Spatial patterns of temporal streamflow variation have been discussed in myriad reports in the hydrologic literature. Diverse collections of streamflow records have provided the data bases for these studies. In undertaking the work reported herein, an attempt was specifically made to first assemble the various USGS streamflow datacollections that have been used in the analysis of surface-water conditions on a meso- or a continental scale. These collections include: (1) the USGS National Water Conditions data set (as constituted for the 1988 "National Water Conditions" monthly reports); (2) the Hardison (1974) data set, which has been used in many flood studies (such as in Landwehr and others, 1978); (3) the MINI-Regions data set (by Matalas and Landwehr, 1978, and used in Thomas, 1981, and Marin, 1983); (4) the Langbein-Slack (1982) data set(also used by Lins, 1985), (5) the Alley-Lins data set (written communication, 1983, and as used in Karl and Riebsame, 1989); and (6) the drought index set used in the 1988-1989 National Water Summary (see Paulson and others, 1991). Stations in the USGS Hydrologic Benchmark Network, which was established to document effects of climate variation on pristine watersheds (see Leopold, 1962; Cobb and Biesecker, 1971; and Lawrence, 1987), also were specifically considered for this endeavor. In addition, stations in the National Stream Quality Accounting Network i.e., NASQAN (see Ficke and Hawkinson, 1975.), which has been used to study continental-scale water quality conditions (see, for example, Smith and others, 1987), were examined. NASQAN was considered for completeness of the effort even though it was known apriori that many station records from this network would be unsuitable for the discernment of the effects of climate variation because the objective of NASQAN is to account for the cumulative impacts on water quality from all activities in the watershed, including anthropogenic effects.
With respect to the National Water Conditions data set, the USGS publishes monthly a report that geographically summarizes patterns of monthly mean discharge conditions at the nationally distributed sites. Currently entitled "National Water Conditions", the publication was begun inOctober 1944, under the title "Water Resources Review." (See discussion in Holmes, 1987.) The stations comprising the data set are referred to as "National Water Conditions Streamflow Index Stations" or just "Index Stations." The composition of the collection has changed over time. For example, the set current in 1988 as used in development of HCDN was slightly amended in 1989; see discussion of changes in "National Water Conditions October 1989." This collection of stations, in its entirety or as subsets of the whole, has served as the source of data for many studies of continental and regional streamflow patterns beginning with those of Harbeck and Langbein (1949). (Also see Busby, 1963; Bartlein, 1982; and Cayan and Peterson, 1989, as examples of uses of this data set.) It should be noted that in some cases, the data used in the monthly report does not reside in WATSTORE. Rather, the data may be furnished by another agency, such as the U. S. Corps of Engineers, or the discharge values are adjusted to account for impoundments, using additional information provided by another agency or one of the USGS District offices. In such cases, the data values in the monthly report reside only in the files of the USGS office that prepared it, namely the Hydrologic Information Unit of the USGS Water Resources Division.
Station lists from all of the above collections were compiled. It was not expected that the data sets would be congruent; however, it was expected that there would be a large overlap among them. That this was not found to be the case suggested that still other appropriate streamflow records for yet other stations might exist within the USGS files. H. C. Riggs (of the Office of Surface-Water of the USGS Water Resources Division) provided an initial review of the various annual State Water Data Reports, and verified that such data did exist. (H. C. Riggs, USGS, oral communication, 1988.)
The effort was then enlarged with the intent of identifying as many as possible of the discharge records held by the USGS whose monthly mean values would be appropriate for the study of the relationship of hydrologic conditions with meso-scale climate variation, as determined by a single set of criteria. Given that the USGS has spent considerable effort in automating the streamflow data files, searching WATSTORE is, for practical purposes, synonymous with choosing from all available daily mean discharge data held by the USGS. Given that daily mean discharge records are available for more than 12,000 surface-water gaging stations, both active and inactive, with in the WATSTORE data base, and given that the criteria required specific information about watershed conditions frequently available only from the originating District office, the help of a surface-water specialist in each District office was enlisted. The criteria by which station records were designated as acceptable for inclusion in the HCDN are as follows.
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.
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