Modeling of Transpiration Rate under Heat and Saturation Deficit Advective Conditions
Location
Space Dynamics Laboratory
Event Website
http://water.usu.edu/
Start Date
3-25-2004 12:05 PM
End Date
3-25-2004 12:10 PM
Description
Exotic riparian vegetation has expanded into western river systems. The amount of water consumed by these plants must be quantified in order to understand and manage the ecosystems and water resources. However, reliable measurements and models of transpiration are lacking for these unique ecosystems which are usually narrow zones surrounded by arid lands. Advection of heat and saturation deficit from arid surroundings and the Atmospheric Boundary Layer (ABL) can enhance evapotranspiration rates by supplying extra energy and introducing larger value of saturation deficit to the vegetation. This study was conducted to study and model the transpiration by a Tamarisk canopy along the Rio Grande in New Mexico. The objective is to determine the environmental factors that govern its transpiration rate by inverting a modified Pennman-Montieth (PM) equation coupled with the ABL. The final goal is to generate a predictive model of daily transpiration by using both modified Pennman-Montieth equation and application of the Ball-Berry stomatal conductance model. Fluxes of sensible heat and latent heat were measured above the canopy using the eddy covariance technique for the growing seasons of 1999 and 2001. Other energy components, net radiation energy flux and ground heat flux, were also measured. Vertical profiles of temperature and humidity were made for several days by radiosonde. Daily water use values for the fully leafed out period averaged 7.4 mm/day and ranged from 4.1 mm/day to 10.2 mm/day for the same period. The seasonal total transpiration amount was around 1000 mm. The relative importance of available energy vs. advection and stomatal conductance in governing transpiration was determined by calculating a coupling or omega factor. The coupling factor was found to vary with time of day, ranging between 0.95 and 0.20. The values generally decreased relative to the wind speed. A weak canopy coupling to the atmosphere was often found in a morning and a strong canopy coupling to the atmosphere in the afternoon. Radiosonde data also showed that saturation deficit in ABL mixed layer was not connected in a morning and it was brought to surface in an afternoon. Canopy conductance was back calculated from the PM equation, and the variation was related to global radiation and vapor pressure deficit. These estimates agreed well with Ball-Berry stomatal conductance model after using calibrated parameters from measured data. The model of transpiration was calibrated using data measured in the Bosque 1999 and applied to Bosque 2001. Measured and modeled daily evapotranspiration values were in good agreement, averaging within 10% on a daily basis. This difference in measured and modeled values was less than uncertainty in the measurement.
Modeling of Transpiration Rate under Heat and Saturation Deficit Advective Conditions
Space Dynamics Laboratory
Exotic riparian vegetation has expanded into western river systems. The amount of water consumed by these plants must be quantified in order to understand and manage the ecosystems and water resources. However, reliable measurements and models of transpiration are lacking for these unique ecosystems which are usually narrow zones surrounded by arid lands. Advection of heat and saturation deficit from arid surroundings and the Atmospheric Boundary Layer (ABL) can enhance evapotranspiration rates by supplying extra energy and introducing larger value of saturation deficit to the vegetation. This study was conducted to study and model the transpiration by a Tamarisk canopy along the Rio Grande in New Mexico. The objective is to determine the environmental factors that govern its transpiration rate by inverting a modified Pennman-Montieth (PM) equation coupled with the ABL. The final goal is to generate a predictive model of daily transpiration by using both modified Pennman-Montieth equation and application of the Ball-Berry stomatal conductance model. Fluxes of sensible heat and latent heat were measured above the canopy using the eddy covariance technique for the growing seasons of 1999 and 2001. Other energy components, net radiation energy flux and ground heat flux, were also measured. Vertical profiles of temperature and humidity were made for several days by radiosonde. Daily water use values for the fully leafed out period averaged 7.4 mm/day and ranged from 4.1 mm/day to 10.2 mm/day for the same period. The seasonal total transpiration amount was around 1000 mm. The relative importance of available energy vs. advection and stomatal conductance in governing transpiration was determined by calculating a coupling or omega factor. The coupling factor was found to vary with time of day, ranging between 0.95 and 0.20. The values generally decreased relative to the wind speed. A weak canopy coupling to the atmosphere was often found in a morning and a strong canopy coupling to the atmosphere in the afternoon. Radiosonde data also showed that saturation deficit in ABL mixed layer was not connected in a morning and it was brought to surface in an afternoon. Canopy conductance was back calculated from the PM equation, and the variation was related to global radiation and vapor pressure deficit. These estimates agreed well with Ball-Berry stomatal conductance model after using calibrated parameters from measured data. The model of transpiration was calibrated using data measured in the Bosque 1999 and applied to Bosque 2001. Measured and modeled daily evapotranspiration values were in good agreement, averaging within 10% on a daily basis. This difference in measured and modeled values was less than uncertainty in the measurement.
https://digitalcommons.usu.edu/runoff/2004/AllPosters/1