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36 3.4 LOSS RATES As previously stated, loss rates are a measure of~the precipi~ation lost to infiltration, eveporation, transpiration, gnd absorptl~n, and also to minor detention storage in the basin. ~ta to deflne or represent loss rates during an extreme event, such as:" ~ PMF, a::e very limited. Therefore, loss rates were estimated u~ng prevlo~s studies and based on information gathered during the JJne 1987 fleld trips through the basin. The loss rates used in previous inflow design and/or pr~bable maximum flood studies were compiled and compared. The ~951 study included a substantial amount of information about loss i~tes.. In that study, soil characteristics, surface geology, and ve4etatlve cover were analyzed to estimate loss rates. Generalized s~::face geology and vegetative maps were prepared for the upper ba~ln and portions of the lower basin. The area in the lower basin b~low the Bridge Canyon dam site was not included in the 1951 study b~cause the original spillway studies for Hoover Dam did not use prr~ cipitation and/or loss rate data to develop the spillway de~lgn peaks and volumes. Examination of other flood studies also' indicated a lack of loss rate information for much of the loier basin, especially in the desert area around Lake Mead. In general, the lower basin areas adjacent to Lake Mead and ,the north side tributaries to Lake Mead were found to be areas of low loss rates and subject to flash flooding. The other tribut~ry areas, especially Kanab Creek, Kaibab Creek, and most of th~.Little Colorado River basin, have somewhat higher loss rates. In gt 1eral, the vegetative cover and loss rates increase with elevation rise. Most of the Little Colorado River basin showed very little evidence of flash flooding or stream channel development. In the upper basin, those areas tributary to Lake Powell were very desert-like with sparse vegetation, and exhibited signs of flash flooding. The loss rates were quite low. Some portions of the lower Green River subbasin have extensive outcrops of Mancos Shale. The upper basin also exhibited the same increase in vegetation and loss rates with elevation increase as the lower basin. Table 3.2 summarizes the field trip observations and relationships between elevation, vegetation, and soils, and the accompanying generalized estimates of "C" values and loss rates. This table represents a refinement of the application of the generalized criteria presented in table 3.3. For this study, the delineated subbasins were further divided into the elevation bands shown on table 3.2. Weighted averages, based on the amount of area contained in each elevation band, were used to determine loss rates and "C" values for each subbasin. The results of this analysis are presented in table 3.1. Many of the loss rates used in the 1951 study were found to still be appropriate, although adjust...ents were made for some subbasins. Table 3.4 is a comparison of the loss rates from the 1951 study with estimates based on observations made in 1987 for areas in the lower basin.