Greenhouse Crop Transpiration and Water Use
Runs an energy balance to daily transpiration, irrigation volume including the leaching fraction, the peak-hour demand.
Example
You enter
- Floor or canopy area (sq ft) 2880
- Daily solar through glazing (Btu/sq ft/day) 1500
- Crop latent fraction 0.5
- Leaching fraction 0.2
- Peak solar intensity (Btu/h per sq ft) 188
- Irrigation hours per day 12
You get
- Transpiration (lb/day) 2057.14
- Crop transpiration 247 gal/day (2,057 lb/day)
- Per square foot 0.0855945
- Irrigation to apply 308 gal/day including 62 gal of leachate
- Peak lb per hour 257.829
- Peak hour demand 30.8962
- Peak to average ratio 1.504
- Moisture into the house air 1972.1
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeRuns an energy balance to daily transpiration, irrigation volume including the leaching fraction, the peak-hour demand, and the moisture entering the house air. The worked house uses 247 gal/day and releases 1,972 pints, which a sealed house has to remove.
Formula or tabletranspiration = solar energy through the glazing x the crop's latent fraction / 1,050 Btu per pound of water; applied = transpiration / (1 - leaching fraction).
Source and editionAn energy balance. The crop's own stage and the irrigation designer govern; UMass Extension sizes the water supply itself at 0.3 to 0.4 gal per square foot of growing area per day, the warmest-day peak, which it says matches evapotranspiration.
Free accessPublic energy balance; the UMass Extension fact sheet is free online.
What governsEstimate. AHJ and licensed professional govern.
API field namesfloor_area_sqft, daily_solar_btu_per_sqft, latent_fraction, leaching_fraction, peak_solar_btuh_per_sqft, irrigation_hours_per_day, transpiration_lb_per_day, transpiration_gal_per_day, gal_per_sqft_per_day, applied_gal_per_day, peak_lb_per_hour, peak_gal_per_hour, peak_to_average_ratio, moisture_pints_per_day
- Latent fraction entered; roughly half for a well-watered crop and much lower for a sparse or stressed onegrower observation
- Supply sizing 0.3 to 0.4 gal per sq ft of growing area per day is the warmest-day peak use and calls it evapotranspiration; it is a conservative supply rule, and an energy-balance transpiration runs well under itUMass Extension, Sizing the Greenhouse Water System (Bartok, 2009)