Time-series data statistics on Water Data for the Nation vs. USGS publications
The statistics generated on this web site may not match values published by the
USGS in various interpretive or data reports for a number of reasons. In general
these relate to the fact that the statistics provided on Water Data for the Nation are generated
from the daily values upon request. As such, it is not possible to exactly mirror
the decisions made by the authors of USGS publications. Some of the most common
reasons for differences between statistical values are as follows.
- The period of record is different between the statistics from Water Data for the Nation and the published statistics.
- Water Data for the Nation allows the user to select the period of record for generating long-term statistics.
Thus the user-selected period may not match the period used for the published values.
Published values are typically generated on the entire period of record, however in some
cases only part of the period or record is used. For example, published discharge statistics
may be computed using only the period after a significant change in the regulatory practices
or storage of a river system.
- The year used may be different
- Water Data for the Nation allows the user to select either water year (October 1 to September 30) or
calendar year (January 1 to December 31) statistics. Published statistics may be
based on either of these years, or sometimes on an entirely separate year such as a climatic year (April 1 to March 31).
- The treatment of partial record may be different
- Water Data for the Nation automatically removes periods of partial record from statistical computations. For example, months
that are not complete are not used for long-term monthly statistics and years that are not
complete are not used for long-term annual statistics. In general this matches the common practice
with published statistics, however at times may be published statistics derived from
- The precision may be different
- The precision (rounding) shown on Water Data for the Nation for the daily values
themselves will usually match any published values.
However the precision of calculated statistics
may not exactly match. Water Data for the Nation rounds calculated
statistical values to one place greater than the standard precision of
any given parameter.The published statistics
may be based on a different rounding standard,
particularly when the daily values data were not collected to the normal
standard precision for that parameter.
- The statistics themselves are different
- Water Data for the Nation currently has a limited set of available statistics.
It may be that the published values are actually totally
different statistics.For example,
Water Data for the Nation only shows statistics based on the daily mean values,
whereas the published values may be based on daily median.
- The published statistics may combine continuous time-series
and discrete data
- Some parameters have historically long periods where only
discrete data were collected combined with a more recent period where
continuous time-series data are available.
Some USGS publications may combine these data using various methods in order
to provide a longer period of record for statistical
analysis than the continuous time-series data alone provides.
A common example of this is when a long historical period of weekly or monthly groundwater level data is
combined with a shorter period of continuous data.
Future versions of Water Data for the Nation should allow for data display and statistics
for combined discrete and continuous data, but currently Water Data for the Nation requires retrieval/display
of each data set independently and displays statistics on continuous time-series