Greenhouse crop transpiration: An implement to soilless irrigation management

Soilless irrigation management requires an accurate and dynamic control due to the low water holding of the substrates and to their limited volume. A multiple regression function derived from the Penman-Monteith equation has been tested to calculate canopy transpiration. The simple function joins two components: a radiative term related with the solar radiation absorbed by the crop (G) and an advective term that includes the air vapor pressure deficit (D). Cucumber canopy transpiration has been recorded during crop ontogeny by means of a scale-lysimeter. The G and D variables explain most of the transpiration variability during day and crop development (R-2 between 0.86 and 0.99), although the coefficient values change along the cycle. Estimation of the radiative and advective components indicated that the advective term has the main influence especially at the beginning of the crop, when leaf area and transpiration by unit ground area are low and their effect on energy dissipation is small. Transpiration rate and air vapor pressure deficit follow a linear relation even when values are higher than 2.5 kPa, under these conditions, the model approach is also satisfactory.