Estimation of ET0 in Alfalfa (Medicago sativa) with the SEG-SRM-V1 System Using the Surface Renewal Method and Validated Using the Penman–Monteith Method

Agricultural producers need affordable tools to estimate reference evapotranspiration (ET0) in field conditions, especially in regions with limited access to complete weather data. In this study, the ET0 for an alfalfa (Medicago sativa) crop was estimated using the SEG-SRM-V1 electronic system, based on the surface renewal method and validated using the FAO-56 Penman–Monteith method. The estimated ET0 values for alfalfa ranged from approximately 4.0 to 8.5 mm day−1 under the prevailing climatic conditions: periods of high temperature (21 °C to 33 °C), as measured by the system in the experimental area, with cloud cover, wind (1 to 8 m/s) and net radiation of 664 W/m2 to 910 W/m2. Comparisons between the two methods yielded determination coefficients of between 0.65 and 0.85, and the values of the errors (MSE, RMSE and MAE) tend to 0, which indicates that the estimates of ET0 measured by the system (SEG-SRM-V1) are close to those obtained using the Penman–Monteith method. Similarly to the performance of open-field systems operating under atmospheric and vegetation cover conditions, these results demonstrate that high-frequency (10 Hz) air temperature measurements provide sufficient physical information to support the estimation of ET0 in alfalfa, while the open architecture of the SEG-SRM-V1 system allows for flexibility and scalability for irrigation management applications in other crop types.