Small Urban Watershed Use of Hydrologic Procedures

Transcription

1 TRANSPORTATION RESEARCH RECOR Small Urban Watershed Use of Hydrologi Proedures VERNON K. HAGEN The Federal Interageny Hydrology has published douments on flood frequeny for gauged and ungauged watersheds. These douments inlude information on the preparation, dissemination, and results of a questionnaire sent to users of hydrologi methods. The questionnaire was irulated by a working group of the Hydrology and pertains to urban watersheds with areas of less than 30 mi 2 Respondents provided information relative to the physial and administrative fators influening their use of speifi hydrologi methods. Although respondents did not ite all available hydrologi methods, most ommonly used methods were mentioned, even though some were not atually used during the period of usage about whih the respondents were questioned. Results of the questionnaire indiated that 86 perent of studies performed by hydrologi methods were performed by one of four methods. More than half of the studies onduted during - a 1-year period used the TR-55 omputer model of the Soil Conservation Servie (SCS). The other three methods in order of popularity were the rational method, TR-20 by SCS, and regression equations by the U.S. Geologial Survey. The questionnaire displayed rather dramatially that users have been opting for less involved methods. The method seletion proess may be due to the fat that mutually aepted guidane on the priority of method use is unavailable. In addition to the results from the questionnaire, supplemental information is provided on the methods being used in state highway departments and the National Flood Insurane Program. Some urrent problems faing administrators regarding the proliferation of hydrologi models and rainfall data douments are also inluded. Within the federal government lead responsibility for oordinating water data aquisition ativities resides with the U.S. Geologial Survey's (USGS's) Offie of Water ata Coordination. The Interageny Advisory Committee on Water ata is omposed of representatives of federal agenies and is divided into funtional subommittees (Figure 1). Among these subommittees is the Federal Interageny Hydrology. Under the purview of the Hydrology is the oordination and the development of guidane for the appliation of hydro logi methods. Two important douments relating to hydrologi methods for flood frequeny analysis were published in the 1980s by the Hydrology. The first doument, Guidelines for etermining Flood Flow Frequeny, ulletin 17, was published initially in Marh 1976 and most reently in Marh 1982 (1). These guidelines provided methods for obtaining the frequeny of flood peak disharges for watersheds with gauged reords of homogeneous data extending for 10 years or more. A follow-on publiation, Estimating Peak Flow Frequenies for Natural Ungaged Watersheds-A Proposed Nationwide Test, was issued by the Hydrology in 1981 (2). The seond doument provided a lassifiation of the proedures and results of a pilot test of several methods. It onluded that a massive nationwide test is needed to provide an authoritative basis ewberry & avis, 8401 Arlington oulevard, Fairfax, Va for proedure seletion and to reommend a national guide for ungauged watersheds. The use of ulletin 17 (1) has been extensive sine its publiation for gauged watersheds. However, its methods apply to speial onditions that are not often found in urban watersheds. Homogeneous peak disharge data are generally not available for watersheds experiening urban development. Urban flood information studies usually require omplete hydrographs for stream and storage routings analyses. Therefore, the guidane in ulletin 17 will generally not suffie in the small urban watershed situation. Although the ungauged watershed report provided information on some available hydrologi methods, it did not inlude information regarding the extent of use for various methods in present-day engineering pratie. Many hydrologi studies are being performed on small urban watersheds for planning, design, land-use. regulations, flood insurane, and other purposes. A potpourri of methods are available, but limited guidane is offered on whih methods are appropriate for various sets of onditions. Thus, it is diffiult for administrators and managers to know when reasonable results are available. With the prevailing problems in mind, the Hydrology formed a working group to evaluate the prospets for providing guidane on small urban watersheds. The purpose of this paper is to identify hydro logi methods ommonly applied to analyses of flood flows in small urban watersheds. This paper is not intended to provide guidane or reommendations on the appliation or seletion