Daily GPS positions
Vertical displacements are collected from continuously operating stations across the United States.
How it works
Measuring water by measuring the ground
The US-GDI connects GPS geodesy with hydrology: changing water mass deforms the solid Earth, and daily GPS observations record that response.
The physical connection
When regional water storage increases
Added water mass depresses the elastic Earth. GPS stations move downward and slightly toward the load.
When regional water storage decreases
As water mass is removed, the elastic surface rebounds. GPS stations move upward and slightly away from the depleted load.
How the US-GDI is produced
Quality control
Offsets, data gaps, and non-hydrologic signals are identified and corrected before hydrologic analysis.
Load inversion
The observed displacement field is converted into spatially continuous hydrologic-load estimates.
Multiscale standardization
Accumulated conditions are evaluated against the historical distribution at windows from 1 day to 4 years.
Choosing a timescale
Rapid response
Highlights short-lived loading and unloading associated with storms, snow, and rapid drying.
Seasonal conditions
Balances event response with the persistence of storage gains and losses across a season.
Persistent storage
Emphasizes longer-term hydrologic drought, multiyear recovery, and unresolved storage deficits.
Reading the products
Hydrologic Load Estimate
- What it shows
- GPS-inferred regional loading expressed in millimeters of equivalent water.
- How to read it
- Positive and negative values describe relative changes in regional hydrologic loading; the map legend provides the active numerical range.
US-GDI Anomaly
- What it shows
- How unusual accumulated storage conditions are relative to the historical distribution for the selected window.
- How to read it
- Blue indicates wetter-than-typical storage and red indicates drier-than-typical storage. D0–D4 and W0–W4 are categories used by the US-GDI, not direct USDM classifications.
Technical reading
Young et al. (2024) demonstrates GPS-based multiscale drought characterization and hydrologic validation in California. White et al. (2022) provides a broader review of GNSS/GPS hydrogeodesy and hydrologic-loading applications.