Monthly Hydrologic Conditions for New York Active

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Statistical analyses that correlate current hydrologic conditions with previous climatic extremes commonly use a statistical measure known as the percentile. A percentile is a value on a scale of 100 and indicates the percentage of recorded values that are equal to or below it. For example, a river discharge at the 90th percentile is equal to or greater than 90 percent of all discharges recorded at this station during all years in which measurements were made. Percentiles are used to characterize the ranges of reported values; for example:

• a percentile greater than 75 is considered above normal conditions, excessive, or wet
• a percentile between 25 and 75 is considered normal conditions
• a percentile less than 25 is considered below normal conditions, deficient, or dry

A variation of the percentile known as the “percent exceedance” is used in some hydrologic studies, particularly those related to droughts. The percent exceedance is obtained by subtracting the percentile scale value from 100 percent; thus a discharge at the 90th percentile is the same as a discharge at 10 percent exceedance (100-90=10). In other words, only 10 percent of the values ever measured at this site exceeded this discharge value.

The usefulness of data in assessment of the effects of climate variability or for the analysis of trends depends on the length of the period of record; therefore the USGS typically uses only stations with at least 30-years of record for analysis of hydrologic trends. All but two of the surface-water stations in the New York USGS hydrologic conditions network have at least 30 years of record, but many of the upstate monitoring wells have a much shorter period of record; therefore the USGS is using records from wells with at least 5 years of record to provide provisional analyses of ground-water conditions, with the caveat that the data do not represent long-term conditions.

*Percent Exceedance is the probability that an event/measurement/calculation will exceed a specified reference level recorded over a period of time or over the period of record.

For Groundwater:

For example, percent exceedance is the percentage of April groundwater levels at a site over the period of record (ever measured in April at that site) that are equal to or greater than the current April water level measurement for that site. An exceedance of 40 percent for April means that 40 percent of the April measurements ever made at this site are higher than the current (WY 2008) April water level measurement at this site (or conversely 60 percent are lower).

For Surface Water:

For example, percent exceedance is the percentage of December monthly mean discharges for the period of record at a site (all December monthly means ever measured at the site) that are equal to or greater than the current December monthly mean discharge for that site. An exceedance of 40 percent for December means that 40 percent of the December monthly mean discharge values ever calculated for this site are greater than the current (WY 2008) December monthly mean discharge (or conversely 60 percent are lower).