Coweeta Hydrologic Laboratory Climate

Mean annual precipitation at Coweeta Hydrologic Laboratory (latitude 35°14' N, longitude 83°26' W) varies from 1798 mm at the base climate station (686 m) to 2373 mm at the high-elevation Mooney Gap climate station (1364 m). Mean annual growing season precipitation, defined as May to October, is 782 mm at the

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Figure 3.1 Precipitation from 1935 to 2001 (standard gauge 19) and streamflow from 1937 to 2001 (reference watershed 18) at Coweeta Hydrologic Laboratory near Otto, North Carolina, United States. The mean total annual precipitation (1798 mm), mean May-October growing season precipitation (782 mm), and mean streamflow (1011 mm) are shown by straight solid lines.

base climate station (figure 3.1). Mean annual streamflow from watershed 18, a low-elevation reference watershed, is 1011 mm or 56% of precipitation (figure 3.1).

Short-duration thundershowers at Coweeta are typical for midsummer and fall with occurrences of large rainfalls stimulated by tropical disturbances near the Atlantic or Gulf coasts. Forty-nine percent of the 133 storms each year have a total precipitation amount less than 5 mm, and 69% of the annual precipitation falls with an intensity less than 10 mm per hour. Snow is a minor part of the annual precipitation, averaging 2-5% depending on elevation. Snow cover rarely lasts for more than 3 or 4 days, even on the upper slopes. Compared with other mountain sites, wind speeds at Coweeta appear to be low and even imperceptible in the valley bottoms.

Periodic droughts occur in the southern Appalachians. The summer drought of 1925 in Asheville, North Carolina (May to August), generated only 32% of mean precipitation (Hursh and Haasis 1931). At Coweeta during the period 1985-1988, a severe drought occurred, totaling 1837 mm of precipitation deficit (mean minus annual precipitation), a 26% reduction, and 1849 mm of streamflow deficit (mean minus annual streamflow), a 46% reduction. A recent drought totals 1246 mm of precipitation deficit from 1998 to 2001, a 17% reduction, and 1349 mm of stream-flow deficit from 1998 to 2001, a 34% reduction.

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