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GJJZ10 39 ThIs report ls concerned only wIth evaporation that occurs from the sur- face of the Arkansas River. In particular, this report Is concerned wi th the incremental Increase In river evaporation due to reservoir water that uses the river as a means of transporting water. There are four basic methods of measuring and estimating evaporation. They are: I. Mass-transfer. An example of this method Is the Thornthwalte-Holzman equation. This method is based on wind speed, vapor pressure differ- ences at different levels, temperatures and coefficients. 2. Energy Budget. This method often takes Into account Items such as Sun and sky short-wave radiation, reflected short wave radiation, long wave radiation exchange with the atmosphere, heat conduction, etc. 3. Water Budget. This approach Is simple in.' theory, i.e., measuring all Inflow to evaporation. But application of this method rarely produces reliable results even under Ideal conditions. 4. Pan Measurements. This method measures evaporation from a pan that Is either sunken In the ground, floating in a lake or exposed on top of the ground. A pan coefficient must be applied to the measured evaporation to obtain actual lake evaporation. Of the four methods, only the last is a practlcal approach for this study. The first three require data and measurements that are not avaIlable for Ar- kansas basin studies. The last method, pan measurements, has been used in the Arkansas Basin. The U. S. Weather Bureau maintains Class "A" evaporation pans at Sugar loaf Reservoir and at the Pueblo CIty ReservoIr. The science of estimating evaporation has not developed to a degree of accuracy where there is a high degree of confidence. After an extensive literature research, we concluded that there has been little research done on evaporation from rivers and streams. Past studies have always been con- cerned with lake and ocean evaporation. Intuitively, and from field obser- vations, and from som~ field tests, we have concluded that the evaporation rate from the Arkansas River Is higher than the evaporation rates from lakes in the basin. This is due to a "splash effect" in the steeper portions of the river where mist is formed by the 1 iteraJly millions of little falls when air becomes entrained In white water and then returns to the atmosphere. Our work has assumed that the evaporation rate from the Arkansas River from Twin lakes to the Colorado Canal is twice the actual lake evaporation for that reach. The lake. evaporation for that reach was .taken as the weighted average of Sugar loaf (1/3) and Pueblo City Reservoir (2/3) pan evaporation rates times 0.7. The o.i being the standard Class "A" pan coefficient. Evaporation rates used in the study are listed in Table V-I.