Subsurface Structure and Flow Regime for Rocky Mountain Hillslopes with Different Geologies

University Of Wyoming, Laramie WY

Investigators

Abstract

Snowmelt from mountainous regions in the western US is a vital source of water for agriculture, industry, cities, and natural ecosystems in the dry inter-mountain basins. The transformation of snowmelt into streamflow in mountainous regions is influenced by the interplay between climate, vegetation, and geology. The role of the subsurface in transporting water from the point of infiltration to the stream is currently poorly understood. This study uses state-of-the-art instruments to image the subsurface of six Rocky Mountain hillslopes with different geologies. In addition, computer simulation models are used to calculate subsurface water flow and storage dynamics and to predict streamflow. An important goal is to find predictable relationships between geology and flow regime. The resulting insights are critical for understanding the impact of current and future climate and land use scenarios on the magnitude and timing of streamflow. Such insights will facilitate better land and water management by agencies, companies, and individuals. Two graduate students and three undergraduate students will gain experience with a broad range of geological and hydrological measurement and computer simulation techniques as part of the project. In addition, a traveling museum exhibit will be constructed in collaboration with the University of Wyoming Geological museum to engage residents and visitors of the Rocky Mountain region in geological and hydrological research. The study combines state-of-the-art geophysical and hydrological measurement and modeling techniques to examine subsurface water flow and storage in hillslopes with three different geologies. The following research questions will be answered: Q1. How different or similar are the weathering zones of the six Rocky Mountain hillslopes selected for this study that represent old granitic surfaces, young volcanic surfaces, and recent glacial deposits over Precambrian rock?, Q2. To what level of detail can subsurface hydraulic properties and flow regimes be determined for the six Rocky Mountain hillslopes?, and Q3. What is the impact of hillslope subsurface model complexity on streamflow and water balance predictions at the watershed scale? Three of the hillslopes are in Wyoming and are currently being measured by the two investigators. The other three hillslopes are proposed in Critical Zone Observatories in Idaho (2) and New Mexico (1). The six hillslopes are paired so that each of the three geologies are represented by two hillslopes. Measurements will consist of shallow seismic refraction to determine subsurface porosity structure, time-lapse electrical resistivity tomography to determine vadose zone water dynamics, and hydrological monitoring to assess water inputs and hillslope hydrologic response. Numerical models will be combined with parameter estimation algorithms to estimate subsurface hydraulic parameters. Integrated watershed modeling will be used to investigate the effect of hillslope model complexity on streamflow and water balance predictions. The study focusses on subsurface hydrological processes and streamflow generation but also contributes to a better understanding of hillslope weathering processes and hillslope ecological functioning. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.