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Review of the New York City Watershed Protection Program (2020) / Chapter Skim
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Appendix B: Critique and Suggestions Regarding Current Water Quality Trend Reporting Approaches
Pages 393-400

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From page 393...
... Such a graphical presentation could be made with each of the sites shown on the same time axis, with the vertical scale repeated for each of the sites. It appears that this data set for this site consists of only six observations, and as such, would probably not yield a strong statistical inference of trend, but even a purely graphical approach would be suggestive to the reader of a progressively worsening situation, if that were in fact the case.
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From page 394...
... The Committee would urge NYC DEP to place less emphasis on the most recent annual value and more emphasis on the long-term temporal pattern of the data. Another example from the 2017 Watershed Water Quality Monitoring Report shows a short-term and subjective perspective in interpretations.
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From page 395...
... total phosphorus, and (3) fecal coliforms for selected stream sites, with 2017 values displayed as a solid dot.
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From page 396...
... rather than commenting on a few years of data at the end of the time series. The 2017 Watershed Water Quality Monitoring Report attempts to convey information about relative levels of trophic state and some indication of trend in all 20 reservoirs in a single figure (see Figure B-4)
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From page 397...
... One way to view those results is to say "it is not statistically significant at alpha = 0.1 so one can ignore it." A better approach is to say that there is moderately strong evidence that the trend is upwards and one can further quantify it by saying that the likelihood that that the trend is truly upwards is 0.944 and the likelihood that the trend is truly downwards is 0.056 (see McBride, 2019 for a discussion of this approach to reporting test results)
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From page 398...
... Repeating this kind of graphical and statistical analysis annually would be a good approach for NYC DEP to communicate with their stakeholders. In summary, the Committee recommends that in future water quality reports annual values should be depicted as time series.
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From page 399...
... SOURCE: Graph generated by the Committee using NYC DEP data.
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Key Terms

  • water quality
  • watershed water
  • watershed protection
  • formal statistical
  • biological assessment

  • logistic regression
  • trophic state
  • trend test
  • monitoring report
  • quality monitoring

  • assessment profile
  • upwards trend
  • dep data
  • time series
  • using nyc

  • state index
  • nearly random
  • downward trend
  • cannonsville reservoir
  • tsi data

  • profile scores
  • strong evidence
  • regression model
  • city department
  • graphical approach

  • displayed makes
  • box indicating
  • median trophic
  • data span
  • annual values


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