The estimation of hydrologic transit times in a catchment provides insights into the integrated effects of water storage, mixing dynamics, and runoff generation processes. There has been limited effort to estimate transit times in southern boreal Precambrian Shield landscapes, which are characteristically heterogeneous with surface cover including till, thin soils, bedrock outcrops, and depressional wetland features that play contrasting hydrologic roles. This study presents approximately 3.5 years of precipitation and streamflow water isotope data and estimates mean transit times (MTTs) and the young water fraction (p y ) across six small catchments in the Muskoka-Haliburton region of south-central Ontario. The main objectives were to define a typical range of MTTs for headwater catchments in this region and to identify landscape variables that best explain differences in MTTs/p y using airborne light detection and ranging and digital terrain analysis. Of the transit time distributions, the two parallel linear reservoir and gamma distributions best describe the hydrology of these catchments, particularly because of their ability to capture more extreme changes related to events such as snowmelt. The estimated MTTs, regardless of the modelling approach or distribution used, are positively associated with the percent wetland area and negatively with mean slope in the catchments. In this landscape, low-gradient features such as wetlands increase catchment scale water storage when antecedent conditions are dryer and decrease transit times when there is a moisture surplus, which plausibly explains the increases in MTTs and mean annual runoff from catchments with significant coverage of these landscape features.

In the densely inhabitated areas surrounding Los Angeles, due to the scarcity of surface storage‐sites, it is essential that all available ground‐water storage‐basins be utilized. In the foothill areas several ideal ground‐water storage‐basins exist, such as the San Gabriel and San Fernando valleys. Each of these basins consists of a coarse granitic material entirely surrounded by mountain ranges with a single confined exit through which all water from the drainage‐area must pass either on the surface or underground. While clay deposits exist in these alluvial fills, they are not continuous and are of small consequence, allowing a direct percolation from the ground‐surface to the water‐table.

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