Ecohydrologic Dynamics of Rock Moisture in a Montane Catchment of the Colorado Front Range

Burns, Ethan F

Graduate Thesis Or Dissertation

Ecohydrologic Dynamics of Rock Moisture in a Montane Catchment of the Colorado Front Range Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/rn301301f

Abstract

Warming across the western United States 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 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. In this study, we quantify the spatiotemporal dynamics of rock moisture in a montane catchment of the Colorado Front Range. Direct measurements of rock moisture were accompanied by intensive monitoring of precipitation, transpiration, soil moisture, tree stress, and groundwater levels to elucidate the role of deep vadose zone moisture in sustaining transpiration and mitigating drought stress. Repeat NMR and neutron probe measurements in six boreholes spanning a catena of hillslope positions showed dynamic rock moisture is widespread. The magnitude of dynamic rock moisture change mirrors aboveground vegetation density, while the depth of dynamic water is confined to the uppermost weathered portions of the deep vadose zone (between the soilsaprolite interface and 5 m). Lower bound estimates of dynamic storage show weathered rock on southern aspects account for at least 9-12 % of the mean annual precipitation. Persistent iii transpiration and discrepancies between estimated soil matric potentials and predawn leaf water potentials suggest rock moisture may mitigate drought stress. These findings provide some of the first direct measurements of rock moisture storage and use in the Rocky Mountains and support previous work that indicates rock moisture use is not just confined to periods of drought or to Mediterranean and semi-arid climates.

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