The Role of Climate in Monthly Baseflow Changes across the Continental United States

Baseflow is the portion of streamflow that comes from groundwater and subsurface sources. Although baseflow is essential for sustaining streams during low flow and drought periods, we have little information about how and why it has changed over large regions of the continental United States. The objective of this study was to evaluate how changes in the climate system have affected observed monthly baseflow records at 3,283 USGS gauges over the last 30 years (1989-2019). We developed a statistical modeling framework to determine the relationship between monthly baseflow and monthly climate predictors (i.e., precipitation, temperature, and antecedent wetness). Overall, we found that baseflow trends and the factors influencing them vary by region and month. In the US Northeast, increases were detected earlier in the year (February and March) and in the summer (May and June), and were likely due to increasing precipitation, warmer temperature, and subsequent changes in snowmelt. Increasing baseflow in the US Pacific Northwest and Midwest were associated with increases in precipitation and antecedent wetness throughout the year. Decreasing trends were located in the US Southeast and Southwest. Baseflow trends in the US Southeast were only detected in March, possibly as a result of decreased precipitation during the spring. On the other hand, decreases in baseflow in the Central Southwestern United States occurred throughout the year. These trends were associated with a lack of precipitation and increases in temperature. Finally, we examined the relationship between monthly baseflow trends and changes in total water storage using monthly Gravity Recovery and Climate Experiment mascon products from the Jet Propulsion Laboratory. In this study, trends in total water storage were strongly associated with baseflow trends across the United States. The spatial and temporal variability in baseflow response to climate reported here can aid water managers in adapting to future climate change. (C) 2022 American Society of Civil Engineers.

Automated summary

This study evaluated the changes in monthly baseflow records over the past 30 years in the continental United States. The study found that baseflow trends and influencing factors vary by region and month. Increases in baseflow were observed in the Northeast earlier in the year and in the summer, likely due to increasing precipitation, warmer temperature, and changes in snowmelt. Increases in baseflow in the Pacific Northwest and Midwest were associated with increased precipitation and antecedent wetness throughout the year. Decreases in baseflow were found in the Southeast and Southwest, with the Southeast seeing decreases only in March possibly due to decreased spring precipitation, and the Central Southwestern region experiencing decreases throughout the year due to lack of precipitation and increased temperature. The study also found a strong association between trends in total water storage and baseflow trends across the United States.