of hydrologi methods for various water resoure purposes, inluding flood frequeny analysis. Information is presented on the appliations of hydrologi methods for small urban watersheds as developed from responses to a questionnaire distributed by the Hydrology. The questionnaire was to provide the first step in developing guidane on the appropriate use of the methods. It is unlikely that onsensus guidane on the appliation of hydrologi methods for small urban watersheds will evolve in the near future; however, groups of experts ontinue to be formed to further this objetive. Sound guidane is an elusive objetive loaked in ontroversy and thus requires time, effort, funding, and ompromise. SUMMARY OF QUESTIONNAIRE From an evaluation of questionnaire responses, the author made some general observations onerning the estimation of hydrologi flood flows for small urban watersheds. Among the most revealing observations are that Pratitioners may not be fully aware of some of the effetive methods available for analyzing small urban watershed flood flows; Pratitioners tend to use methods with whih they have the greatest familiarity;

2 48 TRANSPORTATION RESEARCH RECOR U.S. epartment of Interior Geologial Survey Yater Resoures ivision >--- Offie of Yater ata Coordination Inter ageny Advisory - Advisory Committee Committee on Yater ata for on Publi Use Yater ata (IACY) (ACYPU) Automated Hydrology ata Systems Yater-ata & - Information Exhange - Ground-Water I I Working Groups I Yater-Use - - Sedimentation Information Suhommittee Working Groups Coordinating Counil for Yater- ata Aquisition Methods (IACY Membership) Small Urban Yater sheds Tehnial Working Groups for Reommended Methods FIGURE 1 Organizational hart. Proedures for rural watersheds may be in use without adjusting for the impats of urbanization; Some pratitioners are apparently not using the most reent version of the hydrologi models; and There appears to be a tendeny to use less involved methods. Although these observations are deduted from responses provided on the questionnaire and would require further ommuniation with the respondents to verify and explain the observations, they do point to the need for the development of generally aepted guidelines for hydrologi method appliations. For example, the appliation of proedures developed for rural areas to an urbanized watershed an result in signifiant underestimation of flood flows. Use of something other than the most reent version of a hydrologi model may not seriously affet the results, but it may be an ineffiient means of study. Pratitioners must also use are in the seletion of methods. Less involved proedures may be satisfatory for most situations; however, the onsequenes of ursory estimates are important onsiderations in the level of effort devoted to the omputation of syntheti floods. A possible impliation of the results of the questionnaire is that the onlusions of the report on ungauged watersheds (2) may be influening deisions about the seletion of hydrologi methods. Sine funding of the nationwide test was not provided and does not appear likely, the hydrologi ommunity must deal with the oneption that a nationwide test is the only mehanism for an authoritative national guide on hydrologi methods for ungauged watersheds. Study managers apparently have a diffiult task in onvining deision makers that an in-depth hydrologi method should be used in the fae of the previous onlusion that results annot be sientifially proven to be better than those obtained by a less demanding method. Although sientifi proof of the superiority of some methods over others, given a speifi set of onditions, may not be readily obtainable, tehnial expertise and experiene an be used to provide guidane on whih methods are more likely to give onsistently good results. Therefore, guidane on the seletion and appliation of hydrologi methods for flood estimates of small urban watersheds appears to be needed as soon as pratiable. PROFESSIONAL SOCIETIES Expertise in the appliation of hydrologi methods is available within the publi, private, and aademi setors. Professional soiety membership normally reflets a ross setion of these setors. Journal artiles, ommittee reports, and proeedings of professional meetings offer extensive literature related to hydrologi methods. Many of these douments, as well as publiations of the federal government, are available through the National Tehnial Information Servie, U.S. epartment of Commere, 5285 Port Royal Road, Springfield, Virginia On the basis of questionnaire responses, whih indiate a need for more information on miroomputer hydrologi models, and needs for storm water management praties, the work of two professional soieties is desribed briefly in the following setions.

