Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Civil and Environmental Engineering


Jerald E. Christiansen


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
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 ternperature. Grassi's formula 3a, which expresses evapotranspiration as a function of pan evaporation, was rnodified by deriving a new expression for the temperature coefficient.

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