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 irr igation 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 a re difficult in crisphead lettuce because of high soil evaporation due to frequent irrigation and slow canopy development, A model to estima te ETc for crisphead lettuce using only daily reference evapotranspira tion (ETo) data as an input parameter was developed, The model calcula tes crop transpiration and soil evaporation, and also ground cover dev elopment and daily radiation interception by the canopy. The ETc model showed good agreement when compared with field measurements of ETc ta ken at four different locations in the Salinas Valley and using either the water balance or energy balance methods for determining ETc.