Date of Award

12-1977

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geography

Supervisor

Professor J. A. Davies

Abstract

Procedures to calculate hourly and daily evapotraspiration from vegetated surfaces in the absence of lysimetric or micrometeorological measurements are few. A new for physically-based evapotranspiration models to account for atmospheric and surface control on the process is recognized. The micrometeorological investigation described in this study is a response to this need.

During the growing season of 1974, a measurement program was conducted to study atmospheric and surface control on hourly and daytime evapotranspiration. Data collected include net radiation and soil heat flux densities; vertical profiles of temperature, vapour pressure and windspeed; soil moisture; and leaf stomatal resistance.

The combination model which combines equations dealing with the energetics of evapotranspiration and turbulent transfer was employed in the study of hourly evaporative relationships. In the variant of the model used, atmospheric and surface controls on the process are explicitly considered through the deployment of an aerodynamic resistance to water vapour exchange and a surface resistance to water vapour efflux. Hourly estimates of aerodynamic resistance to water vapour exchange were computed from wind profile data. The combination model was insensitive to aerodynamic resistance errors. Hence, simple parameterization of surface roughness and zero plane displacement could be used with single level windspeed measurements to evaluate aerodynamic resistance. Also stability correction could be ignored for practical applications.

Bulk stomatal resistance was evaluated by residual from the combination model on an hourly basis. Comparison was made with independent estimates developed from stomatal resistance and leaf area index measurements. After canopy development was sufficient to dominate surface energetics, the agreement between the estimations was good. Leaf stomatal and bulk stomatal resistances were found to vary in response to irradiance, soil moisture availability and the vapour pressure deficit of the air. However, the relationship found for the latter is not well defined.

For days when canopy development was sufficient to dominate surface energetics, daytime evapotranspiration totals normalized by equilibrium evapotranspiration are linearly related to soil moisture availability when volumetric soil moisture is below 0.12 mm³ H₂O/mm³ soil. With soil moisture above this threshold limit, evapotranspiration proceeds at a potential rate. These results may prove useful in computing daytime evapotranspiration from cropped surfaces.

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Geography Commons

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