Rivers and Streams - 9 of 24

Rivers and Streams FAQs - 24 Found

How have streamflow records been identified as being appropriate for use in climate research?

Six criteria by which station records were examined for suitability for inclusion in the Hydro-Climatic Data Network (HCDN) were defined as follows:

  1. 1. Availability of data in electronic form -- Because of the functional requirement to handle large quantities of information, data had to be available in electronic format in the USGS national streamflow data base i.e., WATSTORE. It is possible that some files have never been automated, but no such files were discovered during this effort. However, some small special-purpose files kept by a single office, such as that noted for the National Water Conditions report which contains both "adjusted" and measured discharge, may exist. No such files were examined for this effort.

    2. Breadth of coverage -- Records from any station, whether currently active or not, for any water year through water year 1988 were considered. (A water year is defined as the 12-month period, beginning on October 1 and continuing through September 30 and is designated by the calendar year in which it ends.) This criterion insured the broadest possible geographic and temporal coverage with respect to representation of diverse climatic and watershed conditions. (Note that no statement is made concerning the suitability of records for water year 1989 or later.)

    3. Length of record -- A record of at least 20 water years of suitable monthly discharge data was preferred. However, if an otherwise acceptable record was shorter than 20 water years and that record was available for a uniquely located surface-water gaging station, one that was located in an otherwise underrepresented geographic area or climatic condition, then that record was included in the HCDN. Conversely, if a long record was available for a station but only a portion of the record was suitable by the HCDN criteria, then only the unimpaired part of the record was selected for inclusion in the final data set. Explanatory comments are provided if the selected record was shorter than 20 years or if it was less than the entire period of record available for the station.

    4. Accuracy of the records -- The predominant accuracy rating assigned by the District office had to be at least "good" for the record of daily mean discharges in those water years chosen. The occurrence of a few days which are rated "poor" or "fair", such as may be assigned to an estimated value at a time of unusual flow conditions due to ice impairment (see, for example, discussion by Melcher and Walker, 1990) or other such extenuating considerations, were not necessarily sufficient to disqualify a station. Even a few years rated "poor" or "fair" out of a long record generally rated "good" or "excellent" were not reason to disqualify the station. However, a comment is included in the description of the station (given in table 1) if such a qualification exists.

    The rating assigned to a record of daily mean discharge reflects the professional judgment of the office that obtains and prepares the records. The rating of overall accuracy is conditioned on the accuracy of the stage measurements, the stability of the stage-discharge relationship, the accuracy and the frequency with which discharge measurements are made to establish the stage-discharge relationship, and the interpretation of the records. An accuracy of "good" implies that 95 per cent of the daily mean discharge values are assessed to be at least within 10 per cent of the true value. A discussion of the accuracy of the records and the assignment of ratings can be found in the USGS Water-Data Reports for each State, published annually, as well as in the report by Rantz and others (1982).

    5. Unimpaired basin conditions, at least with respect to the computation of a monthly mean discharge value -- There should be no overt adjustment of "natural" streamflow, such as flow diversion or augmentation, regulation of the streamflow by some containment structure, or reduction of base flow by extreme ground-water pumping, nor should the degree of human activity in the watershed, such as changes in land use during the period of record, be so large as to significantly affect the value of monthly mean discharge (computed on the basis of the daily mean discharge) at the station. Even if a station is not presently suitable because of regulation, diversion, augmentation and so forth, but an earlier period of the record available for the station is acceptable by the specified criteria, then the suitable period and only the suitable period was included in the HCDN.

    Although discharge records that were subject to diversion or streamflow transfers of any kind were generally not acceptable for inclusion in the HCDN, an exception was made if the diversion had existed virtually unchanged for the entire period of record -- that is, a diversion may exist and affect the actual value of streamflow, but its effect on the daily mean discharge record is unchanging and should not significantly mask the influence of climate variation in the record. Similarly, stations where streamflow was subject to regulation by dams were generally not acceptable. However, if the regulation was due to a low-head hydropower dam with a reservoir of small storage capacity having only a transient effect on high or low streamflow, with no effect on the monthly mean discharge values, then the discharge record for the station could be acceptable for inclusion in the HCDN. On the other hand, a station in a basin that has undergone a substantial land-cover change, for example from forest to agriculture to urban, is probably not suitable even if the changes were gradual.

    It is noted that a streamflow record was considered suitable for inclusion in the HCDN if the monthly mean discharge values met the criterion for nonimpairment of "natural" streamflow conditions. However, for the majority of records, even the daily mean discharges satisfy this criterion. Consequently, if a record should be considered unimpaired only at a monthly or longer average time step, this qualification is given as a comment which accompanies the identifying information for the station.

    6. Measured discharge values -- The discharge data reported in the records have been obtained by means of standard measurement practices followed by the USGS. Occasionally, some discharge values in the published record will be designated as "estimated". Such a designation arises when the stage height recorder malfunctions, for example, due to ice conditions. If there is an excessive number of estimated values in the monthly record, the assigned rating will be less than "good" and the record will be disqualified by the accuracy criterion. Also, if there is a measured diversion upstream from a surface-water gaging station, such as for irrigation, which is routinely and simply added onto the streamflow measured at the gaging site, then the corrected discharge record was used in the HCDN, but with a comment qualifying the station's record. However, the HCDN contains no records that are "constructed", that is, do not correspond to the flow in any single natural channel, for example, the Four Rivers Index of the Sacramento Basin, nor does the HCDN contain records that are re-constructed using information from other sites or information on activities such as diversions, augmentation, pumping, and regulation, such as the reconstructed record of "natural flow" available for the Colorado River at Lee's Ferry. Neither was any attempt made to extend or "fill in" sections of records with missing values using some computational algorithm. Thus, one need not ask if any patterns to be seen in the data have been introduced by the choice of computational algorithm to extend the records; rather they reflect what was determined from measurements made at the gage at the time represented in the record.

    Special attention was paid to stations that are included in either the Hydrologic Benchmark Network or in the National Conditions Streamflow Index Stations set used in the 1988 monthly National Water Conditions reports. The objectives for defining both of these data collections are allied to the purpose for which the HCDN was constructed, and both have been of particular interest to meso-scale climate studies in the past. If any station in either of these two data sets was deemed not to be acceptable for inclusion in the HCDN by the criteria specified above, then that station was specifically identified as "rejected" for inclusion in the HCDN, with an explanation given for its unacceptability. For example, Crater Lake, Oregon, is a Hydrologic Benchmark station but the station records lake levels, not streamflow. Similarly, the St. Lawrence River at Cornwall, Ontario, near Massena, New York, is a National Water Conditions station even though the flow has been regulated in accordance with an international joint agreement with Canada since 1958. Indeed, there is no actual gaging station at the site; rather, the reported discharge is constructed on the basis of discharge at several points of regulation upstream from the site. Thus, records from these two sites are not included in the HCDN.

Tags: Ecosystems, Groundwater, Climate, Rivers, Water, Streams, Streamgages