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<br />e <br /> <br />,. <br />, <br /> <br />e <br /> <br />~ <br /> <br />CHAPTER 3 <br /> <br />EFFECT OF DATA AVAILABILITY ON CHOICE OF APPROACH <br /> <br />It is the rule, rather than the exception, that the datB available for a <br />particular hydrologic study are not sufficient to give a complete solution to <br />the problem. The available methods of hydrologic analysis are frequently <br />more precise than the data to support them, even though many hydrologic <br />processes are nDt fully understood and completely defined. As a consequence, <br />the hydrologist is often constrained to adopt procedures that are not as <br />complete or as rigorous as desired, but that must be used as a result of the <br />type of data that are available. This chapter discusses the degrees of data <br />availability, the general approach to basin analysis using HEC-l, and the <br />influence of data availability on the study procedure, <br /> <br />3,1 Degrees of Data Availability <br /> <br />Three levels of data availability are considered here: (1) Sufficient <br />data to calculate discharge-frequency curves at some locations, but not at <br />DtherSj (2) SUfficient streamflow data to calibrate precipitation-runoff <br />parameters at some locations, but insufficient data to derive discharge- <br />frequency curves by statistical methods; and (3) No streamflow data available <br />within the basin, <br /> <br />3.2 General HEC-l Approach to Basin Analysis <br /> <br />HEC-l is a generalized precipitation runoff model for single event <br />simulatiDn (Hydrologic Engineering Center, 1973a). Either a unit hydrograph <br />or kinematic wave approach is used to transform rainfall values to runoff. <br />The unit hydrograph is most commonly used, and it is assumed that a single <br />unit hydrograph is appropriate for all magnitudes of rainfall excess. Clark, <br />Snyder, or the Soil Conservation Service unit hydrograph methods can be <br />used, Snowmelt can be included, and several loss rate functions are <br />available. A large basin can be subdivided into subbasins, and stream <br />routing from subbasin to subbasin can be performed with any of several <br />hydrologic routing methods. <br /> <br />11 <br />