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Help using the NWISWeb water-quality data retrieval system

All Data Times In MD-DE-DC Are Eastern Standard Time

This page introduces the water-quality data base, and guides new users through example data retrievals. Although a few definitions are provided herein, more complete definitions are provided on the main NWISWeb help page.

• Recent updates
• Background
• Brief explanation of data base
• Data retrievals-general guidance
• Example data retrievals
• Additional information for technical data analysis
• Data retrievals-precautions

Recent updates

Major enhancements to the water-quality database and the display of these data were incorporated into the NWISWeb system in July 2007. Read the news section for more information.

Background

The NWISWeb water-quality data base is a compilation of the USGS Water Science Center NWIS data bases. It is very large (about 4.4 million samples), with many types of measurements for many different environmental media, from USGS and other agencies. Search criteria are necessary to focus your search. There often is more than one way to locate desired data. The examples provided below are meant to familiarize new users with the system.

Brief explanation of data base

Understanding of a few key elements of the data base is essential to understanding what kind of data you retrieve. Technical data analyses often require more detailed information and coding

Site type describes whether the site is a surface-water, ground-water, precipitation-monitoring, or other site type.

Site name describes the location of the site. Often, the site type can be inferred from the site name. Requesting "list of sites" output formats allows users to view basic information for each site.

Medium type or medium code defines what kind of sample was collected for analysis. This question is particularly important for surface-water sites, which can have samples for several media (water, biological tissue, stream-bottom sediments, suspended sediments, etc.).

Parameter codes are used to identify the water-quality values stored in the data base. Each code is linked to a definition. Parameter-code definitions typically contain information about what was analyzed, what units are associated with the numerical data, and sometimes, how the sample was processed prior to analysis (filtering, for examples). Definitions for each retrieved parameter are provided in the heading of each output.

Data retrievals-general guidance

Retrieving data from this site consists of the following steps (starting from the main NWISWeb water-quality sample page).

• Select search criteria.
• Specify values for your selected criteria, and specify output format (we recommend "list of sites" output for initial retrieval-this is the default output format).
• Preview a "list of sites." From this list you can:
  • Select one or more sites for data retrieval; or
  • Judge whether your site list is acceptable, too long (search criteria not restrictive enough), or too short (search criteria too restrictive). If acceptable, go "Back" and specify an output data format from the bottom of the page. If unacceptable, go "Back" and respecify search criteria.
  • To view a summary of results for each site, retrieve an "inventory" of data for each site. To view the individual results, retrieve a table of data. To retrieve the data in files that can be saved to your desktop, retrieve one of the various tab-delimited data file formats.

Some hints:

• All search criteria that you specify must be true for a sample to be retrieved. For example, if you specify the following criteria: state={Minnesota or Wisconsin}, Site Name={Mississippi}, and Number of observations={20}, then you will retrieve data for all sites in Minnesota or Wisconsin that have the word "Mississippi" in their site name, and that have at least 20 observations in the water-quality data base.
• If you wish to wish to specify a County, you must first go to the State by using the blue banner at the top of the page. Select your State name from the Geographic Area domain; then select "Go" (if necessary). Select the button.
• Selecting multiple states, counties, site types, etc. Selecting (highlighting) multiple items from a domain list is a browser-dependant task. On Windows platforms, holding down the control key and left-mouse-button clicking allows multiple selections. On other platforms (Sun), simple clicking of items (without control key) allows multiple selections.

