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<br />. <br /> <br />Hydrologic Studies <br />In this report, this refers to studies to determine the discharge- <br />probability relationship at various locations or to determine Probable <br />Maximum Flood hydrographs at damsites to evaluate their hydrologic <br />capability during extreme flood events. <br /> <br />Hyetograph <br />A graphical representation of the distribution of rainfall with <br />respect to time. <br /> <br />Imperviousness <br />The degree to which an area will shed water and not allow rainfall <br />to penetrate. For example, paved areas are essentially impervious, <br />while sandy areas are very pervious. <br /> <br />Infiltration <br />Precipitation soaking into the ground. For example, a sandy soil <br />may absorb a great deal of rainfall without leaving an excess for <br />surface runoff. This may be expressed as the total proportion of <br />rainfall absorbed or as an infiltration rate in inches per hour (see <br />"Runoff"). <br /> <br />Log-Pearson Type III <br />A mathematical expression that experience has ,demonstrated provides <br />the best representation of the distribution of flood events as they <br />occur in nature. <br /> <br />Mean Flood <br />The mean flood is an event that has a 50-percent chance of being <br />equalled or exceeded in any given year. Or, in other words, it would <br />be expected that half the annual peak discharges at a given location <br />would be higher in magnitude while the other half would be lower. <br /> <br />MITCAT <br />Massachusetts Institute of Technology Catchment Model--one of <br />several computer programs that can be used as a hydrologic model. <br /> <br />Modified PuIs Method <br />A channel flood routing procedure which is based on the computed <br />or estimated storage-discharge relationship for a given reach of the <br />river or stream. This procedure can also be used to flood route <br />through reservoirs. <br /> <br />N <br /> <br />Actual number or an equivalent number of years in a discharge <br />record. <br /> <br />. <br /> <br />A-3 <br />