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<br />GSlS <br /> <br />During the winters of 1957-67, daily mean <br />streamflow at site Q lA generally was small (median <br />of 16 ft3/s), then streamflow increased in the spring, <br />with more days of large streamflow occurring as a <br />result of snowmelt runoff upstream from Trinidad. <br />The daily median (89 ft3/s) and range in streamflow <br />increased in the summer as snowmelt and rainfall <br />runoff resulted in more days of large streamflows. <br />The daily median (27 ft3/s) and range in streamflow <br />diminished sharply in the fall although rainfall runoff <br />events resulted in some days of large streamflow <br />(fig.5A). <br />During the winters of 1984-92, daily mean <br />streamflow at site Q I generally was zero, although <br />there were times when small releases occurred from <br />Trinidad Reservoir for livestock watering. Reservoir <br />releases generally were small (median of 6.5 ft3 Is) <br />during the spring (fig. 5A), whereas during the <br />summer, relatively large streamflows were sustained <br />as water was released from the reservoir to meet the <br />irrigation demand of crops in the study area. During <br />summer, streamflow generally was greater than <br />186 ft3 Is. Reservoir releases were substantially <br />decreased during fall, resulting in a daily median <br />streamflow of about I ft3 Is. <br />Comparison of daily mean streamflow <br />leaving the study area during 1957-67 to stream- <br />flow leaving during 1984-92 at site Q4 (fig. I), a <br />downstream distance of 159.2 mi, indicated that <br />seasonal streamflow characteristics at this last <br />river station generally were similar for the two <br />periods (fig. 5B). Seasonal streamflow at site Q4 <br />was affected by return flows resulting from irrigation <br />activities farther upstream. During 1984-92, daily <br />median streamflow at site Q4 in winter and spring <br />was larger than during 1957-67 (fig. 5B). During <br />summer, the range and daily median streamflow <br />were larger during 1957-67 because snowmelt and <br />rainstorms from the upper basin resulted in more <br />unregulated streamflow entering the lower basin. <br />During 1984-92, Trinidad Reservoir stored many <br />large daily runoff events, resulting in substantially <br />fewer days of large streamflow being released <br />downstream. Daily median streamflow diminished <br />some in the fall during 1957-67, although the range <br />of daily streamflows was similar to 1984-92. <br /> <br />Downstream daily variations in streamflow <br />at1Purgatoire River below Trinidad Reservoir <br />(site QI), Purgatoire River near Thatcher (site Q2), <br />Purgatoire River at Rock Crossing near Timpas <br />(site Q3), and Purgatoire River near Las Animas <br />(site Q4) from January 1984 through December 1992 <br />ar~ summarized by season in figure 6. Daily stream- <br />flow recorded during 1984-92 at these four sites <br />indicated that variations in streamflow of the <br />Pl)rgatoire River downstream from site Q2 were <br />f*ly similar during 1984-92. Streamflow recorded <br />at! sites Q2 and Q4 were affected by water operations <br />wid return flows from the irrigated areas upstream <br />fr~m these two sites. Streamflow at site Q4 also was <br />pdriodically affected to some extent by return flows <br />from outside the study area, as discussed in the <br />"<Generalized Study Approach" section. Daily <br />m,edian streamflow during winter at sites Q2, Q3, <br />and Q4 (fig. 6) was substantially larger than that at <br />si~e Ql; a likely source for the increased streamflow <br />w:as ground-water discharge to the river reach, <br />d$rived in part, from irrigation return flows. Daily <br />median streamflow during winter at sites Q2, Q3, and <br />Q4 was about the same range, indicating there was <br />little additional ground-water discharge to the river <br />in'reaches 2 and 3 (tributary streamflow was minimal <br />d'. . ) <br />urmg wmter . <br />I <br />, Tributaries in the study area generally <br />dtain relatively shallow uplands on both sides of <br />the valley. Most of the tributaries are ephemeral. <br />s~eams that flow in response to snowmelt or <br />high-intensity precipitation. A recent study for <br />ll11 area adjacent to the Purgatoire River Canyon <br />(yon Guerard and others, 1993) reported that most <br />tributary streamflow was intermittent, of short <br />I <br />d~ration, and resulted from rainfall runoff during <br />~ay through October. Tributary streamflow that <br />r~sults from intense precipitation during spring, <br />s~mmer, and early fall generally is characterized by <br />a ,rapid rise in peak streamflow. Tributary runoff in <br />r~ach 2 contributed a relatively small inflow volume <br />t9 the annual streamflow of the Purgatoire River <br />(yon Guerard and others, 1987). Between 1984 <br />ajtd 1992, total tributary streamflow measured at <br />fiye gaged tributaries between sites Q2 and Q3 <br />! . <br />cpntrlbuted from 0.8 to lOA percent of the annual <br />streamflow measured at Purgatoire River at Rock <br />Crossing near Timpas (site Q3, fig. I). <br /> <br /> <br />12 Evaluation of Streamflow Traveltime and Streamflow Galna and ~o..es along the Lower Purgatoire River, <br />Southeastern Colorado, 1984-92 i <br />