A THEORETICAL EXAMINATION OF GROUNDWATER RESIDENCE TIMES AND INTERBASIN FLOW IN MOUNTAINOUS AREAS

Numerous studies have investigated the residence time of groundwater in a watershed, or catchment transit time, using stream-water environmental tracer time series. These studies have focused primarily on the mean residence time of groundwater in small headwater catchments (<10 km²), and assume simple age distributions in discharging groundwater. Recent studies have drawn attention to the potential importance of waters at the older end of the groundwater age distribution, emphasizing that older waters may compose a non-trivial fraction of streamflow in larger watersheds and may exert a major control on stream-water chemistry. We present a brief overview and theoretical examination of the age distribution in groundwater discharging to streams in mountainous areas in an attempt to address the specific questions of how large and how old the “old” fraction may be. First, potential groundwater age distributions and their old-water components are considered for small headwater catchments using both commonly applied lumped-parameter models and simple numerical models that include different recharge patterns and permeability distributions. Potential age distributions for larger watersheds (up to 1000 km²) are then considered using schematic 3-D numerical models capable of simulating interbasin flow (groundwater flow from lower- to higher-order sub-watersheds). Factors controlling interbasin flow as well as its potential influence on the age distribution in groundwater discharging to lower-elevation streams are investigated.