Guidelines for Frequency of Discharge Measurements during Ice-effected Periods In Reply Refer To: January 26, 1994 Mail Stop 415 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 94.03 SUBJECT: Guidelines for Frequency of Discharge Measurements during Ice-effected Periods The purpose of this memorandum is to provide guidelines for the frequency of discharge measurements during periods of ice-affected streamflow at gaging stations. Office of Surface Water (OSW) technical reviewers have noted during District reviews that the number of discharge measurements during ice-affected streamflow conditions has gradually declined over the past several decades. There is concern that this situation has progressed to the point where the integrity of many winter records has fallen below desirable levels. Determination of daily mean discharge values during winter ice periods is based on discharge measurements and in most cases on comparison of hydrographs for nearby stations. The accuracy of discharge estimates during ice-affected periods is, therefore, directly related to the number and quality of discharge measurements. About one half of all ice-affected discharge measurements are considered "poor," according to Janice Fulford's recent investigation of measurement accuracy reported in U.S. Geological Survey (USGS) Open-File Report 92-493 "Characteristics of U.S. Geological Survey Discharge Measurements for Water Year 1990." There are limitations in improving the accuracy of ice-affected discharge measurements, but more frequent measurements can be made. Several USGS publications provide guidance on discharge- measurement frequency. Water Supply Paper 337 "The Effects of Ice on Streamflow" by W.G. Hoyt, 1913, (p. 67) states: "The number of current-meter measurements necessary to determine the winter flow of a stream depends so largely on local conditions that no definite rule can be given, but the axiom applicable to open-water measurements--the greater the number the greater the accuracy-- applies with equal force to measurements made when ice is present. If the control section of a station is always clear of ice or if the flow is affected only by anchor ice, no more measurements are necessary than are necessary to determine the open-water flow at the same station. If surface ice is present at stations on streams uncontrolled by storage, the number of discharge measurements will depend almost entirely on the temperature; when conditions of temperature and runoff are uniform, one measurement a month may be sufficient to insure good records; when thaws are frequent the number increases rapidly, and during certain periods in the winter, it may be necessary to make measurements almost daily." Water Supply Paper (WSP) 2175, Vol. 2, by S.E. Rantz (p. 360) states: "Because of the variability of the backwater effect, discharge measurements should be made as frequently as is feasible when the stream is under ice cover, particularly during periods of freeze-up and thaw when flow is highly variable. (Procedures for making measurements under ice cover are described in the section in chapter 5 titled, "Current-Meter Measurements from Ice Cover.") In midwinter the frequency of measurements will depend on climate, accessibility, size of stream, winter runoff characteristics, and required accuracy of the discharge record. As a general rule, two measurements per month is the recommended frequency. At stations below powerplants that carry a variable load, it may be necessary to make two measurements during each winter visit--one at the high stage of the regulated flow and the other at the low stage. The backwater effects may be markedly different at the two stages. In very cold climates where winter ice-cover persists and winter discharge shows a relatively smooth recession, fewer winter measurements are needed than in a climate that promotes the alternate freezing and thawing of river ice." In the 1987-88 study of winter discharge record accuracy described in USGS Open-File Report 90-554, "Evaluation of Selected Methods for Determining Streamflow During Periods of Ice Effect" by N.B. Melcher and J.F. Walker, discharge measurements were made at 1-5 day intervals during periods of ice effect on three Iowa streams. These measurements were used to establish a base record which was compared with the results of a variety of computational techniques based on discharge measurements made at a 6-week frequency. Statistical evaluation of results from the commonly used "Hydrographic-and-climate comparison" technique indicated that 60 percent of estimated daily discharges had relative errors greater than 10 percent, and 9 percent had relative errors greater than 50 percent. An acceptable measurement frequency during winter ice periods may vary significantly from stream to stream depending on the relative significance of winter flows to annual discharge, accessibility during winter months, the economics of frequent measurements, and the accuracy goals for given stations. Although judgment that considers these constraints must be exercised in winter data collection, Districts should be knowledgeable of the potential reduced accuracy of discharge estimates with less frequent measurements. In general, the OSW endorses the recommendations given in WSP 337 and 2175 and adds the following guidance: 1. For streams whose surfaces typically stay frozen for all or most of the winter period but respond to temperature changes with change in streamflow, a discharge measurement should be made as soon as possible after the ice sheet formation is safe, and no less frequently than once every few weeks thereafter until breakup occurs. 2. For streams whose surfaces typically stay frozen for the entire winter period and that are characterized by gradual streamflow recessions, a discharge measurement should be made as soon as possible after the ice sheet formation is safe, and at least once every 6-weeks thereafter until breakup. Particular attention is needed to obtain a discharge measurement at minimum flow, which often occurs just prior to breakup. 3. For streams whose surfaces stay frozen for a period, break up and then refreeze, at least one discharge measurement should be made during each frozen period. 4. For streams whose records have good hydrographic comparison, discharge measurements at least every four weeks are recommended and additional discharge measurements should be obtained at the best (index) station, particularly during winter temperature extremes to improve definition of the discharge hydrograph. Districts are encouraged to carefully analyze the quality of ice- affected gaging station records, taking into consideration the number and quality of discharge measurements, groupings of stations used for hydrographic comparisons, applicability of weather records, and to downgrade the accuracy of published records that are derived from limited basic information sources. In execution of data programs, it also is recommended that the Districts' quality assurance plan address winter discharge records, specifying the accuracy goal for various stations or groups of stations in the District. Such accuracy goals may be stated in terms of reduced measurement frequency for those stations. Charles W. Boning, Chief Office of Surface Water WRD DISTRIBUTION: A, B, FO, PO