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<br />~'"' n.' ~ <br />,oJ.! <br /> <br />tion in the epilimnion is partially disrupted downstream <br />from transect 3 as the reservoir surface cools and mixes <br />with the underlying water downstream. This process of <br />fall turnover continues into September as air tempera- <br />tures decrease substantially and can continue into <br />October umil the deeper parts of the reservoir generally <br />are well mixed, Underflow of the Arkansas River helps <br />maintain stratified conditions upstream from transect 3 <br />or 4 throughout the fall and winter. <br /> <br />Specific-Conductance Stratification and <br />Mixing Patterns <br /> <br />Variations in specific conductance in Pueblo <br />Reservoir are a function of the specific conductance <br />and initial routing of the Arkansas River and the ther- <br />mal stratification and mixing patterns within the reser. <br />voir. The specific conductance of the Arkansas River <br />upstream from Pueblo Reservoir varies inversely with <br />discharge. Specific conductance of the river water is <br />lowest in May through early July when discharge is rel- <br />atively large because of snowmelt runoff in the upper <br />basin, Specific conductance gradually increases with <br />decreasing discharge during the late summer and fall. <br />Matching of observed reservoir specific-conductance <br />values with similar values of antecedem specific con- <br />ductance of the Arkansas River inflow provides further <br />infonnation on the effect ofthennal stratification on the <br />initial routing of inflow into the reservoir and mixing <br />patterns within the reservoir. The detennination of <br />stratified or mixed conditions with respect to specific <br />conductance is qualitative; stratification is indicated by <br />a change in specific conductance with depth; well- <br />mixed conditions are indicated by a unifonn or nearly <br />unifonn specific-conductance profile. <br />The specific-conductance profiles for December <br />1986 and March through October 1987 (fig. 5) illus- <br />trate the typical temporal and spatial patterns in specific <br />conductance that were measured from 1985 through <br />]989. These patterns represent those commonly <br />observed in Pueblo Reservoir throughout the year. <br />Specific-conductance stratification patterns are not as <br />well defined as thennal-stratification patterns, With <br />respect to specific conductance, the reservoir generally <br />is well mixed in the winter and early spring (December <br />and March) (fig. 5). The April and May profiles indi- <br />cate specific-conductance stratification is very weak <br />throughout the reservoir. Specific.conductance stratifi. <br />cation becomes well defined in June as underflow <br />becomes the dominant direction of inflow routing, and <br />inflow specific-conductance values decrease at the time <br />of peak snowmelt runoff. Early in the summer, the spe- <br />cific conductance of inflow from the river is relatively <br /> <br />low because of snowmelt runoff; therefore, the under- <br />flow results in smaller conductance values in the <br />hypolimnion. As summer progresses. the inflow spe- <br />cific conductance in the Arkansas River increases as <br />discharge decreases; as a result, the underflow results in <br />larger specific-conductance values near the reservoir <br />bottom from July through October, Fall turnover mixes <br />the reservoir downstream from about transect 4 during <br />August through September, Generally unifonn <br />specific-conductance values in the upper 40 to 70 ft of <br />the downstream part of the reservoir during the late <br />summer and fall indicate these areas are mixing during <br />this time because of fall turnover. Underflow of the <br />Arkansas River maimains relatively strong specific- <br />conductance stratification upstream from about <br />transect 4 during the autumn. Relatively stable <br />specific-conductance values in the river inflow and <br />mixing within the reservoir help maintain nearly uni. <br />fonn, well.mixed conditions from late autumn through <br />the spring throughout much of the reservoir, <br /> <br />Reservoir Residence Times <br /> <br />Residence or retention time is defined as the time <br />necessary for the volume of water in a reservoir to be <br />replaced by inflowing water or the time necessary for <br />the volume of water in a reservoir to be drained by out- <br />flow. When the reservoir is well mixed, the residence <br />time can be calculated by using the following equation: <br /> <br />T = V/Q <br /> <br />(I) <br /> <br />where <br />T <br />V <br />Q <br /> <br />= residence time, in days; <br />= reservoir volume, in acre. feet; and <br />= reservoir outflow, in acre-feet per day. <br /> <br />Theoretical residence times for Pueblo Reservoir <br />can range from a few weeks to more than a year (fig. 6), <br />When the reservoir is thennally stratified, the residence <br />time, as calculated from the above equation. does not <br />accurately represem the actual residence time of water <br />entering the reservoir because of various mixing and <br />circulation patterns that occur within the reservoir, <br />Therefore, the equation for estimating theoretical resi. <br />dence time is val id from about October through March <br />or April, although there is some degree of thennal <br />stratification presem upstream from about transect 3 <br />during part of this time, Less than 7 percent of the total <br />volume of the reservoir is located upstream from <br />transect 3 when storage is at the top of the conservation <br />pool; therefore, the effects of thennal stratification and <br />underflow in this region on reservoir residence times is <br />small. As a result of the relatively strong thennal strat. <br /> <br />PHYSICAL CHARACTERISTICS 15 <br />