Long-term monotonic trends in aquifer and regional groundwater metrics in the United States through 2020

The U.S. Geological Survey (USGS) Water Resources Mission Area (WMA) is working to address a need to understand where the Nation is experiencing water shortages or surpluses relative to the demand by delivering routine assessments of water supply and demand. A key part of these national assessments is identifying long-term trends in water availability, including groundwater and surface-water quantity, quality, and use. This data release contains Mann-Kendall monotonic trend analyses for regional groundwater levels at 53,848 wells located in the conterminous United States, Alaska, Hawaii, and Puerto Rico. The groundwater annual mean water level was calculated for five regional classifications: principal aquifers and secondary hydrology units (Integrated Modeling and Prediction Division, 2000; Qi, 2009; U.S. Geological Survey, 2003), Hydrologic Landscape Regions (HLR) (Wolock, 2003,), Surficial geologic regions (Zell, W.O. and Sanford, W.E., 2020, Soller and others 2004), Hydrologic Unit level 4 (HUC4),  (Wieczorek, M.E., 2012), modified Hydrologic Unit level 4 (HUC4) (Van Meter regions) (Qi and others, 2023).

Annual groundwater levels are computed from groundwater water levels from publicly available data from the National Water Information System (NWIS), the National Groundwater Monitoring Network (NGWMN) and California Open Data Portal. Trend analyses are computed using annual groundwater levels measured throughout the water year, which is defined as the 12-month period October 1 through September 30 and the year corresponds to the calendar year in which the water year ends (for example, October 2019 through September 2020 is referred to as water year 2020). Trends at each regional classification are available for up to four different periods: (i) the longest possible period that meets completeness criteria at each well, (ii) 1981-2020, (iii) 1990-2020, (iv) 2001-2020. Annual mean water-levels for regional classifications for wells screened in unconfined aquifers were determined only when a well's water-level time series was at least 70 percent complete. Additionally, each of these time series must have at least 70 percent complete records in the first and last decade. All longest possible period time series for wells in unconfined aquifer must be at least 10 years long and have annual groundwater levels calculated for at least 70% of the years of the record.

Caution must be exercised when utilizing monotonic trend analyses conducted over periods of up to several decades (and in some places longer ones) due to the potential for confounding trends, such as water-level declines with multi-decadal climatic fluctuations.

The data release contains the following nine large tables:

1. gwl_sites_region.csv ** contains site locations and regional classification for 54,932 wells

2. siteyear_unconfined_annual_mean_all.csv ** contains annual groundwater level data for all available years for 54,932 wells

3. gw_HLR_siteuse.csv ** contains TRUE/FALSE data for if a well was used in the HLR trend calculations

4. gw_HUC4_siteuse.csv ** contains TRUE/FALSE data for if a well was used in the HUC4 trend calculations

5. gw_MH4_siteuse.csv ** contains TRUE/FALSE data for if a well was used in the MH4 trend calculations

6. gw_SurfGeo_siteuse.csv ** contains TRUE/FALSE data for if a well was used in the SurfGeo trend calculations
7. gw_PA_SHR_siteuse.csv contains TRUE/FALSE data for if a well was used in the PA_SHR trend calculations 

8. gwl_region_annualmean_trendout.csv **  contains Mann-Kendall test results for 54,932 wells, including Sen slope estimates of trends in each of the 5 groundwater metrics.

9. gwl_region_annualmean_confband.csv** contains estimated annual values of the median of the distribution of a given annual streamflow metric for a given year along with the 90% confidence bands (5th and 95th percentile) associated with this estimate.