Overview
Aquifer depletion tracking uses USGS National Water Information System (NWIS) groundwater-level monitoring network data, supplemented by NASA GRACE/GRACE-FO satellite gravimetry.
Data Sources
USGS NWIS Groundwater Levels
**Network:** ~20,000 monitoring wells with continuous or periodic water-level measurements**Record length:** Many sites have 30-80+ year records**Access:** https://waterdata.usgs.gov/nwis/gw**Key reference:** Konikow (2013) — USGS SIR 2013-5079, documenting 1,000 km³ total U.S. depletion 1900-2008NASA GRACE/GRACE-FO
**Method:** Satellite measurement of Earth's gravitational field changes, which correlate to groundwater mass changes**Resolution:** ~300 km spatial, monthly temporal**Coverage:** 2002-present (GRACE), 2018-present (GRACE-FO)**Key finding:** Confirmed Central Valley subsidence of 1-2 feet and Ogallala mass loss trends consistent with USGS well dataDepletion Rate Calculation
Method
1. Select representative monitoring wells in each aquifer region (minimum 50 per region)
2. Calculate decadal average water levels for each well
3. Convert water-level change to storage change using specific yield (Sy) for unconfined aquifers or storativity (S) for confined
4. Aggregate by region using Thiessen polygon or kriging interpolation
5. Cross-validate against GRACE satellite data where overlap exists (2002-present)
Key Parameters by Aquifer
| Aquifer | Wells Used | Specific Yield | Depletion Rate (km³/yr) | Critical Threshold |
|---|
| Ogallala | 8,000+ | 0.15-0.18 | ~3.5 | Saturated thickness <10m |
| Central Valley | 2,000+ | 0.10-0.15 | ~5-8 (pre-SGMA) | Subsidence >2m, wells going dry |
| Edwards | 500+ | 0.03-0.05 (karst) | ~0.5-1.0 | Springflow cessation |
| Denver Basin | 400+ | 0.08-0.12 | ~0.3-0.5 | Confined pressure loss |
National Aggregate
Total U.S. groundwater depletion: ~25 km³/yr (accelerating from ~9.2 km³/yr pre-2000)This 3x acceleration is the most robust and defensible statistic in the entire analysisSource: USGS SIR 2013-5079, updated with GRACE-FO data through 2024Critical Timelines
**Central Valley:** 40% depleted, critical threshold projected 2047-2069 depending on management scenario**Ogallala (southern High Plains):** Significant areas already below economic pumping depth; regional exhaustion in southern portions by 2050-2070**Edwards:** More resilient due to karst recharge, but drought-sensitive; springflow cessation events increasingCaveats
Specific yield/storativity estimates introduce ±20-30% uncertainty in volume calculationsSGMA implementation in California may slow Central Valley depletion (too early to measure impact)Localized conditions vary significantly; regional averages mask hotspots and resilient areasGRACE spatial resolution cannot distinguish individual aquifer units in complex geologyClassification
Measured — Based on direct well measurements and satellite gravimetry, with engineering conversions.