# Capture Model Method

## Overview
The stormwater capture model estimates annual recoverable volume from flood-surplus diversions across five major aquifer regions.

## Methodology

### 1. Streamflow Exceedance Analysis
- Source: USGS National Water Information System (NWIS) daily streamflow records
- For each region: identify gauging stations in tributary watersheds overlying target aquifers
- Calculate flow duration curves (exceedance percentiles) from 30+ year records
- Define "surplus threshold" as flow exceeding the 25th percentile (i.e., flows exceeded 75% of the time are protected for downstream/ecological use)

### 2. Diversion Constraints
- **Environmental flow reserve:** No diversion below 7Q10 (7-day, 10-year low flow) + 20% safety margin
- **Legal water rights:** Diversion only from unappropriated flows or junior-right surplus
- **Seasonal windows:** Capture concentrated in wet season (varies by region: Nov-Apr in West, Mar-Jun in Plains)
- **Fail-closed design:** If any constraint is violated, diversion stops automatically

### 3. Treatment Loss Factors
- Pre-treatment screening losses: 2-5%
- Treatment reject/waste stream: 5-15% (varies by source water quality class)
- Injection well efficiency: 85-95% (accounts for clogging, backflush, and maintenance downtime)
- Net recovery efficiency: 70-85% of injected volume (aquifer-dependent)

### 4. Regional Capture Estimates

| Region | Annual Surplus (MAF) | Practical Capture (MAF) | Notes |
|--------|---------------------|------------------------|-------|
| Ogallala | 8-12 | 2-4 | Limited by distance from major rivers; relies on Platte, Arkansas, Canadian tributaries |
| Central Valley | 5-15 | 2-5 | Highly variable; flood years can be 5x dry years; Sacramento/San Joaquin system |
| Basin & Range | 3-8 | 1-3 | Ephemeral washes, monsoon-driven; Phoenix, Tucson, Las Vegas corridors |
| Edwards | 2-4 | 1-2 | Guadalupe, San Antonio, Nueces rivers; well-characterized karst recharge zones |
| Denver Basin | 1-3 | 0.5-1.5 | South Platte, Arkansas headwaters; limited by seasonal window |

### 5. Uncertainty and Caveats
- Climate non-stationarity: historical streamflow records may not represent future conditions
- Practical capture rates depend on infrastructure buildout pace and permitting timelines
- Aquifer injection capacity varies significantly by geology (alluvial >> consolidated rock)
- The "190 trillion gallons" figure represents total stormwater runoff in aquifer regions; practical capture is 10-30 trillion gallons (5-15%) given physical and regulatory constraints

## Data Sources
- USGS NWIS: https://waterdata.usgs.gov/nwis
- USGS Groundwater Decline and Depletion: https://www.usgs.gov/mission-areas/water-resources/science/groundwater-decline-and-depletion
- State water allocation databases (varies by state)

## Classification
**Modeled** — Based on measured streamflow data with engineering assumptions for capture efficiency.