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Predicting Soil Moisture in the Southern Appalachians¹

ABSTRACT

Soil moisture was measured for 3.5 years on forested slopes in the mountains of western North Carolina to develop equations for predicting soil moisture content of watersheds. Predictors used were precipitation and easily measured topographic, seasonal, and soil physical factors; among these, sand content and moisture retention at 1-bar suction were the best predictors of moisture content. Position on slope (height or distance from the stream channel) appeared to be an important factor only in the lower 25% of the slope. The annual cycle of soil moisture in the top 213 cm approximated a sine wave with a maximum during mid-April and a minimum during mid-October. As expected, moisture changes in surface layers were correlated best with rainfall weighted toward days immediately preceding observation; changes in deeper layers were better correlated with rainfall during previous weeks. Seasonal changes in soil moisture content at all depths were greatest upslope and least in the lower slope. Equations developed account for about 88% of the variation in soil moisture (standard error approximately 2–3% by volume) and can be used to predict antecedent moisture distribution for hydrologic and other purposes. Results support the "variable source area" concept of streamflow, but even with the neutron method slope gradients in field moisture were not easy to detect.

¹ Contribution from the Coweeta Hydrologic Laboratory, Southeastern Forest Exp. Sta., FS, USDA, Franklin, N.C., and the School of Forest Resources, Univ. of Georgia, Athens, Ga. Presented before Div. S-1 and S-6, Soil Science Society of America, New York City, Aug. 18, 1971.

² Principal Hydrologist, Forest Hydrology Lab., Pacific NW Forest Exp. Sta., FS, USDA, Wenatchee, Wash. 98801, Professor, School of Forest Resources, Univ. of Georgia, Athens, Ga. 30602, and Principal Hydrologist, Coweeta Hydrologic Lab., SE Forest Exp. Sta., FS, USDA, Franklin, N.C. 28734, respectively.

Received for publication April 24, 1972. Accepted for publication August 15, 1972.