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t, J . . The climate of the area is characterized by low and -G-var iable-pr'ecipitation-:--The-weath'erupat tern-is commonly governed ~ by dry air from the southwest. In the winter brief invasions of C) cold air from the northwest cause sharp drops in temperature. In CJ the spring, summer, and fall the local climate is influenced by ~ moist air from the Gulf of Mexico, which brings most of the growing season precipitation. Ann~l precipitation at Las Animas (elevation 3,893 feet) averages 12.25 inches. About 75. percent of the annual total pre- cipitation falls during the period March through September, and most of this falls during 'thunderstorms. Flows in Muddy Creek are ePhe~meral and occur in response to rain storms over the basin. storm runoff is a function of rain- fall amount, intensity, areal distribution, and location over ,the basin. All of these factors make runoff highly variable from storm to storm. Methods Runoff data are not available for the basin but are available for four similar basins in the area. The topography, soils" precipitation, and vegetation characteristics of these basins are similar to those of Muddy Creek basin. Streamflow records are available on Luning Arroyo and Van Bremer Arroyo from 1967 through 1974. Both are tributary to the Purgatoire River near Model. Rule Creek and Caddoa Creek, both tributary to the Arkansas River near . ~' John Martin Reservoir, were gaged from 1942 to 1946. The drainage ~ areas of these four basins range from 86 to 435 square miles. Flow ~ on three of the streams is ephemeral; however, Van Bremer has a L ~ small year-round f,low sustained by ground water discharge. ~.."l Ii ">"0"'-"" I ~ o\=" 'i'M"",^cJWo.{;L~ ciI-oU4# """ f'A/-....rw> '1 t'l<<Lt- -.J ;t1cAof,ty Cw/tJj iMf _....,f ,>I &)l{ Several methods are available for estimating average annual runoff from ungaged basins when there are streamflow records for other basins in the general area. Runoff is related to several watershed indices including drainage area, soils, elevation, vegeta- tion, and topography. In southeastern Colorado soil, elevation, vegetation, and topography are somewhat similar between watersheds; therefore, differences in drainage area will have the greatest effect on differences in runoff. A second method for estimating average annual runoff is based upon stream channel size and shape characteristics. This ,~eth<:lc:l~~r> ,~en ,devel()p~<1_by ,the_1.kS._!3!=_olClg:j.gal_SlLrvey J,n_recent _~_ years and has been shown to have a lower standard error of estimate than methods based upon drainage basin indices. -2-