AI Data Center Water Use in California: Estimates from Physics and AI Models

A recent post on California WaterBlog digs into estimates of AI data center water use in California, using both first-principles physics and queries to four AI models. The author, Jay Lund, aims to cut through media speculation by producing transparent, physics-based estimates.

Method: From Energy to Evaporation

The calculation starts with known data center characteristics:

• California has ~15 million sq ft (1.4 million m²) of data center floor space.
• Racks dissipate 2–12 kW per square meter of floor space.
• At 100% efficiency, that heat would evaporate 70–420 mm/day per m².
• Real cooling systems (60–90% efficiency) expand the range to 80–700 mm/day per m², which translates to 29–255 meters of evaporation per year per m² — 25–150× more than irrigated agriculture.
• If all data centers used evaporative cooling continuously, total evaporation would be 40–357 million m³/year (32,000–290,000 acre-ft/year).

AI Model Estimates

Lund also prompted four AI models with: “How much water is likely to evaporate from data centers in California per year, assuming they are all using mostly evaporative cooling?”

• ChatGPT: 20–400 taf/year
• Claude: 14.4–21.5 taf/year (assumed less than 100% evaporative cooling)
• Gemini: 2.3–40.5 taf/year
• Co-Pilot: 30–50 taf/year, with a broader range of 10–100 taf/year

The overall range spans 2,300 acre-ft/year to 400,000 acre-ft/year, with the physics-based estimate of 32,000–290,000 acre-ft/year in the middle.

Key Takeaway

AI water use in California is modest relative to other sectors. The article argues that data center water use is “mostly modest” but will be larger in states with more data center activity and less developed water infrastructure. The lack of transparency from AI companies fuels speculation, but physics-based estimates provide a useful baseline.