The third meeting of the Hydrologic Frequency Analysis Work Group was convened at 401 Ninth Street, Northwest, Suite 1000, Washington, DC on May 2, 2001. Martin Becker and Don Woodward, acting co-chairs, conducted the meeting. The following work group members were in attendance:

The agenda for the meeting was primarily to discuss the three initiatives of the work group and to elect a new Chair and Vice Chair.

Flood frequency analysis for ungaged watersheds

One initiative of the work group is to develop guidance for estimating flood frequency for ungaged watersheds. A paper entitled "Evaluation of Flood Frequency Estimates for Ungaged Watersheds" prepared by Thomas, Grimm, and McCuen was distributed to the work group on January 22, 2001 for review. Will Thomas incorporated comments from work group members into an April 8, 2001 version of the paper by Will Thomas. This latest version of the "ungaged paper" was discussed at the May 2, 2001 work group meeting to determine if the paper should be forwarded to the Subcommittee on Hydrology for their review.

David Goldman had the following comments:

• Thought that plus or minus one standard error about regression estimates was too small for judging the reasonableness of rainfall-runoff model estimates. He believes that rejecting the reasonableness of rainfall-runoff model estimates about 32 percent of the time is too large and favors the use of plus or minus two standard errors.

• Expressed the opinion that flood frequency estimates from rainfall-runoff modeling and regression equations should be weighted according to their accuracy.

• Suggested that sensitivity analysis could be used to determine the accuracy of rainfall-runoff model estimates.

• Commented that if rainfall-runoff models were calibrated then these estimates would be consistent with statistically-based estimates.

Don Woodward reacted to David's comments:

• Commented that research was needed on David's suggestions relative to weighting estimates and performing sensitivity analyses to determine accuracy of rainfall-runoff model estimates.

• Believes that the work group needs to complete their initiatives to demonstrate productivity and believes the latest version of the "ungaged paper" is acceptable for submission to the Subcommittee on Hydrology.

Rocky Durrans commented:

• What does it mean to "accept" the ungaged paper? Martin Becker stated that if the Subcommittee on Hydrology agrees that the ungaged paper is worthwhile it would be posted on the Advisory Committee on Water Information (ACWI) web page http://acwi.gov/.

• Suggested that a paragraph be added to the ungaged paper describing future research needed relative to estimating flood frequency for ungaged watersheds.

Jerry Coffey commented:

• In reaction to David's suggestion to weight two estimates, Jerry asked under what conditions does weighting two estimates give an improved estimate. He raised questions about weighting two estimates that display substantially conflicting results and suggested that one or both may be in error. He asked what are the consequences of weighting two bad estimates? (I think it was Jerry who also pointed out that plus or minus one standard error does not address the bias of an estimate.)

Ken Bullard commented:

• Not in favor of weighting regression and rainfall-runoff model estimates since the latter is often calibrated to gaging station data and weighting this estimate with regression estimates may be counter productive.

Will Thomas pointed out three major review comments on the January 18, 2001 version of the ungaged paper that were not incorporated into the April 8, 2001 version:

• Weight two estimates, i.e., weight regression and rainfall-runoff model estimates rather than determine which one is most reasonable (comments made by David Goldman and Rocky Durrans).

• Provide guidance on choosing the appropriate hydrologic region for evaluating flood discharges (comments made by Ken Bullard and Rick McCuen).

• Discuss loss of variance associated with rainfall-runoff model estimates based on continuous simulation (comment made by Rocky Durrans).

Will noted that he did not revise the ungaged paper to include these comments because he believed additional discussion and guidance was needed before inclusion. Based on discussions at the May 2, 2001 meeting, the following revisions will be made to the ungaged paper. The weighting of regression and rainfall-runoff model estimates will not be recommended because there is no agreement that this is a good idea or how this should be done. A few words will be added to the ungaged paper to caution the analyst that all data used in the evaluation should be representative of the hydrologic region where the ungaged watersheds are located. Finally, the loss of variance associated with continuous rainfall-runoff modeling will be briefly discussed. Action item: Will Thomas was asked to revise the ungaged paper within three weeks of the May 2 meeting.

A vote was taken as to whether the ungaged paper should be forwarded to the Subcommittee on Hydrology. All voting members were in favor of forwarding the paper except David Goldman. David indicated that he would submit a minority report expressing his concerns about the ungaged paper. Subsequent to the May 2 meeting, David did provide a minority report Memorandum for Record dated May 12, 2001. Will Thomas forwarded this minority report to work group members on May 18, 2001. When the revised ungaged paper is submitted to the Subcommittee on Hydrology, the minority report from David Goldman will also be submitted.

Frequently Asked Questions relative to Bulletin 17B

A second initiative of the work group is to compile a list of frequency asked questions and responses for Bulletin 17B. Bill Kirby compiled several frequently asked questions and responses and sent these to work group members on April 9, 2001. Most work group members had not adequately reviewed this list of questions and responses by our meeting on May 2, 2001. The work group felt that Bill had done an excellent job in compiling these questions and responses and felt that additional time was needed for adequate review. Action item: Therefore, each work group member is to provide Bill comments on the list of questions and responses by July 16, 2001 and Bill will revise the list appropriately and send out to work group members by August 31, 2001.

Flood frequency for regulated watersheds

A third initiative of the work group is to develop some guidance on flood frequency analysis for watersheds regulated by flood detention structures. David Goldman asked what audience we were addressing with our guidance on regulated flood frequency analysis. He believes that it is not feasible to address issues relative to large reservoirs and we should define methods appropriate for regulated flood frequency analysis where operating rules are not needed. Rocky Durrans offered to draft a paper on methods of regulated frequency analysis for urban detention ponds and provide to work group members by the end of August, 2001.

Election of Chair and Vice Chair

Mike Grimm nominated Will Thomas as Chair. The nomination was seconded and Will was elected the new Chair. Will then nominated Rocky Durrans as Vice Chair. This nomination was seconded and Rocky was elected Vice Chair. Will and Rocky will serve in these positions until September 30, 2002 at which time Rocky will become Chair and serve for two years as described in the Terms of Reference. A new Vice Chair will be elected when Rocky assumes the Chair.

Next meeting

The next meeting of the work group is scheduled for September 10, 2001 at the offices of Michael Baker, Jr. at 3601 Eisenhower Avenue, Suite 600, Alexandria, Virginia. The meeting will begin at noon and lunch will be served.

Will Thomas
July 1, 2001