Improving reference evapotranspiration (ET_o) calculation under limited data conditions in the high Tropical Andes

The computation of the reference crop evapotranspiration (ET_o) using the FAO56 Penman-Monteith equation (PM-ET_o) requires data on maximum and minimum air temperatures (T_max, T_min), relative humidity (RH), solar radiation (R_s), and wind speed (u₂). However, the records of meteorological variables are often incomplete or of poor quality. Frequently, in the mountain areas such as those of the Andes, environmental sensors are subject to harsh conditions, due to the diurnal/nocturnal climatic variability causing challenging conditions for meteorological monitoring, which leads to data loss. For high-elevation landscapes like the Andes, the missing variables of vapor pressure deficit and solar radiation cause a high impact on PM-ET_o calculation. To assess these limitations, several methods relying on maximum and minimum temperature to estimate the missing variables have been considered in the present investigation. Based on data from three automatic weather stations in the high Tropical Andes (humid páramo, 3298 – 3955 m a.s.l.), we found that the calibration and validation of methods were essential to estimate R_s. Using the (De Jong and Stewart, 1993) (R_s-DS) method we retrieved the highest performance, a RMSE between 2.89 and 3.81 MJ m⁻² day⁻¹. Moreover, In the absence of RH observations, replacing the dew point temperature (T_dew) by T_min was a reliable alternative, when apply the method of (Allen et al., 1998) (VPD-FAO) which showed the highest performance with RMSE between 0.08 and 0.12 kPa. These results yielded highly accurate PM-ET_o estimates, with RMSE between 0.29 and 0.34 mm day⁻¹ and RMSE between 0.12 and 0.18 mm day⁻¹, respectively. As expected, when both variables were missing, the ET_o calculation increased its error, with an RMSE between 0.32 and 0.42 mm day⁻¹. A proper estimation of ET_o in the Andean páramo contributes to improved water productivity for domestic and industrial uses, irrigated agriculture, and hydropower.