Location: Soil and Water Management Research
Title: Two source energy balance model to calculate E.T. and ET:Comparison of Priestly-Taylor and Penman-Monteith formulations and two time scaling methods Authors
|Agam, Nurit -|
|Howell, Terry -|
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 31, 2014
Publication Date: N/A
Interpretive Summary: Knowledge of crop water use can be used for irrigation and crop management. Water is lost from a crop in two ways, which are evaporation and transpiration. Evaporation is from the soil and vegetation. Transpiration is from the vegetation. Crop growth and yield are mainly related to transpiration. Evaporation does not contribute directly to crop yield, and is generally considered a loss. Farmers manage crops to minimize evaporation. This is important for water conservation, especially during drought and where irrigation water is limited. Evaporation and transpiration are difficult to measure. Therefore, crop water use models are needed that calculate evaporation and transpiration. Such models can evaluate methods intended to favor transpiration over evaporation. We found that a commonly used crop water use model could calculate overall crop water use with acceptable accuracy. However, the model resulted in evaporation that was too large and transpiration that was too small. Therefore, we made a major revision to the model. The new model did a better job of calculating evaporation and transpiration. This will help farmers reduce evaporation losses from crops. Also, it will increase the chances that crops will survive drought under both irrigated and dryland production systems.
Technical Abstract: The two source energy balance (TSEB) model calculates the energy balance of the soil–canopy–atmosphere continuum, where turbulent fluxes are based on 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. Evaporation (E), transpiration (T), and evapotranspiration (ET) calculated by the TSEB were compared to measurements obtained with microlysimeters, sap flow gages, and weighing lysimeters, respectively, for fully irrigated cotton (Gossypium hirsutum L.) at Bushland, Texas, USA. Radiometric surface temperature (TR) was used to calculate E, T, and ET in the TSEB in 15-min intervals and summed to intervals coinciding with times of measurements. Also, a one-time-of-day TR measurement was used (either 9:45, 11:15, 12:45, 14:15, or 15:45 CST), and E, T, and ET were calculated for the appropriate measurement interval using the reference ET (ETREF) and reference temperature (TREF) time scaling methods. The Priestley – Taylor formulation 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 Penman–Monteith formulation 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. For 24 h ET, the Priestley – Taylor version resulted in maximum RMSE/MBE of 3.2/-1.9 mm, and the Penman – Monteith version had maximum RMSE/MBE of 1.7/0.95 mm. Daytime ET model agreement was very similar for both model versions (RMSE/MBE usually less than 1.1 less than/+1.0m. However, the Priestley–Taylor version consistently calculated negative nighttime ET of up to -2.0 mm. Summed 15-min calculations generally had better agreement with measurements than did the ETREF or TREF methods, and results did not greatly differ for ETREF or TREF. Both time scaling methods were not very sensitive to the TR measurement time used, although morning (9:45 CST) TR measurement times did not perform as well as the other times.