cover (fc) via remote sensing. Days to maturity averaged 75 ± 15 and 89 ± 12 days for romaine and iceberg, respectively. Seasonal lettuce ETc averaged 278 ± 24 mm and cumulative irrigation applied averaged 340 ± 78 mm. Lettuce Kc for sites varied from 0.90 ± 0.13 to 1.19 ± 0.11 and Kcb from 0.20 ± 0.05 to 1.01 ± 0.11 for the initial and mid-season growth stages, respectively. Average days to maturity were 1134 and 810 C-days for growing and enhanced degree day metrics. The study updates information that can guide more efficient lettuce irrigation management.