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on0438 should vary for different times of the year and also for different elevations. As a first approximation of the right order of magnitude. the "dropoutll values shown in Table III have been used as an initial step to illustrate such an adjustment tech- nique. On an annual basis precipitation-year totals corresponding to the water-year runoff totals at Glen Canyon Reservoir should ordinarily include data from September through August. Only very heavy storms in early September contribute to the current Septemberrunoff measured at Glen Canyon. (See September, 1927). Prior to the development of this adjustment table, tests were made on samples of data cover- ing rather small watersheds which have little or no diversion above gaging stations. For instance. the actual September-August precipitation at Fraser for water year 1957 was 28. 08 inches. When these data are adjusted. the net result is 23.37 inches. The runoff for a small 32. 8 square mile watershed measured on St. Louis Creek near Fraser was equal to 21. 58 inches. This was a wet year, and it is believed that some of the moisture was carried over into 1958. From September to August, 1958, the actual precipitation total was 17.23 inches. The adjusted total was only 12. 16. and the runoff was 15. 00 inches. This indicates a benefit in runoff from 1957 precipitation. The two years combined show actual precipitation of 45. 31 inches. The adjusted two-year precipitation was 35.53 inches. and run- off 36.58 inches. Similar relationship problems for small watersheds near Dillon and near Silverton also gave good results for typical near average con- ditions and for wet and dry year extremes. Water- sheds at low elevations studied included the Paria River in Utah and Chevelon Creek on the Little TABLE III 7 Colorado River in Arizona. At these fwo locations the median annual runoff is less than one-half inch. and practically all the annual precipitation must be deducted in the adjustment. The I1dropoutll values as shown in Table III have been used only to illustrate the technique. Further gradation for elevation is recommended. It is also expected that subsequent test and crit- icisms by experienced hydrologists familiar with precipitation and runoff relationships in the Colo- rado River Basin will permit refinement. Subsequent developments in evaporation measurement techniques may give indications of more correct "dropouts" to be applied. b. Value Of "Precipitation Contributing To Runoff" - The effect of making such reductions in observed precipitation amounts as estimates of the losses by evaporation and transpiration are shown in Figure 5. Figure 5 shows that high-altitude stations contribute significantly more runoff than do nearby low-altitude stations. For example, Figure 5 shows more than 16 inches contributing to runoff from Telluride while the nearby station of Montrose yields about only one inch of precipitation contributing to runoff. c. Number Of Storms Contributing To Run- off - Figure 6 shows the number of storm periods that are effective in contributing to runoff after the observed precipitation data are reduced for esti- mated evapotranspiration losses by the values ~hown in Table III. The Number of storm periods contributing to runoff follows a pattern that is sim- ilar to the precipitation contributing to runoff shown in Figure 5. The stations at higher elevations. suchas Telluride, have many more periods each year in which storms contribute to runoff than near- by low elevation stations. such as Delta or Mont- rose. The coefficient of variation for the low- altitude stations is much higher than for the high- elevation stations. AMOUNTS TO BE DEDUCTED (INCHES) FROM INDIVIDUAL STORMS TO ADJUST ACTUAL PRECIPITATION TO "PRECIPITATION CONTRIBUTING TO RUNOFF" Sept Oet Nov Dee Jan Feb Mar Apr May Jun Jul Aug High Level Stations -.5 -.5 I no deduction -. 3 -. 3 -.5 -.5 I cumulative Middle Level Stations -, 7 -.7 -.5 -.2 -.2 -.2 -. 2 -. 5 -.5 -. 5 -.7 -. 7 I cumulative I , Low Leve 1 Stations -. 8 -. 8 -.6 -.6 -, 4 -.4 -.6 -.6 -.6 -. 6 -.8 -.8