ESTIMATING BEDROCK INFILTRATION IN THE DRY CREEK EXPERIMENTAL WATERSHED, BOISE, IDAHO USING THE CHLORIDE MASS BALANCE METHOD

Recharge estimation in mountainous environments is complicated by shallow, variable-depth soils, high variability in slope, aspect and vegetation, complex ground-water flow paths and elevation-dependent precipitation input. Under arid or semi-arid conditions, recharge estimation is further complicated by high rates of evapotranspiration relative to precipitation. This disparity, along with physical variability, renders traditional methods utilizing modeling of evapotranspiration subject to wide margins of error. The chloride mass-balance approach, which traces the mass of chloride delivered from precipitation to groundwater, offers an improvement. The premise for recharge estimation is that change in concentration of chloride in the system is due to evapotranspiration. Chloride in the system is measured in precipitation, stream water and ground water as parameters to calculate the portion of annual precipitation contributing to groundwater recharge. This approach was initially developed in systems where groundwater underlies deep, unconsolidated soils. In mountainous environments it can be difficult to obtain representative groundwater samples. Towards addressing this problem, we have applied the chloride mass-balance method to estimation of bedrock infiltration for Dry Creek Experimental Watershed using varied methods to estimate the concentration of chloride in groundwater.

This watershed encompasses 27.2 km2 on the Boise Front and is characterized by headwater streams, steep slopes and thin, sandy soils. Underlying bedrock is granite and granodiorite of the Idaho Batholith. Precipitation input is measured at several established sites across the watershed with chloride concentration being sampled at the same sites. Groundwater samples are taken from springs and wells within the watershed. Stream discharge is determined from continuous discharge measurements taken at the watershed outlet. Preliminary results suggest that bedrock infiltration comprises 10 to 40% of precipitation received.