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Frequency Asked Questions (FAQ)


  1. How can I check which stations were used in the spatial interpolation? I?d also like to know the number of data years each of these stations has so I can ascertain how much data (both temporal and spatial) the final precipitation frequency estimates are based upon.
  2. The new precipitation frequency estimates differ from some of the other resources that I have, which are more accurate? Which should I use?
  3. What is the difference between an annual maxima series and a partial duration maxima series? Is there a rule-of-thumb regarding basin size or other criteria that favors one method of estimating over the other?
  4. The state-specific page won't load.
  5. The PFDS web site can't be found.
  6. Where can I find historical rainfall information?
  7. When using NOAA Atlas 2 or TP-40, how do I determine estimated depth area reduction factors for area sizes larger than 400 square-miles?
  8. When using NOAA Atlas 2, how do I determine estimated depth area reduction factors for durations less than 30-minutes or greater than 24-hours?
  9. When I try to load a PFDS web page it says it is either missing or I am not authorized to view it. It says HTTP Error 403 ? Forbidden. What am I doing wrong?
  10. I don't understand your adopted terminology in describing precipitation frequency estimates and the different series they are derived from. Can you explain?

Question How can I check which stations were used in the spatial interpolation? I’d also like to know the number of data years each of these stations has so I can ascertain how much data (both temporal and spatial) the final precipitation frequency estimates are based upon.

Answer The actual final quality controlled data from which the estimates were prepared are available from our Precipitation Frequency Data Server (PFDS) web site. You'll note we have both annual maximum series and partial duration data series posted under Time Series. The data includes station information as well as period of record. However because of the method we use (regional approach using L-moments) it is incorrect to assume that the period of record at a single station is indicative of its accuracy. A better approach would be to review the upper and lower confidence intervals on the estimates. Those are also available from the PFDS.
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Question The new precipitation frequency estimates differ from some of the other resources that I have, which are more accurate? Which should I use?

Answer You may want to consult your client or the governing regulatory agency with regards to which precipitation frequency resource to use. NOAA Atlas 14 supersedes all other NOAA/NWS publications and is considered significantly more accurate and reliable. Our precipitation frequency estimates are defacto national standards. They have been endorsed by the Federal Advisory Committee on Water Information's Subcommittee on Hydrology and were funded in part by some state DOTs, state DNRs acting through the Ohio River Basin Commission, some counties, USACE, FEMA, NRCS, and TVA.
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Question What is the difference between an annual maxima series and a partial duration maxima series? Is there a rule-of-thumb regarding basin size or other criteria that favors one method of estimating over the other?

Answer Above an average recurrence interval of about 20 years the values are the same. The difference becomes important for smaller and more frequent events. Partial duration (PD) results diverge from annual maximum (AM) results and can be up to 8-12% higher at about 2 years. Our previous publications (e.g. NOAA Atlas 2) have expressed the results in terms of PD series values and included a discussion in the text to convert to AM values. In the new NOAA Atlas 14 we're trying raise the profile of this difference so that users will be motivated to understand the data they're using. Technically the results from AM series give annual exceedance probabilities, i.e. the chance that a particular threshold can be exceeded on one (or more!!!) occasions in any particular year. The fact that it can be exceeded on more than one occasion means that the reciprocal of the annual exceedance probability is not necessarily the average recurrence interval.. The average recurrence interval is computed based on PD series. See FAQ #10 for more information.
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Question The state-specific page won't load.

Answer The page may require more cache memory to load this page than is available. All browsers have a way to clear the memory cache, so you may want to try this.
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Question The PFDS web site can't be found.

Answer If http://hdsc.nws.noaa.gov/hdsc/pfds/ Does not work, try http://dipper.nws.noaa.gov/hdsc/pfds/
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Question Where can I find historical rainfall information?

Answer Try the National Climatic Data Center (NCDC) at 828-271-4876 or http://lwf.ncdc.noaa.gov/oa/ncdc.html for historical rainfall data. The Regional Climate Centers are also a good source for climate information.
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Question When using NOAA Atlas 2 or TP-40, how do I determine estimated depth area reduction factors for area sizes larger than 400 square-miles?

Answer We would be uncomfortable extrapolating the curves much beyond about 500 sq-mi. The basic assumption behind areal reduction factors is that there is dependence between the point and areal values. The correlation between point estimates reduces as they get farther apart until the values become independent and so the dependence relation between a point and an area breaks down as well.
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Question When using NOAA Atlas 2, how do I determine estimated depth area reduction factors for durations less than 30-minutes or greater than 24-hours?

Answer Given the basin area size in question, from NOAA Atlas 2's depth-area curves (for example, for NOAA Atlas 2, Volume III - Colorado, figure 14), compute the areal estimates for every duration (30 minute, 1 hour, 3 hours, 6 hours and 24 hours) shown in figure 14. Once that is done, plot these values on a duration (x-axis) versus depth (y-axis) linear graph. Then fit a curve to the plotted points, and extrapolate the fitted curve downward to five minutes, 10 minutes, etc. or upward to 36-hours, 48-hours, etc. We recognize you need a number to work with at shorter durations, but the data used create the curves in figure 14 does not confidently support these very short or very long durations. Therefore, use the extrapolated estimates appropriately.
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Question When I try to load a PFDS web page it says it is either missing or I am not authorized to view it. It says HTTP Error 403 – Forbidden. What am I doing wrong?

Answer This is usually caused by your web browser loading an old web page that has been stored in its cache memory. Try returning to the page you tried to load, and force the page to re-load by holding down the CTRL key while you reload/refresh. This will force the page in your computers cache memory to be over-written with the new web page. If the new web page still does not load or work properly, please report it to us.
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Question I don't understand your adopted terminology in describing precipitation frequency estimates and the different series they are derived from. Can you explain?

Answer Precipitation frequency estimates derived from annual maximum series and partial duration series have different statistical meanings and also have different values below about 20-25 years Average Recurrence Interval (ARI). Annual maximum series analysis considers the maximum value in each year. Partial duration series analysis considers the highest values in the entire period of record regardless of which year the values occur. As a result, ARI for annual maximum series is the average interval between years in which a threshold might be exceeded, not between events. ARI for partial duration series is the average interval between the events themselves. Annual exceedence probability (AEP) is the chance that a particular threshold might be exceeded in any particular year (once or more than once) and so only properly applies to results based on annual maximum series. Precipitation depth or intensity estimates derived from partial duration series are higher than estimates derived from annual maximum series for ARIs below about 20-25 years and become increasingly higher as the ARI reduces.
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