Impact of Forest Service Activities on Stream Flow

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TroendlelNankervis/Porth Page 19 5/22/2003 evaluation, 36 years of record were deleted. This represents 1.2 percent of the potential data set. The logic for doing this, and the impact on the double-mass analysis, has been developed in Troendle and Porth (2000). Seasonal stream flows for the 47 Colorado and Wyoming gauging sites were plotted in a double- mass comparison with the stream flow for the Encampment River above Hog Park. The Hog Park gauge had been demonstrated to be a stable record useful for such a screening process (Troendle and Porth 2000). Based on this comparison, many of the stream flow records did not appear suitable for future comparisons (Figure 15) while others appeared quite good (Figure 16). The list was reduced to 11 stream gauges in Colorado and 8 in Wyoming as a result of this analysis. CHERRY CRFEK BELAW CHERRY CREEK LAKE C0.:06713000 8c ENCAMPMENT RN AB HOG PARK CR NR ENCAMPMENT WY0:06623800 w [s. ? ? ? > L 75 ? g O ? 06623800: Cumulative WYRDMF(cfs) Figure 15. Example of an unstable double-mass comparison. If you assume that the stream flow measured at Hog Park is stable, the stream flow from Cherry Creek is quite erratic. No attempt is made to determine the cause of instability. As noted by Troendle and Porth (2000), the double-mass plots do not indicate that change is not occurring at the sites being compazed. What it does indicate is that stream flow at both sites varies similarly, in the case of the comparison in Figure 16, or that one or both sites is erratic as in Figure 15. The next step in the analysis is to assess the stability, and appropriateness, of each site as a reference site. The 19 stream gauges retained for further analysis are listed in Appendix C, Table 1, along with watershed area and length of record. The neact step in the process was to identify some number of reference NRCS snow courses in order to compare snow pack accumulation with stream flow. Ten snow courses in Colorado and 9 courses in Wyoming appeared to be located in the watersheds of concern. We also added the Glade Creek snow course in Wyoming to the list as we knew it was stable and compared well with measured stream flow even though it is in the Little Snake Watershed. The snow course data was downloaded from the NRCS website at http://www.wcc.nres.usda.2ov/water/wmdata.html. Four of the 10 Wyoming snow courses are now snotel (pillow) sites. We found in plotting the data for the 4 manual sites that had been upgraded to snotel sites over time that the data was not consistent through the transition. We dropped those sites from further consideration. We then 0 200000 400000 600000 800000 1000000 1200000 1400000