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.

Isotope‐based hydrograph separation (IHS) is a widely used method in studies of runoff generation and streamflow partitioning. Challenges in choosing and characterizing appropriate tracers and end‐members have, however led to presumably highly uncertain IHS results. Here we tested the effects of end‐member definitions and tracer choices on IHS results in nested Prairie watersheds of varying size and landscape characteristics. Specifically, the consideration of eight potential “new” water end‐members, eight potential “old” water end‐members, and two stable water isotope tracers led to 80 potential IHS results for each stream water sample. IHS‐related uncertainty was evaluated using a Gaussian error propagation method. Results show that choosing an appropriate “new” water end‐member is most challenging during the freshet: highly variable “old” water fractions associated with high uncertainties were attributed to changing conditions from melting snow only to rain‐on‐snow. In summer and fall, it was rather the choice of an appropriate “old” water end‐member that was most problematic. IHS results obtained using δ18O versus δ2H as a tracer were significantly different except in the flattest and most wind‐sheltered watersheds examined. Overall, δ2H‐based IHS results were more uncertain than their δ18O‐based counterparts. Recommendations are therefore made toward careful selection of a tracer and a sampling strategy aimed at characterizing the most appropriate end‐members for IHS, especially when dealing with seasonally cold watersheds.