This study quantifies the separate and combined effects of spatial and temporal variability for waterborne solute transport through catchments. The questions addressed regard whether, when and why the different types of variability may dominate catchment-scale transport. We utilize a versatile numerical solute transport code with a particle-based Monte Carlo time-domain random walk method to simulate waterborne transport through a generic catchment. Simulations are concretized and exemplified using data on spatiotemporal flow-transport variability from direct stream discharge observations and independently calculated advective solute travel time distributions for catchments within the water management district Northern Baltic Proper in Mid-Eastern Sweden. A main conclusion is that the projections of catchment mass loading based on spatial variability alone are robust estimates of long-term average solute transport development. This is especially true when annually aggregated mass load rather than finer temporal resolution of mass flux is considered. Temporal variability yields short-term fluctuations around the long-term average solute breakthrough development, and earlier or later arrival than the latter, depending on the timing and duration of solute input relative to the temporal flow variability. The exact temporal characteristics of future solute breakthrough are thus fundamentally uncertain, but their statistical expectation may be well quantified by accounting only for spatial variability.