Investigating catchment predictors of the fraction of young water variability in streamflow in mesoscale Precambrian Shield catchments in Northeastern Ontario, Canada

Understanding how catchments respond to precipitation under rapidly changing climate and land-uses, is key to long-term management of water resources, including in the understudied northeastern Ontario Canadian shield landscape. However, there is limited knowledge of streamflow generation processes in Precambrian Shield catchments beyond headwater catchments. We estimated the streamflow fraction of young water (Fyw) and damping ratios (DR) in the Sturgeon River-Lake Nipissing-French River (SNF) Basin in northeastern Ontario, Canada using five years of modelled precipitation isotope (δ18O) and 3 to 5 years of streamflow isotope (δ18O) records from sixteen catchments that ranged in size from 27 km² to 6,919 km². The Fyw varied between 16 % and 66 %, with no significant difference between unweighted (Fyw) and flow-weighted (Fyw*) estimates (r = 0.99, p < 0.001). Using Pearson correlation and hierarchical partitioning regression (HP), 70.1 % of total variance in Fyw was predicted by five landscape indices. Median topographic wetness index (TWI), fen + marsh area, and mixedwood forest areas were positively correlated with Fyw, while Fyw decreased with increasing area of lake and high runoff potential soil. The median TWI (slope and catchment area dependent) and percent area of fen + marsh were identified as statistically significant Fyw predictors across the basin, while lake area, mixedwood forest area, and high runoff potential soil were less important variables because they co-varied with the other statistically significantly independent variables (TWI and fen+marsh area). This study highlights how wetlands can play different hydrological roles (water storage versus conveyance) at new hydrological information at a scale (mesoscale) and landscape (Precambrian shield) that are underrepresented and yet critical for regional water management.