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INFILTRATION 1. BACKGROUND THEORY. Infiltration is defined as the physical process of water flowing through the soil surface into the ground. In Hydrologic Engineering applications, infiltration "losses" are deducted from precipitation to compute runoff. Infiltration losses are composed of two primary classifications which include an initial abstraction and an infiltration loss rate. Initial abstraction (or initial loss) is defined as the maximum amount of precipitation that can occur under specific conditions without producing runoff and is comprised of interception and depression storage. Interception is the precipitation which is stored on vegetal cover while depression storage is contained in surface puddles and depression areas. Evaporation during the storm is also part of the initial abstraction, but is normally considered negligible: The infiltration loss rate is the rate at which water passes through the soil surface into the soil. The maximum rate at which water can enter the soil is referred to as the infiltration capacity. It has been demonstrated that the infiltration capacity of a given soil decreases rapidly during continuous rainfall until a fairly definite minimum rate of infiltration is reached, usually within a period of 15 minutes to a few hours. The order of, decrease in infiltration capacity and the minimum rate attained are primarily dependent upon the size of soil pores within the zone of aeration and the conditions affecting the rate of removal of capillary water from the zone of aeration. Decreases in infiltration capacity during a period of intense rainfall are due primarily to: 1) saturation of the soil profile, 2) compaction of the surface due to the energy of rain impact, 3) and in washing of the finer sediments which lodge at points of constriction within the soil profile, and 4) swelling of the clay and the colloid, reducing the size of the internal pores. At the beginning of a period of rainfall, the infiltration capacity of a given soil is related to antecedent field moisture and the physical condition of the soil. Factors which affect the rate of infiltration include rainfall characteristics, soil characteristics, condition of the soil mass, soil surface condition, vegetal cover, and soil moisture. Several methods have been developed to account for infiltration losses which include the Initial and Uniform method, the SCS Curve Number method, Holtan's method, HEC Exponential Method, Horton's method, and the Green & Ampt method. Parameters for each of the methods can be determined by calibration when observed rainfall and runoff data are available, while for some of the methods, 7-1