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<br />Hydrologic Design Manual
<br />for Maricopa County
<br />
<br />Rainfall Losses
<br />
<br />Vegetation Type
<br />hardwood tree
<br />cotton
<br />alfalfa
<br />meadow grass
<br />
<br />Inlerceptlon,
<br />Inches
<br />0.09
<br />0.33
<br />0.11
<br />0.08
<br />
<br />No interception estimates are known fornalural vegetation that occurs in Maricopa
<br />County. For most applications in Maricopa County the magnitude of interception
<br />losses is essentially 0.0, and for practical purposes interception is not considered for
<br />flood hydrology in Maricopa County.
<br />
<br />Depression storage and infiltration losses comprise the majority of the rainfall loss
<br />as illustrated in Figure 4.1. The estimates of these two losses will be discussed in
<br />more detail in later sections of this manual. Three periods of rainfall losses are
<br />illustrated in Figure 4.1, and these must be understood and their implications
<br />appreciated before applying the procedures in this manual. First, there is a period
<br />of initial loss when no rainfall excess (runoff) is produced. During this initial period,
<br />the losses are a function of the depression storage, interception, and evaporation
<br />rates plus the initially high infiltration capacity of the soil. The accumulated rainfall
<br />loss during this period with no runoff is called the initial abstradion. The end of this
<br />initial period is noted by the onset of ponded water on the surface, and the time
<br />from start of rainfall to this time is the time of ponding ITp). It is important to note
<br />that losses during this first period are a summation of losses due to all mechanisms
<br />including infiltration.
<br />
<br />The second period is marked by a declining infiltration rate and generally very little
<br />losses due to other factors.
<br />
<br />The third, and final, period occurs for rainfa~ of sufficient duration for the
<br />infiltration rate to reach the steady-state, equilibrium rate of the soil (fc). The only .f /5
<br />appreciable loss during the final period is due to infiJtration. p..r.i"re. .1,;1.) ;
<br />~ 4S5""''''pt.
<br />The actual loss process is quite complex and there is a goga1leal of interdependence
<br />of the loss mechanisms on each other and on the rainfa)l1iself. Therefore, simplifying
<br />assumptions are usually made in the modeling of raiJ;lfaJJ losses. Figure 4.2 represents
<br />a simplified set of assumptions that can be madeo lfuat surface retention loss is the
<br />S1Illlmation of all losses other than those due to infiltration, and that this loss occurs
<br />from the start of rainfall and ends when the accumulated rainfall equals the magnitude
<br />of the capacity of the surface retention loss. It is assumed that infill:llltion does not occur
<br />during this time. After the surface retention is satisfied, infiltration begins. If the
<br />infiltration capacity exceeds the rainfall intensity, then no rainfall excess is produced.
<br />As the infiltration capacity decreases, it may eventually equal the rainfall intensity. This
<br />would occurat the time of ponding ITp) which signals the beginning of surface runoff.
<br />As illustrated in both Figures 4.1 and 4.2, after the time of pending the infiltration rate
<br />decreases exponentially and may reach a steadY-5tate, equilibrium rate (fc). It is these
<br />'simplified assumptions and processes, as illustrated in Figure 4.2, that are to be
<br />modeled by the procedures in this manual.
<br />
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<br />39
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