Evapotranspiration estimates for greenhouse lettuce using an intermittent nutrient film technique

S. Zolnier, G. B. Lyra, R. S. Gates

Research output: Contribution to journalArticlepeer-review


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 the dynamic 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 the Penman-Monteith model for estimating crop evapotranspiration (ETc) for three cultivars of lettuce (Grand Rapids, Great Lakes, and Regina). Measurements and evapotranspiration estimates were carried out for 30 min intervals. A closed-loop hydroponic system was specially designed for this research so that measurements of ETc could be conducted for short time periods. The hydroponic system was installed in a greenhouse without environmental controls, as is typical for hydroponic lettuce production in Brazil. Measurements of incident solar radiation, relative humidity, air velocity, air temperature, and nutrient solution temperature were stored every minute. The Penman-Monteith model provided excellent estimates of evapotranspiration for leaf area indices (LAI) greater than 0.5. However, the method was not adequate to estimate ETc in the first 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 because a constant value of the surface diffusive resistance (r s) was used. The results of this research can be used to improve the electric energy usage efficiency for hydroponic lettuce production and allow the implementation of automatic systems for adjusting the concentration of nutrients in the irrigation water based on estimates of evapotranspiration.

Original languageEnglish (US)
Pages (from-to)271-282
Number of pages12
JournalTransactions of the American Society of Agricultural Engineers
Issue number1
StatePublished - Jan 2004
Externally publishedYes


  • Hydroponics
  • Model-based control
  • Penman-Monteith

ASJC Scopus subject areas

  • Agricultural and Biological Sciences (miscellaneous)


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