Date of Award

Spring 1-1-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geological Sciences

First Advisor

Shemin Ge

Second Advisor

Robert Anderson

Third Advisor

Roseanna Neupauer

Abstract

In September 2013, the Boulder Creek watershed was subjected to an extreme precipitation event in which 17 inches of rain – nearly the annual average for the watershed – fell in less than eight days. The rainfall event caused extensive flooding within the Boulder Creek watershed. The flood water receded within a few days. A large volume of the precipitation may have infiltrated to the subsurface and contributed to groundwater recharge. Yet, current understanding on flood water infiltration to groundwater is limited.

This study aims to understand how the subsurface responds to extreme precipitation events by quantifying: (1) the volume of precipitation that infiltrates into the vadose zone, (2) the change in subsurface water storage, and (3) the spatial and temporal extent over which the effects of the event can be recognized. This study focuses on a small drainage basin within the Boulder Creek watershed. The study site was actively monitored throughout the storm event, and therefore provides data valuable for hydrogeologic modeling. Data collected at the site include depth to water table, soil moisture, stream discharge, hydraulic conductivity, and precipitation records. The hydrogeologic structure of the drainage basin consists of a thin, sandy soil layer that caps a thick layer of saprolite and weathered bedrock over crystalline basement.

To address these research questions, this study uses the HYDRUS 2D model to simulate variably saturated groundwater flow to simulate. Average annual precipitation and water table elevation records are used to estimate the initial subsurface conditions prior to the event, and measured precipitation at the site is used to force a model response. Results indicate that: (1) 100% of the 20 cm of precipitation was able to infiltrate the subsurface, (2) the water table rose by 4 m across most of the drainage basin, and (3) the water table remained elevated above the pre-storm level for multiple weeks.

Available for download on Friday, May 21, 2021

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