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<br />Contributing Area <br />That portion of a drainage area that contributes to runoff at a <br />given point. Runoff may not occur from parts of a drainage area due to <br />dams, pothole areas that have no outlet, or pervious soils with high <br />infiltration rates. <br /> <br />. <br /> <br />Depth-Area Adjustment <br />As the drainage areas become larger, the average depth of rainfall <br />over the basin becomes smaller at any given return frequency. An <br />adjustment based on an empirical relationship developed by the National <br />Oceanic and Atmospheric Administration was made to all rainfall values <br />used in the study to compensate for this natural phenomenon. <br /> <br />Detention Storage <br />The amount of rainfall and/or rainfall runoff which is intercepted <br />by ditches, ponds, or natural depressions and is therefore removed from <br />the normal surface-runoff pattern. <br /> <br />Discharge-Probability Relationship <br />The chances. (see "Probability") of floods of different magnitudes <br />occurring at a given location. Smaller floods. are likely to occur more <br />often, while greater floods are less common. <br /> <br />Exceedence Frequency <br />The likelihood of an event with a given magnitude being equalled <br />or exceeded in any given year. <br /> <br />Expected Probability <br />An adjustment for the skewed distribution of the sample error <br />around the true population. <br /> <br />Frequency <br />(See "Probability") <br /> <br />Hydrograph <br />A graph or table showing the variation of discharge over a period <br />of time at a given location. A flood hydro graph would shaw a rise in <br />discharge (ascension limb), a peak, and then a decrease in discharge <br />(recession limb), gradually returning to normal levels. <br /> <br />Hydrologic Model <br />A series of mathematical relationships which takes into account <br />factors affecting runoff from rainfall and which is used to make esti- <br />mates of stream discharge based on the runoff volume and the delivery <br />characteristics of the drainage area involved. <br /> <br />A-2 <br /> <br />. <br />