Predicting reference evapotranspiration for screenhouse-grown crops

• ETo was estimated under three different screenhouses and compared to outside ETo.
• ETo reduction was quantified by means of two factors, ζrad and ζadv, related to ETrad, and ETadv respectively.
• ζrad was found to depend linearly on the screenhouse transmittance τ and ζadv on the wind ratio ω.
• A model was proposed for estimating screenhouse ETo based on outside ETrad, and ETadv, τ and ω.

The purpose of the study is to estimate the reference evapotranspiration of screenhouse-grown crops (ETo,in) from routine outside weather data and main screenhouse characteristics. To this aim, all the microclimatic variables required to calculate ETo,in by means of the standard FAO-56 Penman–Monteith formula were measured under three cultivated (sweet-pepper, spring–summer cycle) screenhouses for a two months period observation (Aug–Sept 2012) in a Mediterranean climate (Eastern Central Greece). The screenhouses differed in transmittance to solar radiation (τ) and wind ratio (ω = ratio of inside-to-outside wind speed): (i) a pearl insect-proof screen (IP-1, τ ≈ 0.76, ω ≈ 0.19), (ii) a white insect proof screen (IP-2, τ ≈ 0.58, ω ≈ 0.20), and (iii) a green shade-screen (GS, τ ≈ 0.56 and ω ≈ 0.43). We quantified and analysed at daily scale the evolution of the radiative (ETrad,in) and advective (ETadv,in) components of ETo,in, ETrad,in and ETadv,in were expressed as a function of the corresponding outside values (ETrad,out and ETadv,out) through two reduction ratios (ζrad = ETrad,in/ETrad,out and ζadv = ETadv,in/ETadv,out). ζrad and ζadv could be expressed as linear functions of τ and ω respectively. Based on this finding, we proposed an additive model of the form ETo,in = ζrad ETrad,out + ζadv ETadv,out. The predictive performance of the model was fair (RMSE = 0.11 mm day−1). The proposed model, based only on outside climate data and the knowledge of two screenhouse-related parameters, provides a straightforward way to estimate screenhouse ETo, a necessary step towards the determination of screenhouse-crops water requirements.