Determination of energy partition of a cucumber grown Venlo-type greenhouse in southeast China

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Accurate determination of energy partition inside greenhouses is a key procedure in optimizing efficient water management. In this study, micro-meteorological data and energy fluxes of cucumber were measured during four planting seasons (2018–2021) in a Venlo-type greenhouse to analyze the energy manifestation in different growing stages. The results showed that the latent heat flux (λET) was the primary component of net radiation (Rn), accounting for 61.2–72.6 % of Rn during the whole growing seasons, followed by sensible heat flux (H, 19.8–24.3 % of Rn) and soil heat flux (G, 7.6–14.8 % of Rn). The λET increased while both H and G decreased as the crop grew. Especially, H was larger than λET at midday in the initial growing stage. Energy partition was intensely affected by the leaf area index (LAI). The λET/Rn increased linearly with the increase in LAI, while the H/Rn decreased linearly and G/Rn decreased exponentially until LAI reached 4 m2 m−2. Canopy conductance (Gc), Priestley-Taylor coefficient (α), and the decoupling coefficient (Ω) were calculated to evaluate the controlling factors over cucumber λET. The high values of the α (= 1.15 ± 0.05) and the Ω (= 0.65 ± 0.05) indicated that the cucumber λET was principally constrained by Rn. The Gc was calculated by inverting Penman-Monteith (PM) equation using aerodynamic conductance (Ga) values obtained by the heat transfer coefficient (hs) method based on accurate convection regimes. The Gc varied from 1.42 to 17.98 mm s−1 and high Gc values corresponded to high λET. The λET was estimated by the Ω model with the mean root mean square error (RMSE) and mean absolute error (MAE) equaled 32.65 and 25.73 W m−2, while the determination coefficient (R2) was 0.94. The H was reproduced by the Bulk Transfer (BT) model to validate Ga with the RMSE and MAE equaled 32.57 and 24.95 W m−2, the R2 was 0.86. The results also indicated that the simplified energy balance (EB) method was an optional approach to analyzing energy partition in greenhouses. Consequently, more energy could be saved by avoiding the excessive application of water and thus improving crop water use efficiency in greenhouses.