Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply (2018) / Chapter Skim
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Appendix A: Observed Hydrologic Trends
Appendix A: Observed Hydrologic Trends
Pages 167-178
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From page 167... ...
Given the potential importance of observed recent trends to both the OST PA application for current operations and to the OST simulation (Sim) applica ion for climate change sensitivity studies, t the Committee embarked on a small exploratory analysis of the actual hydrologic records for example watersheds from across the New York City watershed region.
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From page 168... ...
For each of these four seasons, the analysis explored the trends at each streamgage for two variables: mean seasonal discharge and one-day maximum discharge per season.
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From page 169... ...
. At the top of each graph FIGURE A-2 Schoharie Creek at Prattsville, New York, streamflow trends for daily mean discharge for the summer season, 1952–2017.
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From page 170... ...
The results for the spring season are quite different from the summer season. The analysis indicates a virtually trend-free history over this period with a slight indication of a downward trend over the past 20 years, but FIGURE A-3 Schoharie Creek at Prattsville, New York, streamflow trends for daily mean discharge for the spring season, 1952–2017.
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From page 171... ...
Also, it is useful to determine if the trends are relatively similar across the New York City watershed region or if they are substantially different among the different watersheds. The summer increase at all of the sites is primarily focused on the period 1970 to present, which coincides with the start of the rapid increase in greenhouse gas concentrations.
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From page 172... ...
* 88 Rondout Creek near Lowes Corners, NY 1365000 99 40*
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From page 173... ...
, then prudent management of the system would lead to maintaining lower storage levels in those watersheds prone to high turbidity, so that these high-turbidity events could be captured and particulate matter allowed to settle before delivering water to Kensico Reservoir. All other things being equal, this would have an adverse effect on system reliability, because it would decrease the capacity of the system to store water in the wet seasons to satisfy potential needs in drier seasons.
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From page 174... ...
To integrate these findings with those about mean flows, these findings could be used to further modify the historic record by specifically detrending the most extreme discharges as well as detrending the seasonal average discharges to create artificial records that could serve as inputs to OST operated in Sim mode. The question that could be posed might be of the following form: If we project recent trends in mean flows, and recent trends in peak flows, into the future and then run them through OST in Sim mode, do they indicate increases in the frequency or severity of turbidity violations at Kensico and/or increases in the frequency or severity of shortages of water delivered to customers?
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From page 175... ...
68 West Branch Delaware River at Walton, NY 1423000 860 –18 16 148* 88 Neversink River near Claryville, NY 1435000 172 50 –12 135 116 NOTES: The change over the period is expressed as a percentage change over the period.
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From page 176... ...
To address this question in a simple, data-driven manner, the Committee explored the empirical probability distribution of daily streamflows from two different periods using the Schoharie Creek data previously described. The period currently being used by OST (water years 1950–1997)
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From page 177... ...
This may indicate that there has been little overall change in the upper tail of the distribution, and that one day of the highest flow from Hurricane Irene was simply an outlier rather than an indication of a changing distribution. The fact that the HEFS ensembles do not utilize data from this latter period is troubling, given the rather wide divergence between the ECDFs for the two periods.
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