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<br />11:11 Rainfall excess is that portion of the total rainfall depth that drains directly from the
<br />land surface by overland flow. Bya mass balance, rainfall excess plus rainfall loss
<br />equals precipitation. When performing a flood analysis using a rainfall-runoff
<br />model, the determination of rainfall excess is of utmost importance. Rainfall excess
<br />integrated over the entire watershed results in runoff volume, and the temporal
<br />distribution of the rainfall excess will, along with the hydraulics of runoff, deter-
<br />mine the peak discharge. Therefore, the estimation of the magnitude and time
<br />distribution of rainfall losses should be performed with the best practical technol-
<br />ogy, considering the objective of the analysis, economics of the project, and conse-
<br />quences of inaccurate estimates.
<br />
<br />Rainfall losses are generally considered to be the result of evaporation of water from
<br />the land surface, interception of rainfall by vegetal cover, depression storage on the
<br />land surface (paved or unpaved), and infiltration of water into the soil matrix. A
<br />schematic representation of rainfall losses for a uniform intensity rainfall is shown
<br />in Figure 4.1. As shown in the figure, evaporation can start at an initially high rate
<br />depending on the land surface temperature, but the rate decreases very rapidly and
<br />would eventually reach a low, steady-state rate. From a practical standpoint, the
<br />magnitude of rainfall loss that can be realized from evaporation during a storm of
<br />sufficient magnitude to cause flood runoff is negligible.
<br />
<br />Interception, also illustrated in Figure 4.1, varies depending upon the type of
<br />vegetation, maturity, and extent of canopy cover. Experimental data on intercep-
<br />tion have been collected by numerous investigators (Linsley and others, 1982), but
<br />little is known of the interception values for most hydrologic problems. Estimates
<br />of interception for various vegetation types (Linsley and others, 1982) are:
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<br />37
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