Dynamic Response of Watershed Subsurface System to Extreme Rainfall Events

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The episodes of torrential rainfall in September 2013 in the Colorado Front Range caused widespread flooding in the area. While floodwater receded within days, data from subsurface suggested substantial changes in groundwater storage long after the flood. This study seeks to understand the changes in groundwater storage in response to extreme rainfall events. Understanding the effects of extreme rainfall events can provide the basis for predicting aggregated effects over longer temporal and larger spatial scales. By assessing the potential for shallow soils and deeper aquifers to serve as natural storages for floodwaters, this study could provide a scientific basis for water managers to assess the excess water stored during extreme rainfall events and timely utilize the resource when it is released back to streams. The project will have a substantial educational element. Graduate students will be directly involved in research. A two-week undergraduate field course will incorporate some in-situ aquifer tests proposed for this study. A groundwater flow computer model will be created for an undergraduate modeling class project. These field and modeling plans will benefit approximately 90 undergraduate students and offer them the opportunity to learn practical skill sets and a real research experience in water sciences. The project plan also includes recruiting and working with students from underrepresented groups through the RESESS program, an NSF supported summer internship program dedicated to increasing the diversity of students entering geosciences. The overall goal of this research is to better understand the dynamic response of watershed-scale subsurface hydrologic systems to extreme rainfall events. The specific research questions are: (1) How much could precipitation infiltrate into the vadose zone during extreme rainfalls? (2) How much does the subsurface water storage change in the event of extreme rainfall? (3) To what temporal and spatial extent could extreme rainfalls impact subsurface systems? The research plan consists of three components. The first is to collect and analyze hydrologic data that will provide information for conceptualizing the system and the parameters for model calibration. The second is to conduct in-situ and laboratory measurements to characterize the hydrologic properties of the vadose zone and the saturated zone. The third is to develop an integrated vadose-zone and saturated-zone flow model to synthesize data, test hypotheses, and address research questions. The Upper Boulder Creek west of Boulder, Colorado, will be utilized as a test bed because of the availability of existing hydrologic data spanning over the pre- and post-extreme precipitation periods. Using a physics-based mass-balance approach, this study offers a quantitative modeling framework that links precipitation, subsurface flow, and stream baseflow at watershed scales, with an emphasis on rigorous modeling of infiltration processes in the vadose zone. This study will shed new light on infiltration into the vadose zone, dynamic changes in subsurface water storage, and the temporal and spatial extent that a watershed subsurface system could be affected under extreme rainfall events. The study site includes varying geology and geography, making the results transferrable to other regions. This study will advance modeling integrated vadose zone and saturated zone flow through utilizing latest modeling capability and taking advantage of data availability. On the basis of its contribution to rigorous modeling infiltration and site transferability, this study is potentially transformative.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.