Comparison of hydrologic cycle components for pre-storm and storm conditions, Walker Branch Watershed, Oak Ridge Reservation

The hydrology of the West Fork of Walker Branch Watershed, a 38.4 ha forested experimental watershed located on the Oak Ridge Reservation in the Valley and Ridge Province of eastern Tennessee, has been studied using oxygen and hydrogen isotopes, and silica as tracers of stream water source. To assist in characterization of water recharging the watershed, the local meteoric water line was determined to be equivalent to the global meteoric water line. The global meteoric water line describes local precipitation and potential groundwater recharge on the Oak Ridge Reservation and surrounding region.

Within the watershed, several possible storm flow components have previously been identified including groundwater within the bedrock and at the soil/bedrock interface, saturated soil water occurring at shallow depths (storm flow zone), and precipitation. A December 15 - 19, 1992 storm event was examined to determine sources of stream flow. Two component ("new" water and "old" water) and three component (precipitation, soil water, and groundwater) end-member mixing models using oxygen isotopes and silica as tracers have been applied. The two end-member separations yielded a "new" water (event precipitation) contribution of 1 - 16% of total stream storm flow at the base of the watershed, and 3 - 19% of total stream flow at one ephemeral stream site. The three component model applied to stream flow at the base of the watershed indicates that "new" water contributed 15%, soil water 48 - 52%, and groundwater 45-52% of the total stream storm flow. "New" water was determined to contribute 210%, soil water 40 96%, and groundwater 15 - 50% of stream storm flow at the ephemeral stream site. Old soil water and groundwater dominated storm flow at both stream sites. The significant contribution of soil water to storm flow at both stream sites and flow through a subsurface weir support previous suggestions of the importance of a shallow storm flow zone developing within the upper 2 m of the soil column during a storm event. The large role of groundwater in storm stream flow indicates a rapid deeper subsurface flowpath through the soil column.