3 Hagen 49 Amerian Soiety of Civil Engineers The membership of ASCE ontains many engineers who are engaged in hydrologi studies. They inlude pratiing private firms, eduators, and researhers as well as loal, state, and federal agenies. eing onerned about professional integrity and sound engineering praties, ASCE has direted signifiant effort in the field of urban water resoures. Toward this effort ASCE has established several ommittees that deal with the hydrology of small urban watersheds. The ASCE Task Committee on Miroomputer Software has sent questionnaires to vendors regarding available models for urban hydrology analyses and details about their appliation. Results of the survey were presented at the National Conferene on Hydrauli Engineering in New Orleans, Louisiana, August 14-18, Amerian Geophysial Union The Amerian Geophysial Union (AGU) may be desribed more as a sientifi organization than a professional soiety, beause its members and ativities are generally oriented toward researh. However, its Hydrology Committee is one of the strong elements of the organization and is deeply involved in urban hydrology. This ommittee published a monograph (3) on state-of-the-art praties in the field of urban hydrology and storm water management. Another example of AGU Hydrology Committee efforts in urban watersheds is sponsorship of personal omputer workshops on methods for flood frequeny analysis and flood foreasting. This ativity was provided during the International Assoiation of Hydrologial Sienes, Third Sientifi Assembly, May 10-19, 1989, altimore, Maryland. USER'S NEES Present Use of Urban Flood Frequenies Urbanization of watersheds inreases the perentage of impervious land area, with the diret result of inreased volumes of runoff from rainfall events. The inreased volume of runoff may be aompanied by higher flood peaks and inreased, possibly erosive, flow veloities. The onsequenes of this ause-and-effet relationship between urbanization and inreased runoff have required organizations involved in the planning, management, and maintenane of urban areas to beome knowledgeable of hydrologi analysis. Transportation planners and design engineers have a keen interest in flood volume and frequeny with regard to water onveyane through bridge openings and ulverts. Proteting ostly bridge strutures and embankments from damage due to overtopping is a ritial design item. Planning for watershed hanges over the proposed life of strutures is diffiult, but it is an important task. For example, a ulvert in a rural area designed to pass a 100-year flood flow may be adequate to pass only a 50-year flood after urban development of the watershed. In response to inreasing losses from flooding throughout the nation, the U.S. Congress established the National Flood Insurane Program (NFIP) through the National Flood Insurane At of Under the mandate of NFIP, administered by the Federal Emergeny Management Ageny's (FEMA' s) Federal Insurane Administration, studies of 100-year flood hazards have been onduted for all flood-prone ommunities. The results of those studies provide the basis for management of flood risk areas and the provision of flood insurane to homeowners and non-residential building owners within ommunities that partiipate in NFIP. The issuing of building permits, ommunity zoning issues, and land development are among the ativities affeted by flood frequeny analyses onduted as part of a flood insurane study for NFlP. Eonomi justifiation, design, and operations of flood ontrol projets require flood information. Storm drainage failities are usually designed with floods of a speifi frequeny in mind. These ativities and others require information on floods in order to proeed in an orderly fashion. Inorret analysis of floods an involve exessive expenditures or ould result in serious damage to failities based on underestimated flood magnitudes and frequenies. Planning Future Hydrologi Studies The nature and diretion of future urban hydrologi studies should be shaped through insights gained from an examination of the amount of use given to various flood analyses for urban watersheds. Serious questions may evolve from this examination, suh as the following: Are users taking advantage of the state-of-the-art pratie? If not, what are the reasons for limited use of apparently more effetive methods? Is the state of the art adequate for user needs? If not, what diretion should researh and development take? oes user response to the questionnaire reflet real-world appliations of various hydrologi models? If not, what would ' ause the results to be skewed? o the questionnaire results indiate the need for oordinated guidane on the use of hydrologi models? If so, who should provide this guidane and what should be the extent of the guidane? Although this paper is not of the sope needed to answer these and other pertinent questions, ativities of other organizations may help to resolve some of the questions. For example, the Water Siene and Tehnology oard of the National Researh Counil sponsored a ommittee report entitled Opportunities in the Hydrologi Sienes ( 4). That ommittee report addressed items suh as sientifi development to the present, outstanding historial ahievements, intelletual frontiers and sientifi hallenges, new data requirements, qualifiations of the people needed, and an indiation of appliations. ORGANIZATION OF WORK GROUP Early Conepts The Hydrology reognized the need for a working group on methods for analyzing flood flows from small urban watersheds. Ageny interest was soliited by the subommittee hairman, and an initial work group was established in This group inluded FEMA, the U.S. Army Corps of Engineers (USACE), Soil Conservation Servie (SCS), USGS, Agriultural Researh Servie, Tennessee Valley Authority, and FHW A. The early ativities of this group were devoted to the development of a statement of work. The objetive of this ativity was to develop

