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<br />Table 3. Characteristics of weather stations In the Grand Canyon region, <br /> Elevation Record Mean annual Summer Winter <br />Stltlon name. (m) length precipitation (mm) precipitation (%) preclpltltlon (%) <br />Ash Fork .. ],581 4/09-12/98 366 39 39 <br />BrighlAngel RS" 2,726 7/48-12/98 655 23 44 <br />Desert View 2,271 9/60-7/951 347 31 37 <br />Grand Canyon .. 2,204 10/04-12/98 403 <br /> 34 36 <br />Lees Ferry 978 4/16-12/98 148 40 38 <br />Mount Trumbull 1,818 1 0/20-12178~ 299 41 29 <br />Page 1,315 11/57-12/98 169 28 41 <br />Peach Springs 1,613 7/48-12/98 285 37 31 <br />Phantom Rancb .. 834 8/66-12/98 238 31 39 <br />Seligman .. ].618 12/04-12/98 293 <br /> 42 40 <br />Supai 987 6/56-02/78! 212 40 32 <br />Tuba City 1,550 1/00-12/98 164 37 38 <br />Tuweep RS" 1,55] 7/48-12/86' 288 35 39 <br />Williams .. 2,080 10/00-12/98 551 36 <br /> 36 <br /> <br />*All slations are in Arizona (fig. I). <br />tOaily data from September I, 1960, 10 July I, 1975. have been lost at this station, which is not part of the NOAA network. Monthly dal8 is available <br />from Seplember 1960 lO about August 1995 from The National Park Service. <br />'Station discontinued. <br />*'In 1986, Tuweep Ranger Station was discontinued as a cooperative observer station, which records rainfall in 0.25 mm accuracy and reports <br />increments of daily rainfall. A tipping-bucket recording rain gage, which records rainfall in 2.54 nun increments and reports hourly as well as daily <br />rainfall (e.g., U.S. Department of Commerce, 1966), remains in operation. Between 1995 and 1998 the station record is mostly missing. <br />.. Daily precipitation and stonn frequency was analyzed for this station. <br /> <br />precipitation (fig. 3), which is unique in the last <br />century. On a decadal basis (fig, 5), tbe above- <br />average precipitation of the 1980s and 1990s is <br />comparable only to that of tbe 1900s. Annual <br />precipitation from 1940 through 1979 was below <br />average on a decadal basis, as sbown by other <br />regional studies (Hereford and Webb, 1992; Webb <br />and Betancourt, 1992), indicating tbat annual <br />precipitation in the southwestern United States is <br />nonstationary, <br /> <br />Seasonal precipitation also shows considerable <br />interannual and interdecadal variability, Summer <br />(July-September) and winter (November-Marcb) <br />precipitation (fig. 4) has not responded in the same <br />way in eacb year. With notable exceptions, <br />particularly 1983 and 1984, summer precipitation <br />generally bas been above-average when winter <br />precipitation was below average. From 1984 <br />through 1998, summer precipitation was above <br />average in 6 years (40 percent), whereas winter <br />precipitation was above average in 10 years (67 <br />percent). Despite this, the 1980s and 1990s botb had <br />above-average seasonal precipitation because of <br /> <br />exceptional years (e.g., 1997 for summer, 1993 for <br />winter). For summer precipitation, tbe decade of the <br />1930s was similar to the 1980s and I 990s, but the <br />above-average winter precipitation in the last two <br />decades is comparable only to that of the 1900s (fig, <br />5). <br /> <br />STREAMFLOW SEDIMENT YIELD <br /> <br />We used three methods to estimate streamflow <br />sediment yield from the ungaged tributaries to <br />Orand Canyon: (1) a regression equation relating <br />drainage area to sediment yield for all relevant <br />sediment-yield data from nortbern Arizona, (2) an <br />empirical relation developed by Renard (1972), and <br />(3) a new procedure that combines regional flood- <br />frequency analysis with sediment-rating curves, All <br />three methods are compared against regional data to <br />detennine tbeir appropriateness for estimating <br />sediment yield to Orand Canyon, <br />Given the limited amount of sediment data and <br />variety of model types, we found it necessary to mix <br /> <br />STREAMFLOW SEDIMENT YIELD 9 <br />