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<br />PART II.INFILTRATION <br /> <br />1. BACKGROUND THEORY. <br /> <br />Infiitration is defined as the phl'sical process of water flowing through the soil <br />surface into the ground. In Hydrologic Engineering applications, infiitration "losses" are <br />deducted from precipitation to compute runoff. Infiltration losses are composed of two <br />primary classifications which include an initial abstraction and an infiitration loss late. Initial <br />abstraction (or initial loss) is defined as tile maximum amount of precipitation that can <br />occur under specific conditions without producing runoff and is comprised of interception <br />and depression storage. Interception is thEl precipitation which is stored on vegetal cover <br />while depression storage is contained in surface puddles and depression areas. <br />Evaporation during the storm is also part of the initial abstraction, but is normally <br />considered negligible. The infiitration loss rate is the rate at which water passEls through <br />the soil surface into the soil. The maximum rate at which water can enter the soil is <br />referred to as the infiitration capacity. <br /> <br />It has been demonstrated that the infiitration capacity of a given soil decreases <br />rapidly during continuous rainfall until a fairly definite minimum rate of infiltration is <br />reached, usually within a period of 15 minutes to a few hours. The order of decrease in <br />infiijration capacity and the minimum rate attained are primarily dependent upon the size <br />of soil pores within the zone of aeration and the conditions affecting the rate of removal <br />of capillary water from the zone of aeration. Decreases in infiitration capacity during a <br />period of intense rainfall are due primarily to: <br /> <br />1) saturation of the soil profile, <br />2) compaction of the surface due to the energy of rain impact, <br />3) and in washing of the finer sediments which lodge at points of constriction within <br />the soil profile, and <br />4) swelling of the clay and the colloid, reducing the size of the internal pores. <br /> <br />At the beginning of a period of rainfall, the infiitration capacily of a given soil is <br />related to antecedent field moisture and the physical condition of the soil. Factors which <br />affect the rate of infiltration include rainfall characteristics, soil characteristics, condition of <br />the soil mass, soil surface condition, vegetal cover, and soil moisture. Several methods <br />have been developed to account for infiltration losses which include the Initial and Uniform <br />method, the SCS Curve Number method, Hoitan's method, HEC Exponential Method, <br />Horton's method, and the Green & Ampt m~!thod. Parameters for each of the methods can <br />be determined by calibration when observed rainfall and runoff data are available, while <br />for some of the methods, parameters can be determined from field and labcratory tests. <br />When the infiijration parameters are based on labcratory tests, it should be noted that for <br />various reasons, loss rates in natural drainage basins average much less than rates in <br />controlled laboratory experiments wilh similar soils, sometimes an order of magnilude <br />smaller. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7:IS <br />