Soil moisture gradients and controls on a southern Appalachian hillslope from drought through recharge

Soil moisture gradients along hillslopes in humid watersheds, although indi cated by vegetation gradients and by studies using models, have been diffic ult to confirm empirically. While soil properties and topographic features are the two general physiographic factors controlling soil moisture on hill slopes, studies have shown conflicting results regarding which factor is mo re important. The relative importance of topographic and soil property cont rols was examined in an upland forested watershed at the Coweeta Hydrologic Laboratory in the southern Appalachian mountains. Soil moisture was measur ed along a hillslope transect with a mesic-to-xeric forest vegetation gradi ent over a period spanning precipitation extremes. The hillslope transect w as instrumented with a time domain reflectometry (TDR) network at two depth s. Soil moisture was measured during a severe autumn drought and subsequent winter precipitation recharge. In the upper soil depth (0-30 cm), moisture gradients persisted throughout the measurement period, and topography exer ted dominant control. For the entire root zone(0-90 cm), soil moisture grad ients were found only during drought. Control on soil moisture was due to b oth topography and storage before drought. During and after recharge, varia tions in soil texture and horizon distribution exerted dominant control on soil moisture content in the root zone (0-90 cm). These results indicate th at topographic factors assert more control over hillslope soil moisture dur ing drier periods as drainage progresses, while variations in soil water st orage properties are more important during wetter periods. Hillslope: soil moisture gradients in southern Appalachian watersheds appear to be restrict ed to upper soil layers, with deeper hillslope soil moisture gradients occu rring only with sufficient drought.