4 50 TRANSPORTATION RESEARCH RECOR 1471 guidane on the seletion of methods for analyzing flood flows from small urban watersheds. This objetive was to be aomplished by a ontrator who would review methods, onsult with agenies and other users, and prepare a user's manual. The investigation was designed to inlude Watersheds (up to 30 mi 2 ), evelopment (natural to 100 perent urban), and Flow (peaks, volumes, hydrographs). The final produt was expeted to identify methods of analyses and provide guidane on the seletion and appliation of methods. Sine the ontratual arrangement was antiipated to involve onsiderable funding, the separate agenies investigated soures of funds that ould be used to support this ativity. Federal budgets are planned well in advane of reeipt of an atual appropriation from Congress. Thus, a time-onsuming proess is involved, and ompetition for researh-type funding is intense. Questionnaire Status In 1985.the work group onluded that insuffiient funds would be ommitted by the federal agenies to pursue a ontrated study. Laking full funding for planned ativities, the work group agreed to pursue its statement of work on a limited basis with member support. Examining efforts that ould be implemented by the task ommittee, information on the urrent use of methods beame a high-priority item. Thus, the task ommittee began to prepare an appropriate questionnaire that ould be irulated among the pratitioners of urban hydrology. It was agreed that the questionnaire should glean enough information from users to help prepare future guidane on flood methods for urban watersheds. In February 1987 the parent Hydrology approved the task ommittee's reommendation to distribute the questionnaire to organizations experiened in hydrologi modeling. A distribution list was prepared; it inluded the following: Federal agenies on the Hydrology, State agenies involved in water resoures, and Private ontrators performing hydrologi analyses. The distribution list did not inlude eduational and researh institutions beause the information gathering was direted toward pratitioner use rather than researh and development. However, future efforts in guidane on the seletion and appliation of hydrologi models would need to involve this segment of hydrologi experts. The questionnaire was mailed in February Although most responses were reeived in a timely manner, there were a few delays in the reeipt of ompleted questionnaires. RESULTS OF QUESTIONNAIRES Respondent Information Respondents to the questionnaire were grouped by their employers and are listed as federal, state, or private pratitioners in hydrologi analyses. Figure 2 depits the perentage distribution by the soure of responses. It does not indiate the numbers of respondents, beause many respondents provided information on more than one hydrologi method. State (51.7%) FIGURE 2 Soure of responses. Classifiation of Methods Federal (29.6%) Many hydrologi methods were identified in the responses to the questionnaire; however, the list does not inlude all available methods. To onsolidate the responses for data management purposes, all responses were sorted to a lassifiation system ontaining 12 ategories. The individual hydrologi method was used as a lassifiation title when signifiant usage of that method was indiated by answers to the questionnaire. Information on the lassifiation of methods is provided in Table 1 and Figure 3. etails on the numbers of studies performed in the year preeding ompletion of the questionnaire (1986) are given in Table 1. This is a 1-year sample and reflets neither variations in method use from one year to another nor a longer-term summary of method use. Reasons for Seletion of Method Table 2 provides an interesting omparison among methods regarding the most prevalent reasons given for using a partiular hydrologi method. Review of suh information is important in planning a strategy for obtaining guidane on the seletion of hydrologi methods. Obviously, an individual user will not benefit from seletion guidane if an ageny or ustomer ditates the method that is to be used for a speifi study. The user and lient may, however, benefit from appliation guidane. Although the results of this ativity provide good general information, tehnial information suh as reliability and reproduibility of results is probably best obtained via an expert systems approah. Response Regarding Physial Features The United States has a wide variety of physial features that influene the amount and rate of runoff from a watershed. Several of these features were inluded in the questionnaire, and responses reflet the peuliarities of different parts of the ountry. Physial feature~ inluded in the questionnaire were watershed size, slope of terrain, type of soil over, density of vegetation, and annual preipitation. Responses on this aspet are not inluded in this paper; however, a areful review of this information by experts in hydrologi analysis ould provide insight on the proper use of a hydrologi method for the environment in whih it was used. Suh information is important to the ritiality of ompleting guidane on the seletion and appliation ofhydrologi methods. When physial features

