Moisture and energy conditions within a sloping soil mass during drainage


  • John D. Hewlett,

  • Alden R. Hibbert


In an effort to explain the source of nonstorm streamflow in deep-soiled areas of the southern Appalachians, a 3×3× 45-foot inclining concrete trough was constructed on a 40 per cent slope. The structure was packed with a natural sandy-loam soil to a bulk density of 1.3, and after thorough soaking was covered with plastic to prevent evaporation. Outflow was measured at the base, and soil moisture tension and content were recorded for 145 days. As shown by piezometers, the larger pores were substantially emptied in 1.5 days, during which outflow occurred according to the expression, Q = a1T−b1 where T is time in days since day zero and a1 and b1 are constants. After a 5-day transition period, the logarithm of drainage for the next 80 days was again linearly related to the logarithm of time, but with new constants,a2 and b2, expressing unsaturated flow from the entire soil mass. An additional 60 days of flow occurred, deviating more and more from the log-log relation as the limits of drainage were approached. Soil moisture content and tension substantiated the theory that the entire unsaturated soil mass was contributing to outflow throughout the experiment. It is concluded that unsaturated flow in the earth mantle of steep watersheds cannot be ignored in hydrograph analysis, since it may well be a primary mechanism for sustaining baseflow.