Quantifying changes in age distributions and the hydrologic balance of a high-mountain watershed from climate induced variations in recharge

Changes in the spatial distribution and flow-paths of water in a high-mountain, headwaters watershed are evaluated using an integrated hydrologic model based on a heterogeneous domain in the Rocky Mountains of Colorado in the United States. The model framework simulates both surface and subsurface flow and age. Four different recharge scenarios that span the range of recharge changes across North America due to climate change are compared. The changes in the distribution of water are quantified in terms of stream flow and groundwater level changes. Composite age distributions that represent the total time spent in the terrestrial hydrologic system are used to link these flow perturbations to alterations of residence time. This integrated approach permits delineation of the time spent in the surface water system, the vadose zone, and saturated groundwater. For the range of recharge scenarios considered, the biggest changes to the hydrologic system manifest in the vadose zone, which then propagate into the groundwater system and heavily affect the composite age distribution. Overall, this approach shows that composite age distributions and domain specific age distributions provide an excellent accounting of the changes in the distribution of water mass and the extent of characterization is far more descriptive than only considering stream flow or groundwater levels.