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r� <br />L <br />Roscoe to North Platte reach. This is one way to determine if a gage's data is not reliable due to <br />problems with the location of the gage due to timing or the data reliability of the gage due to <br />other factors. In other words, when there is a residual loss, after the evaporation loss was <br />subtracted, from Julesburg to Roscoe that is essentially equal to a gain from Roscoe to North <br />Platte, this may simply be due to a gaging error. For example, if there are no gains or losses and <br />both the Julesburg and North Platte gages both show 500 cfs, but the Roscoe gage shows 600 cfs, <br />it would appear that there was a 100 cfs gain from Julesburg to Roscoe and a 100 cfs loss from <br />Roscoe to North Platte. Although it is possible that there really was a 100 gain, then a 100 cfs <br />loss, if the number of times the gain equals the loss is high, the chance of having a gage <br />problems are high. The percentage of time when this residual reversal of X cfs in the Julesburg <br />to Roscoe reach was equal to a —X cfs residual in the Roscoe to North Platte reach was calculated <br />and is shown in the following graph. <br />The graph shows after 1992 the likelihood of the residual reversing increased significantly and <br />that after 1994 the likelihood increased even more. There is not enough data to determine which <br />event (bridge construction or change of administration) has had the greater effect. When the <br />results are compared to the same analysis done with data from the Paxton gage one can see that <br />the gage reliability is sub -par. <br />An analysis was done to determine whether the physical conditions of the basin had changed <br />between the periods of record for the two gages and was the cause of the increase in residual <br />reversal probability. Previously, the South Platte River had a consistent chance of losing in both <br />the upstream and downstream reaches (see graphs in Appendix B). Monthly mass - balance <br />analysis of the Roscoe gage data shows that today the upstream reach has a greater chance of <br />losing than the downstream reach. The daily residual reversal analysis shows that the residual <br />changed from gaining to losing more often than losing to gaining (at a ratio of approximately <br />2:1, the Paxton analysis showed a ratio of 1:9). This is completely opposite from what could be <br />expected and shows that the gage has never performed up to standards. I believe that this gage <br />can be used in the interim, but that it should be improved or relocated as soon as possible. <br />10 <br />