Warming across the western US continues to reduce snowpack, shift melt dates, and increase atmospheric demand, leading to uncertainty about moisture availability in upland forest ecosystems. As many of these forests are characterized by thin or absent soils and extensive rooting into weathered bedrock, deep vadose zone water is thought to be a central determinant in controlling late season water availability and may mitigate water stress during a changing climate. A key impediment to understanding the role of the deep vadose zone as a reservoir, lies in the challenge of quantifying the plant available moisture held here and its relationship to snowmelt and rainfall timing. As a result, few direct measurements of rock moisture exist, and questions remain about what controls moisture availability to plants during the growing season. In this study, we quantify the spatiotemporal dynamics of rock moisture and its role in sustaining transpiration and mitigating drought stress in montane catchment of the Colorado Front Range. We find reductions in dynamic rock moisture during the 2021 growing season are concurrent with drying soil moisture, persistent transpiration, and in some cases reduced water stress. Lower bound estimates of dynamic storage show weathered rock on southern aspects account for at least 9-12% of the mean annual water inputs. These findings provide some of the first direct measurements of rock moisture storage and use in the Rocky Mountains and supports previous work that indicates rock moisture use is not just confined to periods of drought or to Mediterranean and semi-arid climates.
Geography Graduate Student, CU Boulder