Streamwater ages in nested, seasonally cold Canadian watersheds

Abstract

The mean transit time (MTT) is an important descriptor of water storage and release dynamics in watersheds. Although MTT studies are numerous for many regions around the world, they are rare for prairie watersheds where seasonally cold or dry conditions require adequate methodological choices towards MTT estimation, especially regarding the handling of sparse data records and tracer selection. To examine the impact of such choices, we used timeseries of δ 18 O and δ 2 H from two contrasted years (2014 and 2015) and relied on two metrics and two modelling methods to infer MTTs in prairie watersheds. Our focus was on nested outlets with different drainage areas, geologies, and known run-off generation mechanisms. The damping ratio and young water fraction (i.e., the fraction of streamflow with transit times lesser than 3 months) metrics, as well as the sine-wave modelling and time-based convolution modelling methods, were applied to year-specific data. Results show that young water fractions and modelled MTT values were, respectively, larger and smaller in 2014, which was a wet year, compared with that in 2015. In 2014, most outlets had young water fractions larger than 0.5 and MTT values lesser than 6 months. The damping ratio, young water fraction, and sine-wave modelling methods led to convergent conclusions about watershed water storage and release dynamics for some of the monitored sites. Contrasting results were, however, obtained when the same method was applied using δ 2 H instead of δ 18 O, due to differing evaporation fractionation, or when the time-based convolution modelling method was used. Some methods also failed to provide any robust results during the dry year (i.e., 2015), highlighting the difficulty in inferring MTTs when data are sparse due to intermittent streamflow. This study therefore allowed the formulation of empirical recommendations for MTT estimation in prairie environments as a function of data availability and antecedent wetness conditions.

Water age in stormwater management ponds and stormwater management pond‐treated catchments

“…Although all three water age metrics produced the same youngest and oldest water sites for and δ 18 O data results, DR produced a similar but not exact ranking to MTT and Fyw. This agrees with non‐urban studies that found that all three water age metrics produce the comparable results and water age ranks (e.g., Bansah & Ali, ; Kirchner, ). These results demonstrate that although DR is a useful tool to perform a quick estimation of MTT, Fyw estimations better align with MTT estimations when using either δ 2 H or δ 18 O data in urbanizing catchments.…”

“…The water age computations were applied using the δ 18 O and δ 2 H tracers at each sample location. The DR (Equation ) uses the damping of precipitation isotope signals in stream flow to characterize general trends and distributions of MTT (Bansah & Ali, ; Soulsby et al, ; Tetzlaff et al, ): where CV represents the coefficient of variation.…”

Using isotopes to understand the evolution of water ages in disturbed mixed land‐use catchments

“…We used a relatively long (>5 years) isotopic time series in this study, albeit at a relatively coarse (~weekly) resolution to characterize water ages. It is well known that travel time distributions are nonstationary and vary with hydroclimatological conditions, whereby wetter conditions (e.g., annually, seasonally, or even on an event basis) are typically associated with shorter MTTs and younger water (Harman, ; Peralta‐Tapia et al, ; Segura, James, Lazzati, & Roulet, ) and larger young water fractions (Bansah & Ali, ; Remondi, Kirchner, Burlando, & Fatichi, ). Moreover, despite our efforts to sample higher flows, the relatively coarse sampling resolution could be under‐representing some important short‐term processes (Aubert & Breuer, ; Birkel et al, ; Cayuela, Latron, Geris, & Llorens, ), especially as MTTs are short (Timbe et al, ).…”

Special issue on Canadian Geophysical Union 2018

“…Bansah and Ali () carried out a study to examine streamwater ages and mean transit times (MTT) in nested, seasonally cold Canadian Prairie watersheds using stable isotopes of water sources from two contrasting years. Results showed that most outlets in 2014 had young water fractions larger than 50% and MTT values <6 months.…”

“…However, the location of the trench is an important consideration, with trenching higher in the basin and in flat terrain contributing more to baseflow, suggesting that this should be considered in Andean landscape and water resources management. Bansah and Ali (2019) carried out a study to examine streamwater ages and mean transit times (MTT) in nested, seasonally cold Canadian Prairie watersheds using stable isotopes of water sources from two contrasting years. Results showed that most outlets in 2014 had young water fractions larger than 50% and MTT values <6 months.…”