Effects of the Evaporative Cooling Method on Tomato Yield and Water-Use Efficiency in a Semi-Arid Climate

In greenhouses with evaporative cooling systems, the amount of water used for cooling can exceed plant water consumption, so caution is advised in water-scarce regions. Since the semi-arid region where the study was conducted receives low rainfall (an average of 380.4 mm per year), the amount of water used for plant and evaporative cooling makes water use management a critical issue. For this purpose, a study was conducted between May and July 2023 in Kırşehir, Türkiye, using two physically identical, side-by-side, polyethylene-covered high tunnels (5 m × 3 m × 2 m) with external shading nets; one operating with natural ventilation (NV) and the other with direct evaporative cooling (DEC). The study determined the effects of these applications on the indoor climate, the morphological and quality characteristics of tomatoes, plant water consumption, and water-use efficiency. During the study period, the highest cooling effect measured in the DEC application was 9.6 °C, the relative humidity effect was 29.3%, and the cooling efficiency was 67.6%. In the NV application, the highest cooling effect was 5.0 °C, and the relative humidity effect was 13.2%. As a result of the findings, the DEC application made a positive contribution to the morphological (stem diameter, plant height, and number of leaves) and quality parameters (width, length, weight, pH, titratable acidity, and total soluble solids) of tomatoes compared to the NV application. Daily plant water consumption per unit area was 114.7 L m−2 in the NV application, 88.1 L m−2 in the DEC application, and 117.5 L m−2 for cooling. The amount of water used for irrigation in the NV application was approximately 23.2% higher than in the DEC application. In the study, total yield (TY) was 2355.5 g m−2, and marketable yield (MY) was 2240.8 g m−2 under NV application. In the DEC application, TY was 5721.5 g m−2 and MY was 5529.8 g m−2. Accordingly, TY decreased by 58.8% and MY decreased by 59.5% in the NV application compared to the DEC application. Total water-use efficiency (TWUE) was 20.5 g L−1 in the NV application, while marketable yield water-use efficiency (MWUE) was 19.5 g L−1. In the DEC application, TWUE was 42.2 g L−1 while MWUE was 39.6 g L−1. Accordingly, compared to the DEC application, TWUE decreased by 51.4%, and MWUE decreased by 50.8% in the NV application. Furthermore, considering the water used for cooling in the DEC application (irrigation + cooling), CTWUE was 27.8 g L−1, and CMWUE was 26.9 g L−1. Therefore, the water used for DEC reduced CTWUE by 34.1% and CMWUE by 32%. The results indicate that while evaporative application has positive contributions to cultivation, it also reduces water-use efficiency; therefore, its use should be considered in regions with limited water availability. Collecting rainwater and using it in irrigation and as cooling water after filtration will be important for sustainable greenhouse farming in these regions.