Evaluation Of High-Frequency Mean Streamwater Transit-Time Estimates Using Groundwater Age And Dissolved Silica Concentrations In A Small, Forested Watershed

Many previous investigations of mean streamwater transit times (MTT) have been limited by an inability to quantify their dynamic nature. Here, we draw on (1) a linear relationship (r² = 0.97) between groundwater ³H/³He ages and dissolved silica (Si) concentrations, combined with (2) predicted streamwater Si concentrations from a multiple regression relation (R² = 0.87) to estimate MTT at 5-min intervals for a 23 yr time series of streamflow (water year (WY) 1986 thru 2008) at the Panola Mountain Research Watershed, Georgia. The time-based average MTT derived from the 5-min data was ~8.4×2.9 years and the volume-weighted (VW) MTT was ~4.7 years for the study period, reflecting the importance of younger runoff water during high flow. The 5-min MTTs are normally distributed and ranged from 0 to 15 years. Monthly VW MTTs averaged 7.0 × 3.3 years and ranged from 4-6 years during the typically high streamflow period in winter to 8-10 years during the low streamflow period in summer. The annual VW MTTs averaged 5.6×2.0 years, and ranged from ~5 years during wet years (2003, 2005) to >10 years during dry years (2002, 2008). Stormflows are composed of much younger water than baseflow, and although stormflow only occurs ~17% of the time, this runoff fraction contributed 39% of the runoff over the 23 year study period. Combining the 23-yr VW MTT (including stormflow) with the annual average baseflow for the period (~212 mm) indicates that active groundwater storage is ~1000 mm However, the groundwater storage ranged from 1040 to 1950 mm using WY baseflow and WY VW MTT. The approach described herein may be applicable in other watersheds underlain by granitoid bedrock, where weathering is the dominant control on Si concentrations in soils, groundwater, and streamwater.