<![CDATA[Despite advances in greenhouse technology, few studies have quantified the direct relationship between solar radiation and lysimeter-measured crop evapotranspiration (ETc) in cherry tomato under tropical greenhouse conditions. This study aimed to fill this gap by determining ETc of cherry tomato (Solanum lycopersicum var. cerasiforme) using a weighing lysimeter (5 min intervals, November 28, 2023 to January 6, 2024) and analysing its relationship with solar radiation under different sky conditions. The experiment was conducted in a greenhouse containing 32 plants arranged in two planting beds and irrigated with a drip system. All environmental and crop parameters were monitored through sensors integrated into a digital dashboard. Daily solar radiation ranged from 4.7 to 13.6 MJ m⁻² d⁻¹ (mean 9.0 MJ m⁻² d⁻¹), whereas ETc ranged from 2.03 to 3.19 mm day⁻¹ (mean 2.60 mm day⁻¹). A significant positive correlation was found (r = 0.717, R² = 0.514, p < 0.001), yielding the overall regression ETc = 0.069 × Rad + 2.004. A critical threshold of 8 MJ m⁻² d⁻¹ was identified: under sunny conditions (Rad ≥8), ETc = 0.074 × Rad + 2.000 (R² = 0.446, p < 0.001); under cloudy conditions (Rad <8), the near-zero slope (0.009, R² = 0.314) supported using mean ETc = 2.41 mm day⁻¹. Diurnal analysis revealed a 30–45 min ETc lag on sunny days, informing optimal irrigation timing (before 11:00–12:00). Water use efficiency was 33% higher on cloudy days (0.364 vs. 0.273 mm MJ⁻¹ m²). These empirical models can be directly embedded into automated irrigation systems, supporting precision irrigation practices in greenhouse cherry tomato cultivation under tropical conditions. Future research should focus on integrating VPD and physiological crop parameters to further improve ETc prediction accuracy under low-light conditions.]]>
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