Effect of Cooling Water Temperature on Nutrient Solution Cooling Efficiency and Lettuce Growth in Hydroponic Systems

This study investigates the detailed impact of nutrient solution temperature on the growth performance of lettuce (Lactuca sativa) in a hydroponic NFT system under typical arid climate conditions. A mathematical model was developed to predict the thermal behavior of the nutrient solution based on both environmental variables and reservoir cooling mechanisms. The experiment was conducted in a fan-pad cooled greenhouse located in Safaga, Egypt, using three distinct nutrient solution temperature treatments: 17.4 ± 0.3°C, 20.8 ± 0.2°C, and 26.8 ± 0.4°C. Results showed that the optimal temperature (20.8°C) improved shoot fresh weight (190.2 g/plant), root dry weight (11.4 g/plant), and root-to-shoot ratio compared to the sub-optimal and supra-optimal growth conditions (p < 0.05). The proposed model demonstrated exceptionally high predictive accuracy (R² = 0.94; RMSE = 0.62°C), validating its reliability. These findings strongly support the integration of low-energy cooling strategies in improved nutrient management protocols for large-scale hydroponic systems in extremely hot climates.