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<br />The basic sequence followed in the situation where no streamflow is <br />available for the basin is shown in Figure 3.1. The basin average rainfall <br />associated with a particular frequency of occurrence is determined, either <br />from historical records or using methods for developing hypothetical- <br />frequency events (see Chapter 4). Basin average loss rates are set. These <br />may be calculated from regional values; they may be estimated through direct <br />transfer from historical events; or Dther methods may be used. <br /> <br />The computed time distribution of basin-average rainfall excess (Le., <br />the rainfall mirus losses) is then used with a unit hydro graph and the <br />general basin routing model to compute the runoff hydrograph at the basin <br />outlet. The Clark, Snyder, or SCS unit hydrograiXls may be used with the <br />hydrograph parameters determined by calibration using measured events (see <br />Chapter 5) or by using parameters determined with regionalization techniques <br />(see Chapter 6), Alternatively, the kinematic wave options of HEC-l can be <br />used to develop a basin model. <br /> <br />/ <br /> <br />This process is repeated for several storms of different frequencies. <br />The computed flow for each event can be associated with the frequency of <br />occurrerce of the rainfall, and a discharge-frequency relationship can be <br />established, This relationship should be adjusted to match events of known <br />frequency. <br /> <br />3.3 Influence of Data Availability on Study Procedure <br /> <br />The type and amount of data that are available for the study strongly <br />influence how the study is conducted. This is illustrated here for the case <br />where no streamflow data are Bvailable in the basin (Case 3). Three possible <br />levels of study procedure are discussed, starting with the most extensive <br />investigation effort. <br /> <br />Procedure A - Extensive study <br /> <br />(1) Obtain Basin Parameters <br />Determine locations in a hydrologic region similar to the study basin <br />where discharge has been measured during several (usually 5 to 10) major <br />storm events. Collect streamflow and rainfall data for these events at as <br /> <br />12 <br /> <br />e <br /> <br />e <br /> <br />e <br />