Drainage Design Manual, Hydrology - Flood Control District of ...

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Drainage Design Manual for Maricopa County Hydrology: Rainfall Losses investigators (Linsley et al. 1982), but little is known of the interception values for most hydrologic problems. Estimates of interception for various vegetation types (Linsley et al. 1982) are: Vegetation Type Interception, inches Hardwood tree 0.09 Cotton 0.33 Alfalfa 0.11 Meadow grass 0.08 FIGURE 4.1 SCHEMATIC REPRESENTATION OF RAINFALL LOSSES FOR A UNIFORM INTENSITY RAINFALL No interception estimates are known for natural vegetation that occurs in Maricopa County. For most applications in Maricopa County, the magnitude of interception losses is essentially zero. Interception is considered for flood hydrology in Maricopa County, but for practical purposes an actual value is not assigned. 4-2 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Rainfall Losses Depression storage and infiltration losses comprise the majority of the rainfall loss as illustrated in Figure 4.1. The estimates of these two losses will be discussed in more detail in later sections of this manual. Three periods of rainfall losses are illustrated in Figure 4.1, and these must be understood and their implications appreciated before applying the procedures in this manual. First, there is a period of initial loss when no rainfall excess (runoff) is produced. During this initial period, the losses are a function of the depression storage, interception, and evaporation rates plus the initially high infiltration capacity of the soil. The accumulated rainfall loss during this period with no runoff is called the initial abstraction. The end of this initial period is noted by the onset of ponded water on the surface, and the time from start of rainfall to this time is the time of ponding (T p ). It is important to note that losses during this first period are a summation of losses due to all mechanisms including infiltration. The second period is marked by a declining infiltration rate and generally very little losses due to other factors. The third, and final, period occurs for rainfalls of sufficient duration for the infiltration rate to reach the steady-state, equilibrium rate of the soil (f c ). The only appreciable loss during the final period is due to infiltration. The actual loss process is quite complex and there is a good deal of interdependence of the loss mechanisms on each other and on the rainfall itself. Therefore, simplifying assumptions are usually made in the modeling of rainfall losses. Figure 4.2 represents a simplified set of assumptions that can be made. In Figure 4.2, it is assumed that surface retention loss is the summation of all losses other than those due to infiltration, and that this loss occurs from the start of rainfall and ends when the accumulated rainfall equals the magnitude of the capacity of the surface retention loss. It is assumed that infiltration does not occur during this time. After the surface retention is satisfied, infiltration begins. If the infiltration capacity exceeds the rainfall intensity, then no rainfall excess is produced. As the infiltration capacity decreases, it may eventually equal the rainfall intensity. This would occur at the time of ponding (T p ) which signals the beginning of surface runoff. As illustrated in both Figure 4.1 and Figure 4.2, after the time of ponding the infiltration rate decreases exponentially and may reach steady-state, equilibrium rate (f c ). It is these simplified assumptions and processes, as illustrated in Figure 4.2, that are to be modeled by the procedures in this manual. August 15, 2013 4-3
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