Soil moisture and evapotranspiration (ET) are important components of boreal forest hydrology that affect ecological processes and land-atmosphere feedbacks. Future trends in soil moisture in particular are uncertain, therefore accurate modeling of these fluxes and understanding of concomitant sources of uncertainty are critical. Here, we conduct a global sensitivity analysis, Monte Carlo parameterization, and analysis of parameter uncertainty and its contributions to future soil moisture and ET uncertainty using a physically-based ecohydrologic model in multiple boreal forest types. Soil and plant hydraulic parameters and LAI have the largest effects on summer soil moisture at two contrasting sites. We report best estimates and uncertainty of these parameters via a multi-site Generalized Likelihood Uncertainty Estimation approach. In future scenario simulations, parameter and global climate model (GCM) choice influence projected changes in soil moisture and evapotranspiration as much as the projected effects of climate change in a late-century, high-emissions scenario, though the relative effect of parameters, GCM, and climate vary between objective and study site. Saturated water content, as well as the sensitivity of stomatal conductance to vapor pressure deficit, have the most statistically significant effects on change in evapotranspiration and soil moisture, though there is considerable variability between sites and GCMs. In concert, the results of this study provide estimates of: (1) parameter importance and statistical significance for soil moisture modeling, (2) parameter values for physically-based soil-vegetation-atmosphere transfer models in multiple boreal forest types, and (3) the contributions of uncertainty in these parameters to soil moisture and evapotranspiration uncertainty in future climates.