Date of Award:

5-1964

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Department name when degree awarded

Irrigation and Drainage Engineering

Committee Chair(s)

Jerald E. Christiansen

Committee

Jerald E. Christiansen

Abstract

The principal objectives of this study were to compare several formulas for estimating pan evaporation and evapotranspiration and to derive new formulas or modify existing formulas that will better fit the data for central Iraq. Computed evaporation was compared with measured evaporation by determining the differences and the ratios. The pan evaporation formulas were also compared by expressing them all in the form
Ev = K CRCTCWCHCDTCSCM

Three pan evaporation formulas were modified. For the Meyer and Utah formulas, the constants were changed and expressions were derived for monthly coefficients. For the Blaney-Criddle formula, an expression for k was derived in which k is a function of the mean temperature, wind velocity, and the month.

For potential evapotranspiration, two formulas were modified to give reasonable values for January and July. For the Blaney-Criddle formula, an expression for k was derived in which k is a function of temperature. Grassi's formula 3a, which expresses evapotranspiration as a function of pan evaporation, was modified by deriving a new expression for the temperature coefficient.

From these modified formulas, potential evapotranspiration and pan evaporation can be estimated from climatic data without applying any judgment factor, as is ordinarily necessary in using the Blaney-Criddle formula.

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