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CHAPTER 7 HYDROLOGIC METHODS PART I - PRECIPITATION 1. BACKGROUND THEORY. Synthetic or lypothetical storms are 1requently used by hydrologists and IJI1~lineers to generate synthetic flood events for evaluation of existing flood conditions. and for study of the effects of flood-mitigation components. The degree of dependency on synthetic flood events during Cl flood evaluation depends generally on the number of gages and the length of hydrologic record available. I=or reservoir studies there is typically at least one long-record stream gage near the dam site. and thus the storage required tor reservoir food control is usually computed by period-of-record simulation teclll1iques using recorded data. Channels, levees, and other lo<:a I protection projects are more likely to be sized by synthetic storm-flood analyses since these projects often extend over many miles of the stream, and it is rare that gage information would be available throughout the reach. Hypothetical storms are also used to ensure that rare storm.flood events are included in an overall assessment of tile proposal. Since large storms that cause major floods are infrequent, the available runoff recorc quite possibly may not reflect the occurrence of these rare events. Analyses of major reselvoirs, although relying mainly on recorded stream data, also require the use of synthetic storms (Standard Project and Probable Maximum Storms) to evaluate the safety of major dam componen~s, such as the spillway and outlet works, and guard against overtopping of tile dam. Synthetic precipitation data are also used for analyses of flooding throughout a basin, testing of the hydrologic effects of alternative land uses and flood plain regulations, and evaluation and design of flood control components. This is usually done with a model of the watershed to simulate the rainfall-runoff process. with the rain':all specified by hypothetical storms. 2. DATA SOURCES. The primary sources of hypothetical storm information for the United States are various technical publications (TP) of It]e National Weather Service (NWS) and hydrometeorological reports (HMR) of the National Oceanic and Atmospheric Administration (NOAA). All publications for development of hypothetical frequency storms feature generalized isopluvial rainfall maps and/or regression equations. Other methods. such as statistical analysis of nearby long-record rain gages to derive hypothetical storms of particular frequencies in lieu of the NWS and NOAA procedures. are used extensively in some parts of the United States but not discussed here Colorado Flood Hydrology Manual DRAFT 7.1