Crop growth and water use model for lettuce

Crop evapotranspiration (ETc) by crisphead lettuce varies considerably with weather conditions and irrigation management. Accurate estimates of ETc will enable irrigation applications to be matched more closely to crop requirements, which will reduce excessive applications of irrigation water. This is important in the Salinas Valley, Calif., where overpumping of ground water is associated with seawater intrusion and nitrate contamination of local aquifers. Accurate assessments of ETc are difficult in crisphead lettuce because of high soil evaporation due to frequent irrigation and slow canopy development, A model to estimate ETc for crisphead lettuce using only daily reference evapotranspiration (ETo) data as an input parameter was developed, The model calculates crop transpiration and soil evaporation, and also ground cover development and daily radiation interception by the canopy. The ETc model showed good agreement when compared with field measurements of ETc taken at four different locations in the Salinas Valley and using either the water balance or energy balance methods for determining ETc.