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() S 2.~1 The availability and completeness of streamflow data for tributary streams in the area varied seasonally and spatially by river reach. Analysis of daily mean streamflow data at two gaged tributaries in reach I [sites Tl and T2 (fig. I)] from 1957-67 indicated that tributaries had streamflow a large percentage of the time, but tributary streamflow generally contributed less than 10 percent of the mainstem streamflow (table 2). Analysis of daily mean streamflow data at five gaged tributaries in reach 2 [sites TI-T7 (fig. I)] from 1984--92 indicated that tributaries had streamflow between 38 and 58 percent of the time, but contribu- tions that exceeded 10 percent of mainstem flow occurred infrequently (table 2). A large percentage of the time, gaged tributary inflow in reaches I and 2 contributed less than 10 percent of the mainstem streamflow (table 2), indicating that these seven gaged tributaries generally were not a source of substantial streamflow (inflow) to the lower Purgatoire River. Daily precipitation measured between 1984 and 1992 at National Weather Service stations in various parts of the study area were checked for correlations with tributary streamflow measured in reach 2; the relation would have been used to adjust daily streamflow contributions for the ungaged tributaries in reaches I and 3. The correlation analysis indicated that there were temporal and spatial differences in daily precipitation among the collection sites in the study area. Because of this variability affecting the data, daily precipitation was not correlated closely enough to estimate ungaged tributary streamflows or to determine, with reasonable. accuracy, on which days rainfall runoff occurred in a river reach. srREAMFLOW TRAVELTIMES Traveltime information is valuable in water- resources management to determine when a reservoir release or flood wave would arrive at a downstream Idcation. In this study, traveltime information was needed for time adjustments to the streamflow at the ubstream site in a reach; these time adjustments were used to improve the accuracy of estimates of stream- flbw gain and loss. Hourly streamflow data collected frOm 1990 to 1994 were used to evaluate the relation of traveltime to streamflow at four river stations [sites QI, Q2, Q3, and Q4 (fig. I)]. The estimates of reach traveltime were derived by determining the tiine, in hours, for distinguishable streamflow peaks a~ the upstream site to arrive at the downstream site (peak-to-peak traveltime). Suitable streamflow hydrographs used in assessing traveltimes in a reach consisted of periods of reasonably steady antecedent streamflow, followed by a distinguishable increase and decrease in streamflow. Hydrograph periods were selected so that the ungaged inflows were njinimal during the intervals used for analysis. :tv,Iost of the hydrograph periods that were selected for analysis included from 3 to 10 days of data. The tracking of a specific streamflow peak down the lower PUrgatoire River often was difficult because of large fluctuations in streamflow that occurred due to large <1lversions for irrigation and various ungaged return flpws in a river reach. Traveltimes were not evaluated oh a seasonal basis because the time-of-travel of s\reamflow peaks within a river system is largely dependent on discharge, antecedent streamflow conditions, and the general morphology of the river. Table 2. Percentage of time daily streamflow occurr/1d at selected tributary streams for the upper and middle reaches of the study area, 1957~7 and 1984-92 I [Reach 1 extends from site Ql to site Q2 (see fig. 1) and includes ~o gaged tributaries, and reach 2 extends from site Q2 to site Q3 (see fig. 1) and includes five gaged tributaries;~, greater than] Season 1 Reach 1 (January 1957-December 1967) Percentage of Percentage of time . time tributary tributary atreamflow was streamflow lesa than 10 percent of, occurred malnatem streamflow i Winter Spring Summer Fall 71 75 78 67 91 63 68 90 Reach 2 (January 1994-Decsmber 1992) Percentags of Percsntags 01 time time tributary tributary streamflow was streamflow lesa than 10 percent of occurred malnatem streamflow 58 >99 58 99 41 93 38 >99 IWinter is December-February, spring is March-May, su~er is June-August. fall is September-November. 14 Evaluation of Streamflow Traveltime and StreamflOW Gains and Losses along the Lower Purgatoire River, Southeastern Colorado, 1964-92