5 Hagen 51 TALE 1 Classifiation of Methods TR-55 10, Rational 4, TR-20 1, USGS Rural Regression F.quations 1, USGS Urban Regression F.quations FHW A Small Rural Watersheds HEC Log-Pearson Type ill SCS Hand Methods Syntheti Flood Frequeny Other Runoff Hydrograph Models Misellaneous TOTAL , and the use of hydrologi models are onsidered, other aspets not addressed by the questionnaire should also be onsidered. Some of these important aspets are watershed shape (long and narrow basins have longer response times than short and wide basins); many methods do not have the apability to ombine and hannel route subbasin runoff; others do not provide the ability to route runoff through storage. CONCLUSIONS FROM RESPONSES After examining the responses to the questionnaire, the author drew some general onlusions regarding the use of hydrologk methods as applied to small urban watersheds. Other analysts experiened in hydrologi models may arrive at different onlusions. Some of the more obvious onlusions are as follows:. The number of studies performed in a I-year period (20,000 plus) indiates that there is a great deal of ativity in hydrologi analyses of small urban watershed. Calibrated rainfall runoff models suh as TR-20 and HEC-1 onstitute a small perentage (12 perent) of all models used. w (.) a: w a w 40 "' :::> ~. z TALE 2 Reasons for Seletion of Method TR-SS Rational TR-20 CLASSIFIE METHO USGS Rural Regression Equation USGS Urban Regression Equation FHW A Small Rural Watersheds HEC l Log-Pearson Type ill SCS Hand Methods Syntheti Flood Frequeny Other Runoff Hydrograph Models Misellaneous Legend: A - Eonomi Feasibility - Required by Regulations C - Speified in Contrat RESPONSE RANKING s 6 E F A F E F A E F - Unique Environment E - Organi7.ational Familiarity F - Ease of Use Pratitioners of hydrologi analysis are interested in the proedures being used by their peers, beause 94 perent of the respondents requested opies of the result(s) of the survey. Ease of use was the most prominent response regarding the reason for seletion of methods. Respondents generally do not view small urban watersheds as unique environments beause this reason for method use ranked midway in responses. Results of the survey onfirm the need for guidane on the seletion and appliation of hydro logi methods for flood frequeny analysis being applied to small urban watersheds. A P CLASSIFICATION FIGURE 3 Use of hydrologi methods. OTHER ASSESSMENTS OF HYROLOGIC METHOS Sine the Federal Interageny Hydrology survey of hydrologi methods was summarized in 1989 and has not been

6 52 TRANSPORTATION RESEARCH RECOR 1471 published for general information, it appeared prudent to supplement the results of that survey with additional and more reent information. ata from two individual assessments are provided for omparison. State Highway epartments Probably thousands of hydrologi omputations for sizing highway water passages (ulverts and bridges) are made eah year. Sizes are generally based on speifi quantiles (peak flows and their assoiated exeedene frequenies). This survey onduted by the Maryland State Highway Administration in 1990 inluded 45 states and the hydrologi methods used by their highway departments. Table 3 gives the hydrologi methods ited and the number and perentage of states that use them. Some interesting observations from this survey are that Eighteen states indiated the use of only one hydrologi method in their engineering analyses of waterway openings; Three states use five different hydrologi methods; and Thirty-three states use hydrologi methods that produe only peak flows (hydrographs are not available from the methods seleted). This survey did not inlude information on the reasons that speifi hydrologi methods were seleted. However, from examination of the methods seleted, it appears that simpliity of use, onsisteny of results, and nonuse of hydrographs are important onsiderations. eause of the limitations of this survey, it appears that state highway departments do not beome involved in detailed hydrologi analyses. This assumption may be orret for most of the states surveyed; however, it is ertainly not true for the Illinois epartment of Transportation (IOT). IOT is an example of a state highway organization that has a leading role and responsibility for sound hydrologi analyses within the state. For those states that do not have a lead ageny for oordinating and reviewing hydrologi analyses, IOT serves as an example of how state highway organizations an provide this important servie. Although several federal agenies and other state agenies may be performing hydrologi studies within a state, there may be no state ageny with responsibility or adequate funding to oordinate studies and to maintain some level of onsisteny throughout the state. National Flood Insurane Program Most of the ommunities in the NFIP have base (100-year) floods inluded in their flood insurane studies (FISs). Thus, FEMA is urrently more in a revision mode than in its earlier development mode insofar as hydrologi studies are involved. The majority of revision studies involving hydrologi analyses are performed by engineering firms that represent owners of property within the designated 100-year floodplain. These property owners are seeking relief from NFIP by onvining FEMA that their estimate of the 100-year flood is more orret than the estimate in the FIS. ewberry & avis (&) serves as a tehnial evaluation ontrator for FEMA Regions 1 through 5. In this role & reviews the tehnial adequay of hydrologi studies in revision requests as well as those studies performed by FEMA's study ontrators who update FISs. A survey of the hydrologi