Dissertation
On the detection and attribution of changes in baseflow across the U.S. Midwest
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2021
DOI: 10.17077/etd.006063
Abstract
Baseflow is the portion of streamflow that comes from groundwater and other delayed sources. Although baseflow is essential for sustaining streams during low flow and drought periods, we have little information about if it has changed across the U.S. Midwest. The region represents one of the most intensely cultivated areas in the world, and it also experiences extreme climatic events such as flooding and droughts. Climate change and anthropogenic influences present challenges for water resources management throughout the 21st century. Examining long-term changes in baseflow is critical for effective management of water supply and water quality. The objective of this thesis is to determine changes in monthly baseflow and to identify the role of climate and land use in driving baseflow response. I first examine the relationship of baseflow and water quality by evaluating the sources and timing of nitrate loads for eight agricultural watersheds in Iowa. I focus on long-term water quality data where grab samples have been collected from 1987 to 2019. I model the relationship between nitrate load and streamflow’s two components (i.e., baseflow and stormflow) to quantity loads, and develop a method for estimating loads on days when monitoring data are not available. Four regression models relate daily nitrate load to daily total streamflow, baseflow, and stormflow. The models that perform best include baseflow and stormflow separately in their model formulations. Overall, baseflow is a better predictor for smaller nitrate loads during low flow months, and stormflow captures peak loads in the spring.
Chapters 4 and 5 examine the role played by climate and land use change in observed monthly baseflow changes. I focus on 458 U.S. Geological Survey (USGS) stream gages that have at least 50 years of daily discharge data across the U.S. Midwest. The results of the Mann-Kendall trend test reveal that baseflow has increased in the last 50 years, especially in warmer months (May to September). Although increasing baseflow trends are widespread throughout the region, decreasing trends are prominent but limited to Kansas, Nebraska, and in the Great Lakes region. To describe changes in monthly baseflow, I develop a statistical modeling framework to select a set of predictors that represent the role of climate (precipitation, temperature and antecedent wetness) and land use (harvested acres of corn and soybean). The models capture well the variability in monthly baseflow across the region, with an average correlation coefficient between the observational records and the median of the fitted distribution of 0.70 among all months. My results indicate that precipitation and antecedent wetness are the most important predictors, where the latter was selected the most often. Temperature is an important predictor during the spring when snow-related processes are the most relevant. Agriculture was frequently selected in the Cornbelt region during the growing season (from March to August).
Chapter 6 examines the impacts of climate and agriculture on baseflow through the end of the 21st century. I use the models developed in Chapter 5 to evaluate three scenarios. The first scenario is based on downscaled and bias corrected Global Climate Model (GCM) outputs and the Representative Concentration Pathway (RCP) 8.5, and agriculture is held constant. In the next two scenarios, climate is held constant to isolate the impact of agriculture on baseflow. In terms of agricultural changes, I consider scenarios representative of either increases or decreases with respect to the production of corn and soybean. Changes in the climate system point to increases in baseflow that are likely a result of increased precipitation and antecedent wetness. Seasonally, warmer temperature in the winter and spring (February to July) is expected to cause increasing trends in baseflow. Changes in land use show that agriculture would either mitigate the impact of climate change or possibly amplify it. Expanding corn and soybean areas would increase baseflow in the Corn Belt region. On the other hand, converting land back to perennial vegetation would decrease baseflow throughout the entire year. Despite its simplicity, this chapter provides basic information to understand where to expect adverse effects on baseflow and improve land management practices in those areas.
Finally, Chapter 7 builds off the statistical framework developed in Chapters 4 and 5 to evaluate the role played by changes in the climate system in the observed monthly baseflow records. I analyze 3,496 USGS streamflow gages across the continental United States from 1989 to 2019. I develop statistical models to determine the climate drivers (precipitation, temperature and antecedent wetness) responsible for historical changes in monthly baseflow. Overall, I found that baseflow trends vary by region and season. In the U.S Northeast, increases are detected earlier in the year (February and March), and are likely due to warmer temperature and changes in snowmelt. Increasing baseflow in the U.S. Pacific Northwest and Midwest are associated with increases in precipitation and antecedent wetness.
Decreasing trends are located in the U.S. Southeast and Southwest. Baseflow trends in the U.S Southeast are only detected in March which could be a result of decreased precipitation during the spring. On the other hand, decreases in baseflow in the central-southwestern United States occur throughout the year. These trends are associated with a lack of precipitation and increases in temperature. Finally, I examine the relationship between monthly baseflow and basin wetness using changes in monthly Gravity Recovery and Climate Experiment (GRACE) satellite. Differences in model selection reported here can aid water managers in adaptation to future changes in climate and land use.
Details
- Title: Subtitle
- On the detection and attribution of changes in baseflow across the U.S. Midwest
- Creators
- Jessica R Ayers
- Contributors
- Gabriele Villarini (Advisor)Allen Bradley (Committee Member)Darren Ficklin (Committee Member)Christopher Jones (Committee Member)Witold Krajewski (Committee Member)Keith Schilling (Committee Member)Larry Weber (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Civil and Environmental Engineering
- Date degree season
- Spring 2021
- DOI
- 10.17077/etd.006063
- Publisher
- University of Iowa
- Number of pages
- xvii, 185 pages
- Copyright
- Copyright 2021 Jessica R Ayers
- Language
- English
- Description illustrations
- color illustrations, color maps
- Description bibliographic
- Includes bibliographical references (pages 144-166).
- Public Abstract (ETD)
Baseflow is the part of streamflow that comes from groundwater, and it is important because it sustains streams between precipitation events and during dry seasons. Baseflow also has consequences for water quality. In this thesis, I first examine the relationship between baseflow and nitrate in Iowa. For eight agricultural watersheds, I relate baseflow and nitrate loads to quantify loads on days when water quality data is unavailable.
Chapters 4 to 6 examine the impact of climate and land use on monthly baseflow changes in the U.S. Midwest using a statistical modeling framework. Precipitation and antecedent wetness are the strongest factors, pointing to the role of water availability and infiltration in driving baseflow. Temperature is important in the winter and spring where snow-melt processes are most relevant. Agricultural intensity is selected in the Corn Belt region from March to August. Furthermore, I evaluate three climatic and agricultural scenarios to understand potential changes in baseflow through the 21st century. Climate change suggests increases in baseflow that are likely from increases in precipitation and antecedent wetness. Increases (decreases) in agriculture magnify (mitigate) the impact of climate.
Finally, Chapter 7 expands on the methodology developed in Chapters 4 and 5 to understand changes in baseflow across the continental United States. The results of this thesis show the differences in regional climate and land use that can aid water managers in decision making for water availability, food security, and economic growth.
- Academic Unit
- Civil and Environmental Engineering
- Record Identifier
- 9984097170302771
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