Abstract
Reference evapotranspiration (ET₀) is a key parameter for irrigation scheduling and agricultural water management, but estimating it using the FAO-56 Penman–Monteith method requires solar radiation data, which are often unavailable in many regions. This study evaluated the performance of the Hargreaves–Samani (HS) and Valiantzas (VAL) solar radiation models for estimating ET₀ in Valapadi, Salem district, Tamil Nadu, using 25 years (2000–2025) of meteorological data obtained from the NASA POWER database. Results indicated that the mean measured solar radiation was 23.48 MJ m⁻² day⁻¹, while HS and VAL models estimated 23.40 MJ m⁻² day⁻¹ and 22.27 MJ m⁻² day⁻¹, respectively. The mean ET₀ calculated using measured radiation was 5.78 mm day⁻¹, closely matched by HS-based ET₀ (5.79 mm day⁻¹), whereas VAL slightly underestimated ET₀ (5.58 mm day⁻¹). Statistical analysis showed strong agreement with measured ET₀ values (HS: R² = 0.894, VAL: R² = 0.982). These results demonstrate that both models can effectively estimate ET₀ under limited data conditions, supporting irrigation planning in data-scarce regions.