EVAPOTRANSPIRATION ESTIMATES FOR GREENHOUSE LETTUCE USING AN INTERMITTENT NUTRIENT FILM TECHNIQUE

Abstract

Lettuce production using the nutrient film technique (NFT) in Brazil has increased considerably since 1990.However, the procedure used to supply the nutrient solution is based on interval timers, which does not account for thedynamic environment. In general, nutrient solution is recirculated for 10 to 15 min followed by a resting period of 15 to 20 min,which is used for saving electricity and to provide oxygen to the roots. The objective of this work was to evaluate thePenman-Monteith model for estimating crop evapotranspiration (ETc) for three cultivars of lettuce (Grand Rapids, GreatLakes, and Regina). Measurements and evapotranspiration estimates were carried out for 30 min intervals. A closed-loophydroponic system was specially designed for this research so that measurements of ETc could be conducted for short timeperiods. The hydroponic system was installed in a greenhouse without environmental controls, as is typical for hydroponiclettuce production in Brazil. Measurements of incident solar radiation, relative humidity, air velocity, air temperature, andnutrient solution temperature were stored every minute. The Penman-Monteith model provided excellent estimates ofevapotranspiration for leaf area indices (LAI) greater than 0.5. However, the method was not adequate to estimate ETc in thefirst two weeks after transplanting (LAI < 0.5). During this period, the model sometimes over- or underestimated ETc,depending on the cultivar. Additionally, the model overestimated ETc early in the morning and late in the afternoon becausea constant value of the surface diffusive resistance (rs) was used. The results of this research can be used to improve the electricenergy usage efficiency for hydroponic lettuce production and allow the implementation of automatic systems for adjustingthe concentration of nutrients in the irrigation water based on estimates of evapotranspiration.