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<br />The hydrologist's role in calculating elevation-frequency <br /> <br /> <br />relationships consists primarily of calculating the probability of various <br /> <br /> <br />amounts of storm runoff entering a stream in a given period of time. The <br /> <br /> <br />hydrologist's primary tool in establishing this relationship is the unit <br /> <br />hydrograph, The hydro graph measures the workings of the hydrologic <br />system, To make these calculations, the hydrologist needs historical <br /> <br />records of rainfall, stream gauge levels and extent of flooding, and a <br /> <br />land use study to determine how existing conditions may differ from <br /> <br />conditions that led to historical events, <br /> <br />Hydrograph analysis then continues through a six step process, as <br /> <br />follows: <br /> <br />1) The amount of rainfall is measured over time, <br /> <br />2) The amount of rainfall that is lost to infiltration into soils <br /> <br />and other permeable surfaces over time is, computed. <br /> <br /> <br />3) The amount of excess rainfall is calculated by subtracting the <br /> <br /> <br />amount of infiltration losses from the amount of rainfall runoff, <br /> <br /> <br />4) The unit hydrograph is used to translate the amount of rainfall <br /> <br />over time to the amount of surface runoff. <br /> <br />S) The base flow is computed by calculating the amount of runoff <br /> <br /> <br />that would generally be present from all previous storms, <br /> <br /> <br />6) Total runoff is then computed by adding the amount of direct <br /> <br /> <br />runoff to base flow. <br /> <br />V-16 <br />