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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />2484 <br /> <br />illustrates the strengths ana weaknesses of each type of equation. <br /> <br />Temperature based methods require the least amount of data, are <br /> <br />easiest to use, and thus have the greatest general acceptance. <br /> <br />Combination equations more accurately reflect the climatic <br /> <br />variables driving evapotranspiration processes and thus are more <br /> <br />likely to give satisfactory results when applied to an unknown area. <br /> <br />They are preferred for estimates over short time periods, daily to <br /> <br />weekly. For longer time intervals and seasonal project-level <br /> <br />water use estimates, the simpler methods may perform as well as the <br /> <br />more complicated methods. Use of temperature-radiation and <br /> <br />combination methods should not necessarily be rejected, however, <br /> <br />because of insufficient data. In many cases data can be estimated or <br /> <br />interpolated for the project area with good results. <br /> <br />Selection of an appropriate method for evapotranspiration estimates <br /> <br /> <br />is as much a matter of engineering judgement as data availability. <br /> <br /> <br />Factors to be considered in method selection include the following: <br /> <br /> <br />Intended use of evapotranspiration estimates <br /> <br />Desired time increment for computation <br /> <br />Data availability <br />Performance of the various methods in areas with similar <br /> <br />climates, latitudes and elevations <br /> <br />General acceptance of the equations <br /> <br />Availability of research data, lysimeter studies, or calibrated <br />equations in the project area <br /> <br />1- 21 <br />