POTENTIAL SHIFTS IN GROUNDWATER DISCHARGE AS SNOWMELT-DOMINATED WATERSHEDS WARM

Climate warming may alter the quantity and timing of groundwater discharge to streams in high alpine watersheds due to changes in the timing of snowmelt recharge and the duration of seasonal freezing in the subsurface. It is imperative to understand the effects of warming on groundwater discharge to streams in alpine watersheds as streamflow originating from these watersheds is critical to water resource availability for downstream users. This study evaluates how climate warming may alter groundwater discharge due to changes in snowmelt and seasonally frozen ground using one of the first 2D coupled flow and heat transport model with freeze and thaw capabilities for variably saturated media. The model is applied to evaluate a representative snowmelt-dominated watershed in the Rocky Mountains of central Colorado, USA, based on snowmelt time series that were reconstructed from a 12-year dataset of hydrometeorological records and satellite-derived snow-covered area.

Model analyses indicate that groundwater constitutes 53% of streamflow over a yearly cycle and that the duration of seasonal freezing in the subsurface controls groundwater discharge to streams, while snowmelt timing controls groundwater discharge to hillslope faces. Climate warming causes changes to subsurface ice content and duration, rerouting groundwater flow paths. For a 50-year scenario with a warming trend of 4.8°C/100 years and unchanging snowmelt recharge, groundwater discharge to streams increases while groundwater discharge to hillslopes decreases. For a suite of 50-year scenarios with the same warming trend and five unique snowmelt recharge conditions, total annual groundwater discharge increases by an average of 7% in the spring (March-May) and decreases by an average of 9% in the summer (June-August). These changes may have significant impacts to local ecology and downstream hydrology.