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Equilibrium, Potential and Actual Evaporation from Cropped Surfaces in Southern Ontario

J. A. DaviesDept. of Geography, McMaster University, Hamilton, Ontario, Canada

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C. D. AllenDept. of Geography, McMaster University, Hamilton, Ontario, Canada

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Abstract

Micrometeorological data collected over cropped surfaces at Simcoe in southern Ontario are used to evaluate various evaporation rates. Data collected in 1971 from two plots of perennial ryegrass, one of which was irrigated, show that daily potential evaporation (λPE), taken as the evaporation rate from the irrigated plot, is related to an “equilibrium” rate (λPES) by λPE = αλPES, where α = 1.27. This value of α is very similar to that reported by Priestley and Taylor. Previous data for other crops support this value. Data from elsewhere are included to show why the value should be a best estimate of α for air temperatures between 15 and 30C. It follows that at temperatures over 25C the surface net radiation should be a good estimate of λPE. Actual and equilibrium evaporation values for the dry grass plot in 1971 compared closely on moderately dry days, confirming previous results. An approach to calculating daily evaporation is presented which utilizes λPES and surface soil moisture.

Abstract

Micrometeorological data collected over cropped surfaces at Simcoe in southern Ontario are used to evaluate various evaporation rates. Data collected in 1971 from two plots of perennial ryegrass, one of which was irrigated, show that daily potential evaporation (λPE), taken as the evaporation rate from the irrigated plot, is related to an “equilibrium” rate (λPES) by λPE = αλPES, where α = 1.27. This value of α is very similar to that reported by Priestley and Taylor. Previous data for other crops support this value. Data from elsewhere are included to show why the value should be a best estimate of α for air temperatures between 15 and 30C. It follows that at temperatures over 25C the surface net radiation should be a good estimate of λPE. Actual and equilibrium evaporation values for the dry grass plot in 1971 compared closely on moderately dry days, confirming previous results. An approach to calculating daily evaporation is presented which utilizes λPES and surface soil moisture.

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