Two-Source Energy Balance Model to Calculate E, T, and ET: Comparison of Priestley-Taylor and Penman-Monteith Formulations and Two Time Scaling Methods

The two-source energy balance (TSEB) model calculates the energy balance of the soil-canopy-atmosphere continuum, where transpiration is initially determined by the Priestley-Taylor equation. The TSEB was revised recently using the Penman-Monteith equation to replace the Priestley-Taylor formulation, thus better accounting for the impact of large and varying vapor pressure deficits (VPD) typical of advective, semiarid climates. This study is a comparison of the Priestley-Taylor and Penman-Monteith versions of the TSEB (termed TSEB-PT and TSEB-PM, respectively). Evaporation (E), transpiration (T), and evapotranspiration (ET) calculated by the TSEB-PT and TSEB-PM versions were compared to measurements obtained with microlysimeters, sap flow gauges, and weighing lysimeters, respectively, for fully irrigated cotton (Gossypium hirsutum L.) at Bushland, Texas. Radiometric surface temperature (T-R) was used to calculate E, T, and ET in both TSEB versions in 15 min intervals and summed to intervals coinciding with times of measurements. In addition, a one-time-of-day T-R measurement was used (9:45, 11:15, 12:45, 14:15, or 15:45 CST), and E, T, and ET were calculated for the appropriate measurement interval (i. e., daytime, nighttime, and 24 h) using the time scaling methods based on reference ET (TSCET) and reference temperature (TSCTEMP). Measured average values of E, T, and ET during the study period were 0.94 mm (24 h), 6.9 mm (7:00 to 22:00 CST), and 7.2 mm (24 h), respectively. The TSEB-PT consistently overestimated E and underestimated T, with RMSE/MBE of up to 2.8/1.8 mm and 4.1/-3.9 mm, respectively. In comparison, the TSEB-PM greatly reduced discrepancies between calculations and measurements, with respective RMSE/MBE for E and T of only up to 1.5/0.79 mm and 1.3/+/- 0.76 mm, respectively. For 24 h ET, the TSEB-PT resulted in maximum RMSE/MBE of 3.2/-1.9 mm, and the TSEB-PM had maximum RMSE/MBE of 1.7/0.95 mm. Daytime ET model agreement was very similar for both model versions (RMSE/MBE usually <1.1/<+/- 1.0 mm). However, the TSEB-PT consistently calculated negative nighttime ET of up to - 2.0 mm. Summed 15 min calculations generally had better agreement with measurements than did the TSCET or TSCTEMP methods, and results did not greatly differ for TSCET or TSCTEMP. Both time scaling methods were not very sensitive to the T-R measurement time used, although morning (9:45 CST) T-R measurement times did not perform as well as the other times.