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<br />Section 4.04. Probable maximuM ground conditions <br /> <br />Ground conditions that affect losses during the probable maximum <br />storm should be the MOst severe that can reasonably exist in conjunction <br />with I1IlIxillllllft probable precipitation. Lowest loss rates that have been <br />observed'might be used if there is reasonable assurance that the entire <br />range of possible losses has been experienced. However. loss estil1llltes <br />are subject to major uncertainties. and there are cases where negative <br />loss rates are computed simply because of inadequate precipitation data. <br />Accordingly, some allowance IlIlIst be made for this uncertainty. and loss <br />rates that are conservatively low should be selected for probable I1IlIximum <br />flood computation. <br />Where it is possible for the ground to be frozen at the start of a <br />rainflood or snowmelt flood. it can be concluded that zero or near-zero <br />loss rates should be used for probable Maximum flood computation. <br />There may exist I seasonal variation in minimum loss rates. in which <br />case. rates selected should be those representative of most extreme con- <br />ditions for the season for which probable maximum runoff is being com- <br />puted. <br /> <br />Section 4.05. Probable maximum snowpack <br /> <br />As in the case of standard project snowpack. it is not feasible to <br />compute maximum snowpack accumulation from winter precipitation. temper- <br />atures and snowpack losses. Probable maximum snowpack for floods that <br />are primarily snowmelt floods should be estimated from observed snowpack <br />data and should be considerably larger than standard project snowpack. <br />It may be satisfactory to add a factor such as 25 percent to standard <br />project snowpack in order to obtain probable maximum snowpack. <br />In the case of rainfloods that have some snowmelt contribution. <br />snowpack used for probable maxillllllR rainflood computation should be the <br /> <br />4-08 <br />