USING REMOTE SENSING & MODELING FOR EVALUATING HYDROLOGIC FLUXES, STATES, & CONSTITUENT TRANSPORT PROCESSES WITHIN AGRICULTURAL LANDSCAPES
Title: Application of the Priestley-Taylor Approach in a Two-Source Surface Energy Balance Model
Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 17, 2009
Publication Date: February 25, 2010
Citation: Agam, N., Kustas, W.P., Anderson, M.C., Norman, J.M., Colaizzi, P.D., Howell, T.A., Prueger, J.H., Meyers, T.P., Wilson, T.B. 2010. Application of the Priestley-Taylor Approach in a Two-Source Surface Energy Balance Model. Journal of Hydrometeorology. 11:185-198.
Interpretive Summary: This study investigates the utility of using the Priestley-Taylor (PT) approximation, originally developed as a simple regional potential evapotranspiration (ET) model, exclusively for the vegetation component as part of a two-source (soil + vegetation) modeling scheme using thermal-infrared remotes sensing. This thermal-based land surface modeling scheme is being applied over the continental U.S. for mapping ET and drought. Using micrometeorological data from agricultural crops and natural vegetation in a variety of climates, the PT parameter applied exclusively for the vegetation canopy is shown to be fairly conservative at its traditional/historical value of 1.3, except under extreme advective (low humidity or high vapor pressure deficit) conditions, such as observed in irrigated agricultural areas in arid climates where the PT value reached nearly a value of 2. For the natural vegetated sites, the PT coefficient tended to be lower at ~0.95 on average and not strongly dependent on the vapor pressure deficit. However, overall, for most conditions, the use of PT~1.3 in the thermal-based two-source model for ET estimation provided reliable ET estimates, and hence is a robust approach for many landscapes. Techniques using standard weather station observations and remote sensing information are being developed to adjust the PT value for extreme conditions. Further testing of the model’s ability in estimating ET and the behavior of PT under different climate and vegetation conditions are planned.
The Priestley-Taylor (PT) approximation for computing evapotranspiration was initially developed for conditions of a horizontally uniform saturated surface, sufficiently extended to obviate any significant advection of energy. Nevertheless, the PT approach has been proven efficient within the framework of a thermal-based Two-Source Model (TSM) of the surface energy balance, when applied only to the canopy component of the latent heat flux and not to the bulk system. The objective of this research was to investigate the behavior of the PT parameter when applied to solely the vegetation component in view of its utility in the TSM. Micrometeorological flux measurements collected at multiple sites under a wide range of atmospheric conditions were used to implement an optimization scheme, assessing the value of the PT parameter for best performance of the TSM. Overall, the findings suggest that within the context of the TSM, the optimal canopy PT coefficient for agricultural crops appears to have a fairly conservative value of ~1.3 except under very high vapor pressure deficit (VPD) conditions where its value increases. For natural vegetation (primarily grasslands), the canopy PT coefficient assumed lower values on average (0.95), particularly in arid environments. The dependency on VPD for the sites with natural vegetation did not significantly affect the agreement between TSM estimates of latent heat flux and measured fluxes because the LAI at the more arid sites was typically small. This analysis provides some insight as to why the PT approach, initially developed for regional estimates of potential evapotranspiration, can be used successfully in the TSM scheme yielding reliable heat flux estimates over a variety of land cover types.