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<br />from the sample or observed discharge values. <br /> <br />Advantaoes <br /> <br />Disadvantaoes <br /> <br />1. Generally provides most accurate <br />means of discharge"frequency analysis. <br /> <br />1. Usually not a stream gage exactly at <br />site of interest. <br /> <br />2. Well documented, standardized <br />procedures (Bulletin 17B). <br /> <br />2. Assumes probability distribution <br />model fits population. <br /> <br />3. Assumes sample data is <br />representative of population, <br /> <br />4. Assumes parameters of population <br />can be determined from sample data. <br /> <br />6.1.3 Rainfall-Runoff Modellino. For situations where historic streamflow data are <br />inadequate or none):istent, a rainfall-runoff model is commonly used for flood-runoff <br />estimates. Generally" a rainfall-runoff model must be used if it is intended to evaluate the <br />effects of either landlJse changes or structural modification in the watershed. Rainfall- <br />runoff models are generally divided into two classes: single event and continuous <br />streamflow modelling.. Single-event modelling involves simulating the runoff response of <br />the hydrologic system to precipitation for a single unique storm event. Continuous <br />streamflow modelling involves continuously accounting for sustained periods of precipitation <br />over a basin with spec:ial emphasis on adjusting the soil moisture, evaportranspiration, and <br />subsurface flows during periods of no precipitation. The primary interest In flood hydrology <br />is the flood hydrograph, so consequently, single-event modelling is usually the procedure <br />used for analysis. <br /> <br />Single-event modelling usually follows the steps listed below, <br /> <br />(1) Determination of subbasin average precipitation <br /> <br />(2) Dfltermination of precipitation excess from infiltration losses <br /> <br />(3) Transformation of precipitation excess into a runoff hydrograph <br /> <br />(4) Routing of stream flows <br /> <br />(5) RElservoir routing <br /> <br />(6) Combination of hydrographs <br /> <br />ColoraM Flood <br />Hydrology Manllal <br /> <br />6.2 <br /> <br />fFIff <br />