Use of small-scale evaporation pans for evaluation of whole plant evapotranspiration

The degree to which evaporation of water from petri dishes can be used to predict evapotranspiration from hydroponically grown lettuce in greenhouses was investigated. The relationship between crop evapotranspiration and dish evaporation was found to be linear, with an adjusted coefficient of multiple determination (r²) of 0.57. Adding a term for CO₂ concentration to the relationship improved the adjusted coefficient to 0.90. Severity of tipburn also was related to dish evaporation rate, but as a step function. Dish evaporation rates greater than 20 mm/day corresponded to the least tipburn on the crops. The crop coefficient varied in a manner consistent with that of field crops, except for a sharp drop at the time of plant respacing. The pan coefficient showed no noticeable trends with respect to time, and had a mean value of 0.17. Comments and suggestions in the text suggest how this information can be applied to the design of air distribution systems for greenhouses.