Indices of dynamic (hydrometric-based) and total storage (isotope-based) were strongly intercorrelated for basins draining the Oak Ridges Moraine in southern Ontario, Canada. Storage indicated by these indices was greatest in basins with steeper, forest-covered headwaters underlain by permeable sand and gravel deposits with thick and relatively uniform unsaturated zones. Our results provide the foundation for initial predictions of relative differences in streamflow response to regional changes in climate and land use/land cover.

The subsurface hydrological behavior of volcanic ash soils (Andosols) resembles that of a “wet layered sponge” in which vertical flow paths are dominant despite the formation of a perched water layer below the root zone.

We used 30 years of chloride measurements in rain, snow and streamflow from 12 catchments in the Turkey Lakes Watershed (Ontario, Canada) to estimate mean travel times. We found that wetland cover was associated with shorter travel times during wet periods and longer travel times during dry periods. Forest harvesting reduced mean travel times. Stream water quality was partly related to travel time but also influenced by biological processes.

Measured groundwater ages at several locations downslope in different depths yield a single overall groundwater age stratification representative for the catchment. The groundwater age stratification reveals a subsurface discharge zone. By applying a simple analytical approximation, two-measure can be extracted: (1) The extent of the subsurface discharge zone in relation to the recharge zone and (2) the overall recharge volume.

We present a novel model for the characterization of the water or solute age, within a hillslope by its average value and its variance. The methodology which merges the simplicity of solutions one-dimensional with the complexity of physically based models, is suitable for experimental data set analysis and theoretical investigations.

• Combinations of different flow paths or contrasting water velocities can create multimodal age distributions in complex hydrological systems.
• A single tracer measured in inflows and outflows may not allow an accurate determination of all peaks of the age distributions.
• Accurate simulations of tracer concentrations require accurate age distributions in which all peaks are determined.

Nitrate removal in catchments is typically controlled by the long residence times in the subsurface environment. Estimates of nitrate removal are here provided from comparison with conservative chloride and are shown to be well anti-correlated with the fraction of young water in the stream.

Isotopic tracers, applied in a small watershed (3.4 km²), indicate that the catchment storage has a preference to release young water for stream flow generation, resulting in contrasting ages for stream water and groundwater of 1.3 ± 0.5 year and 8.2 ± 1.7 year, respectively. Young water fraction, calculated through the seasonal variation of water stable isotopes in precipitation and stream water, yielded a value of 14%, which is an important fraction, considering the dry characteristic of the Mediterranean climate of this area.

High-frequency discharge-dependent stream water isotope sampling of highly responsive runoff in a Mediterranean headwater research catchment showed that any discrete sampling rate will underestimate young water fractions associated to the highest discharges; this loss may be assessed by using detailed flow records (figure). Our findings show that catchment's young water fraction and its discharge sensitivity metrics highly depend on the dynamic precipitation forcing, which is why young water fractions should be characterized using annual means and variances.

1. The tracer-aided hydrological model can be used to assess the spatio-temporal variability of water transit time and young water fractions in karst critical zone.
2. Strong hydrological connectivity between the land surface and underground conduits leads the drastic increasement in young water fraction of runoff after heavy rain.
3. Strong mixing and drainage of small fractures accelerate the old water release during high flows.

Cycle of pre-drought, drought, and recovery of “green” and “blue” water fluxes and soil water ages of grassland and forested sites.

Mean transit times estimated using tritium in Australian headwater catchments at baseflow conditions are years to decades. The long mean transit times result from a deeply-weathered regolith coupled with high evapotranspiration rates. The large catchment storages buffer the streamflow against the impact of short-term climate variability.

StorAge Selection functions are implemented to investigate age-dependent selection in a eucalyptus forested catchment. A strong preference for young water by evapotranspiration is needed to successfully reproduce oxygen-18 in streamflow. Streamflow selection behaviour from storage indicates a system comprised of several long and short subsurface flow paths.

Snowpack disrupts seasonal cycles of isotopic variability, preventing accurate estimation of F_yw in catchments with overwinter snowpack Monte-Carlo simulations produce a range of possible F_yw for a given measured A_r/A_p, and together with an independent estimate, show that ~1/5 of streamflow in the study catchment is less than 1 year old.

• Transpiration represents 65% of actual ET and relates to LAI, slope, and soil properties.
• The stream water ages are dominated by young water.
• Higher soil mean water age (MA) are related to more developed soil sand MA of ground water increases towards the bottom of the catchment.

This paper proposes a framework to quantify the young water fraction of streamflow (F_YW) in snow-dominated catchments based on stable water isotopes and a simple snow accumulation model. Based on three case studies from the Alps, the challenges and pitfalls of F_YW estimation in presence of seasonal snow cover are discussed, with ensuing recommendations for best practices. The results confirm previous findings that seasonal snow cover leads to low F_YW.

In this study, we used two time thresholds ${\mathrm{\tau }}_{\mathrm{yw}}$ and ${\mathrm{\tau }}_{\mathrm{ow}}$ to divide the stream water age into young water, middle-aged water and old water in runoff and took the Nagqu watershed (with an area of 16 900 km²) as an example to calculate stream water age. The results revealed that in the Nagqu watershed, an average of 23% of the runoff was less than 51.6 days old, whereas an average of 55% of the runoff had ages ranging from 51.6 to 365 days.