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2. A far more confusing picture emerges from the analyses of the mean annual precipitation levels in the study area. The time series plot (Figure 4.9) shows high interannual variability; the five~year running means reveal dry periods near the turn of the century, in the early 1930s, in the mid-1950s through the early 1960s, and in the mid-to-late 1970s. The early 1980s were characterized by unusually high precipitation totals in the area.. averaged data. At all three Utah divisions, the Student's t shows a statistically significant rise in precipitation levels (Table 4.3); however, rainfall totals decline noticeably, although not significantly, at the two Wyoming divisions. As a result, the area-averaged annual precipitation totals display a modest increase in the Student's t that is not significant. Linear trends in the area-averaged data through the entire period or through either subperiod are also not significant (the trends are very close to zero). In addition, the Bartlett test does not reveal the existence of any statistically significant changes in the variability of annual precipitation totals over the study period. 3. The patterns in the PHDI values are similar to the patterns in the annual precipitation data. The plot of annual PHDls (Figure 4.10) shows a set of dry and wet periods described for the precipitation data. The Student's t test results show that at all three Utah divisions, the mean PHDI values have risen significantly indicating a shift to a more moist regime. However, the two divisions in Wyoming display statistically significant decreases in the mean PHDI levels. Student's t test results for the area..averaged data show a slight, non~significant, decline in the mean PHDI values. No significant linear trends were found in the area-averaged data over the 1895..1962 subperiod, the 1963-1989 subperiod, or for the entire 1895-1989 study period. In addition, no changes in variability were found in any of the PHDI data. 4.5.5. Conclusions The purpose of this investigation was to determine if climatic changes have occurred, or are occurring, in the Upper Green River /Y ampa River basins. The analyses clearly show significant increases in temperature throughout the study area. However, these analyses are based entirely on the annual temperature values. The more detailed description of the rise in temperature (e.g., seasonality, maximum verses minimum temperature) is beyond the scope of this preliminary study. Nonetheless, a strong, highly statistically significant, warming signal appears in the temperature data of the study area. The precipitation and Palmer Hydrological Drought Index (PHDI) patterns are much more complex. The area-averaged annual precipitation and PHDI values do not show statistically significant changes over the 1895-1989 study period. However, imbedded within these area-averaged data are a number of significant changes. The three Utah climate divisions all show a tendency to the 1963-1989 subperiod to be significantly more moist than the 1895-1962 subperiod. Amazingly, the two Wyoming climate divisions display significant decrease in moisture levels over the same time intervals. Therefore, one must conclude from our preliminary study, that significant climate changes have occurred and are occurring within the Upper Green River/Yampa River basins. A tabular summary of climatic data is included in Appendix F. 4-12 I I I I I I I I I I I I I I I I I I t