Spectral Analysis to Quantify the Response of Groundwater Levels to Precipitation — Northwestern United States

Open-File Report 2020-1007
Prepared in cooperation with National Oceanic and Atmospheric Administration’s National Integrated Drought Information System
By:  and 



Persistent atmospheric patterns that lead to wet and dry seasons and droughts over periods of months to decades and longer-term climate change over periods of decades to millennia affect groundwater resources. Changes in groundwater storage and the resulting groundwater discharge from most aquifers is relatively slow and steady compared to the variability of daily precipitation. The response of groundwater levels to precipitation can be complex because of a combination of processes that include evapotranspiration, surface runoff, and infiltration of net recharge from precipitation through the vadose zone. Typically, this response is delayed and results in a change in groundwater storage reflected in a time series of groundwater levels. Understanding the relations between variations in precipitation and changes in groundwater storage is essential to water resources planning. The objectives of this study were to (1) characterize the relation between precipitation and responses in groundwater levels at seasonal to decadal scales and (2) to develop methods that are transferable on a continental scale to any groundwater-level record. Spectral analysis was applied to daily precipitation and groundwater levels for eleven monitoring wells in the northwestern Unites States with records ranging in length from 5.9 to 23.9 years. The analysis provided a quantitative characterization for each monitoring well that met both objectives and indicated that maximum and minimum precipitation rates generally occurred in December and August, respectively. Maximum groundwater levels occurred from February to August, and minimum values occurred from January to December. The lag in the annual peak response of groundwater to peak precipitation ranged from 2.2 to 8.8 months, with a median value of 5.3 months. Groundwater responses to wet and dry seasons were evident in the relatively high amplitude frequencies of 10 and 20 cycles per decade (cpdec). A high amplitude frequency at 1 cpdec represents a drought cycle that resulted in larger groundwater level changes than typical seasonal water level fluctuations.

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Methods
  • Lag in Groundwater Response
  • Synthesis of Results for all Sites
  • Possible Alternative Methods
  • Summary and Conclusions
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Spectral analysis to quantify the response of groundwater levels to precipitation — Northwestern United States
Series title Open-File Report
Series number 2020-1007
DOI 10.3133/ofr20201007
Year Published 2020
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Washington Water Science Center
Description iv, 18 p.
Country United States
State Oregon, Washington
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details