An independent view of water storage

Water managers already track many important parts of the hydrologic system, including precipitation, snowpack, streamflow, reservoir levels, groundwater observations, drought indices, and water demand. GPS-based hydrologic assessment adds a different kind of information, an independent measurement of how water storage is changing across the landscape.

Existing high-precision GPS stations move slightly up and down as water mass is added to or removed from a region. When snow, soil moisture, reservoirs, groundwater, and surface water increase, the added weight causes small but measurable motion of the Earth’s surface. When water storage declines during drought or dry seasonal conditions, the land responds in the opposite direction. These movements are typically only millimeters in size, but they can be measured with enough precision to provide useful hydrologic information.

This makes GPS valuable because it responds to stored water, not only to recent weather. A wet storm, wet month, or improved precipitation index does not always mean that a basin has recovered from drought. Groundwater, reservoir storage, snowpack, soil moisture, and deeper basin storage may remain depleted even after meteorological conditions improve. GPS helps provide an independent view of whether the landscape is actually gaining, losing, or retaining water relative to historical conditions.

GPS-based assessments are especially useful when interpreted alongside existing water-management datasets. Precipitation and snowpack help describe water inputs. Streamflow, reservoirs, groundwater wells, and demand records help describe specific parts of the managed system. GPS adds a storage-sensitive regional layer that can help connect these observations and identify whether hydrologic conditions are consistent with drought development, drought recovery, wet-year recharge, reservoir and snowpack loading, or unusual seasonal water loss.

Unlike many satellite-based water-storage products, GPS stations can provide frequent observations at local to regional scales using infrastructure that already exists across much of the United States. The result is not a replacement for existing tools, but an additional line of evidence for assessing regional water-storage conditions and supporting water-management decisions.