ASCE-EWRI standardized Penman-Monteith evapotranspiration (ET) equation performance in southeastern Colorado

The performance evaluation of the PM ETsz equation was done for different atmospheric stability conditions for individual years and for all years combined. Also, the evaluation was carried out for multiple alfalfa cutting cycles. The results obtained showed that the equation underestimated measured ETr (−0.04 mm/h (−14%)) for stable atmospheric conditions. Nonetheless, the ETsz values matched closely to those of the lysimeter for unstable atmospheric conditions. Furthermore, somewhat large scattered ETrs values (RMSE of 0.11 mm/h (25.5%)) were observed for neutral atmospheric conditions. The ETrs comparison produced acceptable results for daily time step for years 2009-2011. However, the daily ETrs comparison was not acceptable for the 2012 dataset, which included effects of severe heat advection. Moreover, it was found that the ETsz equation overestimated ETr, the MBE was 0.05 mm/h (18.1%) for the first alfalfa cutting cycle and slightly underestimated (up to −0.02 mm/h (−5.7%)) ETr for the second, third and fourth cutting cycles. Therefore, adjusting (reducing) the value of the constant Cd in the ASCE Standardized ETrs equation for stable atmospheric conditions is recommended; and either adjusting (increasing) the Cd value or developing a different set of crop coefficients for the first cutting cycle of alfalfa, which ultimately helps in improving agricultural water use efficiency.