Example data retrievals

• How do I locate water-quality data for sites near my home? This is a simple request, but there are several ways to tackle the problem:
  • Locate samples using my county as a search criterion.
    • In the blue banner on the main NWISWeb page, select a Data Category of "Water Quality," select your state's name under "Geographic Area," and click "Go."
    • Select the button.
    • At the Search Criteria page, select "County" as one of your search criteria. Select other search criteria to focus your search (Common examples include site name, site type, and medium type). Click "Submit."
    • Specify your county and other search criteria. Optionally, select "Output Format" (or leave as default to preview a site list).
    • Did I get too many sites? Depending on the County in which you live, there may be many sites; finding the site or sites of specific interest to you can take some work. Here are some alternatives: (1) scan the site list, and click on the site you desire; (2) go back and modify your search criteria-would site name or site type (for example) help narrow the search? (3) go back to the Search Criteria page, and select "Save file of selected sites to local disk for future upload," edit the site-list file on your computer (keeping only those sites of interest), and upload it using the "File of site ID's" search criterion; (4) select sites based on a latitude-longitude box (rather than county), described in the next example.
    • Once satisfied with a your search criteria and site list, go to the Search Criteria page and specify your data output format in the bottom "Output Format" box.
  • Locate sites using a latitude-longitude box.
    • Construct a latitude-longitude box that encloses your area of interest. Determine the northern and southern boundaries (latitudes) of the box; then determine the western and eastern boundaries (longitudes).
    • How do I determine latitude and longitude coordinates? You can estimate your coordinates using various maps (including most USGS maps), a GPS, or do it online using the USGS MapFinder. In general, avoid being overly precise; coordinates rounded to the nearest arc-minute, ten minutes (or tenths of a degree, for decimal coordinates) are usually sufficiently precise. Also, avoid specifying too large an area-a 1x1 degree area, for example, may yield many undesirable sites.
    • Choose "Lat-Long box" as a selection criterion at the main page, and enter the appropriate values on the subsequent [criteria specification] screen.
• How do I retrieve data for multiple states? First, select "United States" as your geographic area in the blue banner; select "state" as a search criterion, along with other necessary criteria. Select the desired states from the pick list at the next page.
• How can I limit the number of parameters I retrieve? Use parameter group period of record table as an output criterion, and select the group of interest. This will provide a tab-delimited file of results within that group of parameters.

Additional information for technical data analysis

• For technical data analyses, it may be necessary to know what sampling and analytical methods were used, what the quality of the data has been for the time period, and what agencies collected and/or analyzed samples. Data quality (precision and bias) depends on sampling and analytical methods used. Methodological changes-usually improvements-occur over time within an agency, and data from different agencies may be generated using different sampling and/or analytical methods. Therefore, coding of methods and agencies is important.
• Quality Assurance / Quality Control (QA/QC) documentation.
  • Field and laboratory protocols, and changes in those protocols, are documented in numerous reports and technical memoranda. These range from project-specific reports to national protocols. Current protocols are published in the USGS National Field Manual and other publications (http://water.usgs.gov/owq/Fieldprocedures.html).
  • Summary of National QA/QC documentation through 1995.  (Or see USGS Open-File Report 96-337).
  • More technical memoranda, which document protocols.  
  • National QC programs are run by the Branch of Quality Systems.
  • USGS Water Science Center offices may be contacted for project-specific QA/QC reports; and QC data.
• Fixed value codes are used to define further information about a sample.
• Agency codes. The collecting agency code and analyzing agency (00028) are provided with each data retrieval.
• Sampling method information.
  • Sampling method (82398)
  • Sampler type (84164)
  • Sample purpose code (71999) (samples collected for NAWQA and NASQAN, for example, are identified using this code. These National programs typically have nationally consistent protocols).
• Analytical method information
  • Parameter code definition
  • Method code

Data retrievals-precautions

• The data you have secured from the USGS NWISWeb database may include data that have not received Director's approval and as such are provisional and subject to revision. The data are released on the condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use.
  • USGS endeavors to ensure the accuracy of all water-quality data provided to the public. As part of our efforts to improve the timeliness and relevance of our data-collection efforts, provisional data are provided which may not have been reviewed. The majority of results are not changed during the review process, but errors do occur. The "data-quality indicator" (DQI) is provided to guide data users. Results are initially coded with a DQI of "S" (presumed satisfactory). After review by a project hydrologist, the DQI is usually changed to "R" (reviewed and accepted). Laboratory results are initially reviewed at the laboratory and subsequently by the hydrologist. Sometimes, comprehensive reviews of quality-assurance data cause revisions to previously reviewed data, therefore an "R" DQI coding does not guarantee that results will never be updated.
  • To view additional data-quality attributes, output the results using these options: one result per row, expanded attributes.
• Known coding problems
  • Missing codes - Many of the codes mentioned above that could be used to help with the technical analysis of the data are not mandatory and so, may not be populated. This is particularly true of the older samples.
  • Missing information - Some of the options for sample selection in NWISWeb, such as drainage area, are not mandatory fields and so, may not be populated. A search based on drainage area will miss many sites.
• For further information about the water-quality program and access to technical help with the use of the data, go to the Office of Water-Quality home page.