Precipitation Impacts on Groundwater Levels in the Ephemeral Holgate Lake: A MODFLOW Inquiry

Abstract

ii ACKNOWLEDGMENTS Dr. Johnson, for generously sharing her brain space and lab space. And for pulling me out of rabbit holes.Linley Mescher, for all the MODFLOW collaboration.PP and JMV, for the late night dinners and endless support.And to RC, for keeping the lights on.iii ABSTRACT Holgate Lake, an ephemeral lake located in southeast Portland, has a history of flooding and inundating residential areas. The appearance of the lake is hypothesized to be a function of precipitation-driven changes in groundwater levels. A model was developed using MODFLOW-NWT, a U.S. Geological Survey (USGS) modular hydrologic program, to analyze parameters contributing to the appearance of the lake. The model was most sensitive to changes in aquifer properties including hydraulic conductivity and storativity. The model displayed low sensitivity to changes in precipitation, evapotranspiration, and conductance of surface waters. Results from the analysis contribute to a better understanding of the surface-groundwater system, and could be used to assess flooding risks in the Holgate Lake area with additional model calibration.iv