methods being used in review ases was TALE 3 Use of Hydrologi Methods by State Highway epartments. Regional Regression Equations (USGS) Rational (Mulvaney's Equation) ulletin 17- (Interageny 18 Hydrology ) other Methods (Used by One 7 16 State) TR-55 (SCS) 6 13 TR-20 (SCS) 4 9 HEC-1 (USACE) 7 FHWA Proedures 7 onduted during August 1993 for Regions 1 through 5. Results of that survey are inluded in Table 4. An earlier survey of the hydrologi methods used during FEMA's development mode would have resulted in a table more onsistent with that showing the results of the state highway department survey. Less use of rainfall runoff models suh as HEC-1 and TR-20 would have been reported. Although the FEMA survey shows HEC-1 as the most prominent hydrologi model used, in many instanes the HEC-1 model uses the urve number loss funtion and unit hydrograph proedure from TR-20. HEC-1 does not inlude the Att-Kin routing proedure from TR-20 as one of its routing options. Therefore, it is diffiult to obtain exatly the same results from the two models unless hydrograph routing is not involved. Another observation not available from the survey is that the Kinemati wave option available in HEC-1 is seldom used in hydrologi studies submitted to FEMA in the eastern half of the United States. The reasons for the lak of popularity of this valuable urban hydrology proedure are not readily apparent. However, unfamiliarity with the proedure and more intense input requirements ould be deterrents. RAINFALL ATA AN AVAILALE MOELS Those who are part of water resoure programs being administered by various federal, state, and loal organizations fae important deisions regarding the use of hydrologi models and rainfall data. The traditional rainfall probability relationships published by the National Weather Servie (NWS) many years ago are being updated TALE 4 Hydrologi Methods Used by FEMA, Regions 1to5 '''~.EijgpiQJ.A91g!~q?st?;,:' l'ffl!m~'.:()ij;~u?.st HEC-1 (USACE) Regional Regression Equations (USGS) ulletin 17 (Interageny Hydrology ) TR-20 (SCS) Rational (Mulvaney's Equation) 10 7 TR-55 (SCS) 6 Others (used in one study) 6 TOTAL

7 Hagen 53 in parts of the United States. However, funds are not available for a omplete revision of these important douments. In fat, several years of study and revision would be needed to omplete suh an undertaking if all needed funding was available, thus the need for deisions on the use of new data developed by a group other than NWS. The state of Illinois has direted that its ulletin 70 be used when performing hydrologi studies within the state. Other entities have performed rainfall probability studies for use in lieu of the NWS douments. Although speial rainfall studies may provide more up-to-date information, they an reate problems for federal or state programs insofar as onsistent results are onerned. This is espeially true when in-depth detailed analyses are not involved. evelopment of new hydrologi models also auses onern for administrators and tehnial experts attempting to beome profiient in the use of the large array of models available. When the new models are developed within federal agenies that support the models by orreting errors and adding improvements, onerns are less dramati. However, when model development is by private individuals or through researh grants, there is muh less opportunity for ontinuing support and improvement. FEMA has established rules for the use of hydrologi models in NFIP; however, many of those requesting revisions submit proposed hanges that are based on the use of models that do not omply with FEMA rules. This an ause property owners onsiderable expense in redoing the studies or going through the proess of obtaining FEMA' s approval for the use of the model. Many federal agenies, suh as the USGS, USACE, SCS, and FHW A, have their own hydrologi models that were designed for the speifi needs of the agenies. However, many of these models are used by the private setor, often with some add-on modules that inrease the total number of models available for use. Thus, organizations without their own hydrologi models are onfronted with the diffiult task of seleting or approving models for use in their programs. Presentations on speifi hydrologi models, su.h as these being made for TR, are useful for those individuals evaluating models for use or approval. Although these presentations are only a sample of the available hydrologi models, sorting out the pros and ons of the different models is a formidable task. Two of the presentations involve models used extensively as indiated by the surveys that were onduted; however, the other two models are more involved, and their use is probably limited to individuals highly trained in their appliation. Interest in SCS and USACE models will be related more to their pratial appliations, whereas interest in the other two models (R3M and HSPF) will tend toward sientifi appliations. REFERENCES 1. Guidelines for etermining Flood Flow Frequeny, ulletin 17. Hydrology, Interageny Advisory Committee on Water ata, Reston, Va., Marh Estimating Peak Flow Frequenies for Natural Ungaged Watersheds A Proposed Nationwide Test. Hydrology Committee, U.S. Water Resoures Counil, Washington,.C., Water Resoures Monograph Urban Stormwater Hydrology (. F. Kibler, ed.), Hydrology Committee, Amerian Geophysial Union, Washington,.C., Opportunities in the Hydrologi Sienes (P. S. Eagleson, ed.), Water Siene and Tehnology oard, National Researh Counil, Washington,.C., Publiation of this paper sponsored by Committee on Hydrology, Hydraulis, and Water Quality.