Predicting reference evapotranspiration for screenhouse-grown crops
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 (tau) and wind ratio (omega = ratio of inside-to-outside wind speed): (i) a pearl insect-proof screen (IP-1, tau approximate to 0.76, omega approximate to 0.19), (ii) a white insect proof screen (IP-2, tau approximate to 0.58, omega approximate to 0.20), and (iii) a green shade-screen (GS, tau approximate to 0.56 and omega approximate to 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 (zeta(rad) = ETrad,in/ETrad,out and zeta(adv) = ETadv,in/ETadv,out). zeta(rad) and zeta(adv) could be expressed as linear functions of tau and omega respectively. Based on this finding, we proposed an additive model of the form ETo,in = zeta(rad) ETrad,out + zeta(adv) 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. (C) 2014 Elsevier B.V. All rights reserved.