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• Why aren't data available for the site I am interested in?
• Why aren't data available for the time period I am interested in?
• When will more discharge data be available?
• Where are the instantaneous data for the period between that available on NWISWeb and from the archive?
• What is a unit value?
• Why does the instantaneous peak in IDA differ from the flood peak in the USGS Peak Flow File (or the peak on NWISWeb)?
• How is daylight saving time handled in IDA?

Why aren't data available for the site I am interested in?

The station may not have any readily available instantaneous data, or the USGS Water Science Center operating the site may not have populated the archive for that station yet.

Why aren't data available for the time period I am interested in?

The instantaneous data may never have been available electronically and may only exist in paper records, or it may have been lost due to the failure of an old archival media.

When will more discharge data be available?

All USGS Water Science Centers are encouraged to make as much historical instantaneous data available as soon as possible. Advances in computer memory capacity now allow for storage of virtually all historical instantaneous values, and most Centers have reloaded all readily available electronic data from backup media. Further processing of historical data involves recovering from the original paper records, which is extremely time intensive and expensive. This cost is not covered by the operational budget of the existing data collection program and thus must be done as time and resources permit, or must be borne by those requesting historical data. Users with needs for specific periods and or stations are encouraged to contact the local Water Science Center.

Where are the instantaneous data for the period between that available on NWISWeb and from the archive?

Currently this site serves instantaneous data only for periods that have been approved and published. This typically occurs anywhere from 6-18 months after the data is originally collected. NWISWeb currently serves instantaneous data for the most recent 31 days. Data between the two periods is in the process of being corrected and reviewed. Eventually these data will also be available, but currently the provisional status and potential for repeated change do not make it appropriate for loading to the archive, and the volume does not allow it to be made available via NWISWeb. Users who require these data should contact the local Water Science Center.

What is a unit value?

Internally the USGS has commonly referred to instantaneous data as "unit values." This is as opposed to "daily values", which are a statistical measure of the instantaneous data that occurred on a given day, such as the daily maximum, minimum, or mean. On this site the term "unit values" is not used, however those who contact a USGS Water Science Center should be aware of the term when discussing the availability of instantaneous data.

Why does the instantaneous peak in IDA differ from the flood peak in the USGS Peak Flow File (or the peak on NWISWeb)?

This is likely because the peak in the Peak Flow File has been officially revised but the instantaneous data value in IDA has not been modified to reflect that revision. Typically, the USGS will revise a flood peak, which is an instantaneous value, whenever it is found to be in error by more than 10%. The revision criteria, however, for published daily values is much broader, and sometimes a daily value will not be revised until it is in error by as much as 50%. Because the instantaneous data in IDA is tied to the published daily values, in instances where the peak for the year in the Peak Flow File is revised but the corresponding daily value is not, the instantaneous peak in IDA will reflect the published daily value and not the revised peak in the Peak Flow File. In general, users should use peaks in the Peak Flow File in such cases because the Peak Flow File represent officially approved and published data and IDA data does not. Users may also wish to check with the local USGS office for confirmation. The revision criteria for streamflow have recently been changed such that for any future revisions the instantaneous data and daily values will be revised along with the peak in the Peak Flow File to ensure data consistency.

How is daylight saving time handled in IDA?

All times are shown in IDA using the time settings (the time zone and whether or not to switch to daylight time) stored with each station. For sites that are set to use daylight time, this means that the IDA output file will switch from standard to daylight time as appropriate (which will result in a 23 hour day) and from daylight to standard time (which will result in a 25 hour day). Normally the time settings of each station reflecting the local usage where the station is located, however that is not always the case given changes in local usage over time. Some USGS Water Science Centers have also consistently used standard time regardless of local usage of daylight time. The time zone of each instantaneous value is clearly shown in the tz_cd column of the IDA output files.