STABLE ISOTOPE APPLICATIONS IN A STUDY OF POTENTIAL CLIMATE-CHANGE IMPACTS ON HYDROLOGIC SYSTEMS—SAGEHEN BASIN, CALIFORNIA

As part of a multiple-method study (including dissolved gases, ³⁵S, and gravity measurements) of groundwater recharge processes in Sagehen basin, California, stable isotopes of oxygen and hydrogen were measured in soil water, groundwater, stream water, fresh precipitation, and bulk snowmelt to help understand groundwater systematics in the basin. Of special interest were 1) quantifying the importance of cool-season precipitation to groundwater recharge, and 2) estimating groundwater contributions to streamflow.

Cool-season precipitation contribution to groundwater was examined by collecting fresh precipitation, bulk snowmelt (plus any rain that fell during the snow accumulation and melt period), soil water from different depths, and groundwater. Data from the first year of monitoring show that cool-season precipitation penetrates at least 1.5 m into the soil column (based our deepest soil-water samples) with little to no mixing, dominating the soil-water profile in the spring/early summer. In contrast, while unmixed warm-season precipitation dominates the near-surface (up to at least 0.3 m below surface) soil water in the late summer/early fall, it appears to be responsible for at most 6 to 25% of the soil water present at 1.5 m below surface, depending on the measurement site.

Groundwater contribution to streamflow was estimated by collecting stable isotope, CFC-12, and radon samples (along with EC measurements) at 1 km intervals along a reach of Sagehen Creek during a low-flow period. A model of groundwater inflow into the stream was run, constrained first by CFC-12 concentration, then by δ¹⁸O. These two tracers were valuable to apply to this problem in tandem, because their values in groundwater and stream water at the study site are appreciably different, and their values are the result of independent processes and thus the two parameters are not directly correlated. The two tracers suggest nearly identical groundwater inflow to the stream; inflow appears to be highly dependent on location (ranging from nearly zero to ~2 m³/d per meter of channel), but overall, groundwater inflows appear to be extremely important during the low-flow period, contributing about 75% of the total flow at the bottom of the study reach. These estimates are consistent with inflows interpreted from the radon and EC data.