# Monitoring Wells Method

## 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 1900-2008

### NASA 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 data

## Depletion 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 analysis
- Source: USGS SIR 2013-5079, updated with GRACE-FO data through 2024

## Critical 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 increasing

## Caveats
- Specific yield/storativity estimates introduce ±20-30% uncertainty in volume calculations
- SGMA implementation in California may slow Central Valley depletion (too early to measure impact)
- Localized conditions vary significantly; regional averages mask hotspots and resilient areas
- GRACE spatial resolution cannot distinguish individual aquifer units in complex geology

## Classification
**Measured** — Based on direct well measurements and satellite gravimetry, with engineering conversions.