Analysis of Streamflow Trend in the Susquehanna River Basin, USA - Hydrologic Time Series Analysis: Theory and Practice - page 188

9.4 Results and Discussion
9.4.1 Annual Streamflow Time Series
The annual minimum, median and maximum streamflows over the period of
1940 to 2006 are analyzed in this study and the results are based on the
detailed analysis of Sinnemahoning Creek at Sinnemanhoning, Pennsylvania,
and summaries of the detailed analysis of the other watersheds.
Figure 9.4 shows the annual minimum streamflows at Sinnemahoning
Creek and Fig. 9.5 demonstrates the test results. The individual trend tests are
usually significant if the beginning year is in the 1960s or before and the
ending year is in the 1970s or later. The long-term trends are usually significant.
As discussed in 'Methodology', this finding suggests that a long-term change
in annual minimum streamflows at Sinnemahoning happened during a short
period around 1970. For the trends of shorter periods, there are often no
significant trends if both beginning and ending years are before 1970. If the
beginning year is in the mid-1970s to mid-1980s, there are often significantly
decreasing trends, which show that the annual minimum streamflows in
Sinnemahoning are relatively high in that period.
Other than Deer Creek at Rock, Maryland, and Dunning Creek at Belden,
Pennsylvania, all the remaining watersheds show sensitive annual minimum
streamflows as what have been seen in Sinnemahoning Creek and West Branch
Susquehanna River at Bower, Pennsylvania. These watersheds are distributed
in the western and middle portions of the Susquehanna River Basin (Fig. 9.6).
On the other hand, Deer Creek and Dunning Creek do not show the abrupt
increased annual minimum flows which occurred around 1970.
Moreover, the annual median streamflows at Sinnemahoning Creek are
shown in Fig. 9.7 and the results of the trend pattern identification are shown
in Fig. 9.8. While only some of the trend tests are significant if the beginning
year is before the mid-1960s and the ending year is after the mid-1970s, those
tests often have p -values of less than 0.10. If the significance level of 0.10 is
adopted, the pattern is very like the one seen in the annual minimum streamflow.
This indicates that the abrupt increase in annual median streamflows in
Sinnemahoning Creek is of a lesser magnitude than the abrupt increase in
annual minimum streamflows. Pine Creek at Cedar Run, Pennsylvania, West
Branch Susquehanna River at Bower, Pennsylvania, and Frankstown Branch
Juniata River at Williamsburg, Pennsylvania, share similar trend patterns with
Sinnemahoning Creek in that all have relatively sensitive annual median
streamflows. The watersheds with relatively sensitive annual median
streamflows are located in the western portion of the Susquehanna River
Basin (Fig. 9.9).