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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />. and accumulated in proportion to the percent of aerial coverage for each type <br />of surface. <br /> <br />. <br /> <br />2.3 Infiltration <br />The flow of water into the soil surface is called infiltration. In urban <br />hydrology much of the infiltration occurs on areas covered with lawn grass. <br />Urbanization can increase or decrease the total amount of infiltration (2). <br />Soil type is the most important factor in determining the infiltration <br />rate. When the soil has a large percent of well-graded fines, the <br />infiltration rate is low. In some cases of extremely tight soil there may be, <br />from a practical standpoint, essentially no infiltration. If the soil has <br />several layers or horizons, the least permeable layer will control the maximum <br />infiltration rate. The soil cover also plays an important role in determining <br />the infiltration rate. Vegetation, lawn grass in particular, tends to <br />increase infiltration by loosening the soil near the surface. Other factors <br />affecting infiltration rates include: slope of land, temperature, quality of <br />water, age of lawn and soil compaction (5). <br />As the rainfall continues the infiltration rate decreases. When a <br />rainfall occurs on an area that has little antecedent moisture, that is, the <br />ground is dry, the infiltration rate is much higher than it is with a high <br />antecedent moisture, resulting from previous storms or irrigation. Although <br />ant'lcedent precipitation is very important when calculating runoff from <br />smaller storms in non-urbanized areas, the runoff data from the urbanized <br />basins indicates that antecedent precipitation has a limited effect on runoff <br />in the urbanized portions of the Urban Drainage and Flood Control District. <br /> <br />2.3.1 Horton's Equation <br />There are many infiltration model s in use by the urban storm runoff <br />hydrologists. These models vary significantly in complexity. Because of the <br />cl imatic condition in the semi-arid region and because runoff from urban <br />watersheds is not very sensitive to infiltration refinements, the infiltration <br />model proposed by Horton (40 & 41) was found to provide a good balance between <br />simplicity and reasonable physical description of the infiltration process. <br />Horton's infiltration model is described by Equation 2-1 and is illustrated <br />graphically in Figure 2-2. <br /> <br />. <br /> <br />5-1..84 <br />