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<br />Section 3.03. Standard project loss rates <br /> <br />As discussed in Section 2.08, loss rates during major storms, <br />consisting principally of infiltration, can best be estimated from <br />records of observed storms and floods. Loss rates for use in computing <br />standard project floods should correspond to those considered reason- <br />ably likely to occur during storms of such magnitude, estimated on the <br />basis of rates observed in floods that have occurred in the basin or in <br />similar areas. <br />As discussed in Volu.! 4, SOMe loss-rate functions are complex and <br />cannot be expressed in a single parameter. Loss rates change with <br />precipitation intensity and with changing ground conditions during the <br />storm. These rates of change can be different during different storms <br />in the same basin. Accordingly, selection of a severe loss rate func- <br />tion for standard project flood computation may require use of a sim- <br />plified index. <br />One index of loss-rate severity is the "infiltration index," which <br />is the uniform loss rate that would produce the same volume of runoff <br />as that which actually occurs with the complex function. Use of this <br />index ignores the variation of average loss with average precipitation <br />intensity over a drainage basin, but often produces adequate results. <br />A better index might be the initial value of loss coefficient in the <br />following formulas that relate loss to rainfall intensity and accumu- <br />lated loss during the storm: <br /> <br />L . K R E <br />K . K e-CtL <br />o <br />in which: <br />L . loss rate <br />K . loss coefficient <br />R . rainfall rate <br />E . elPirical constant between 0.0 and 1.0 <br /> <br />3-07 <br />