PUBLICATIONS--Guidelines for Publication of Streamflow Statistics in the Annual State Data Reports In Reply Refer To: March 4, 1992 WGS-Mail Stop 415 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 92.07 SUBJECT: PUBLICATIONS--Guidelines for Publication of Streamflow Statistics in the Annual State Data Reports Changes in the format of the Water Resources Division (WRD) annual State data reports were described in Office of Surface Water Technical Memorandum No. 91.12. Guidelines for revising the format of these reports are contained in WRD Memorandum No. 92.04. The purpose of this memorandum is to revise, reemphasize, and supplement existing guidelines, particularly those provided in WRD Memorandum No. 92.04, relative to the publication of the summary streamflow statistics. The U.S. Geological Survey (USGS) publishes daily mean discharges for many regulated streams--that is, streams where streamflow has been altered by human activities, such as the construction of reservoirs or diversions, modification of stream channels, or urbanization of watersheds. Streamflow data are needed for regulated as well as unregulated streams to make various water- management decisions. Generally, reservoirs and diversions account for most of the major changes in a watershed that significantly affect streamflow statistics. However, stream channel modification such as straightening and paving, or watershed modifications such as an increase in impervious area in a watershed as a result of urbanization, major loss of vegetation caused by major forest fires, land-surface disruption caused by surface mining, or a change in agricultural practices or irrigation diversions can also significantly affect streamflow statistics. Publication of streamflow statistics for a watershed without documenting the time periods for when streamflow conditions have changed abruptly can be misleading and inappropriate. Changes in watershed conditions that alter streamflow should be documented briefly in the Remarks paragraph of the annual State data report. The following guidelines are intended to improve the consistency of streamflow statistics published in the annual State data reports across the Nation. 1. Guidelines described in WRD Memorandum No. 92.04 state that at least 5 years of data should be used to define the annual streamflow published in the annual State data reports. This is consistent with the traditional guidelines used for publishing the mean annual streamflow which has been considered an estimate of the long-term mean. With the concurrence of the Assistant Chief Hydrologist, Scientific Information Management, this criterion for computation and publication of mean annual streamflow in the summary of streamflow statistics in annual State data reports is herein rescinded. The reason for this change in the guidelines is that the mean annual streamflow for a designated time period and that for the current-year are provided primarily for comparison statistics purposes. Therefore, upon receipt of this memorandum, no restriction on the length of record used for computing the mean annual streamflow or any other statistic for any designated time period is to be applied. The summary of streamflow statistics published in the annual data reports is provided mainly for comparison of streamflow statistics between two periods. The values are not intended to be estimates of long-term streamflow statistics or of streamflow population values because no inference of frequency or probability of exceedance or nonexceedance is specified. Statements that the summary of streamflow statistics represents any specific watershed condition should be avoided. 2. WRD Memorandum No. 92.04 states that streamflow statistics for periods of unregulated or regulated periods flow conditions or both can be published in the annual State data reports. This option is reemphasized in this memorandum. The selection of the appropriate period(s) of regulation or nonregulation should be determined by District personnel who are most knowledgeable about watershed conditions. As noted above, the time period of regulated streamflow should be identified in the Remarks paragraph of the data report. In the context of this memorandum, regulated streamflow is used generically to mean regulation of streamflow by reservoirs, diversions, or stream channel and (or) watershed modifications. 3. Different streamflow statistics are affected to varying degrees by regulation. For example, reservoir storage, without accompanying diversions, might not significantly affect annual total streamflow or annual and monthly mean streamflow but might significantly affect peak flow, low-flow, and daily-flow duration statistics. Under these conditions, it might be advisable to publish only the annual and monthly statistics. In summary, District personnel are responsible for deciding what streamflow statistics should be published. 4. WRD Memorandum No. 92.04 also states that the publication of peak flows above base is optional but that the entry of these data into the Peak Flow File is mandatory. This memorandum reemphasizes the requirement to enter peak flow above base into the Peak Flow File. A canvass of cooperating agencies in 1975 and 1981 indicated a continuing demand for peak flow above base. Even if the District decides not to publish peaks above base information. Therefore, even if the District decides not to publish these data, quality assurance procedures should be implemented to ensure that accurate data are entered into the Peak Flow File in a timely manner. In summary, the above guidelines are intended to reemphasize, revise, and supplement existing guidelines, such as those in WRD Memorandum No. 92.04. The basic objectives are to publish meaningful statistics representative for either regulated or unregulated streamflow conditions (or both), to avoid publishing statistics that could be misleading or incomplete, and to avoid making any interpretive statements regarding streamflow statistics that represent any specific watershed condition. The publication of these summary statistics should provide the public useful data for comparing current-year streamflow statistics against those for a previous designated period. Charles W. Boning Chief, Office of Surface Water