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Methods · Virginia

Water Ledger methods: Virginia

Model version 1.6 · data snapshot 2026-11-02 · two-page brief (PDF) · snapshot manifest

Virginia has a calculator on waterledger.org. The same data and estimates are available through the open waterledger Python packages, for example wl.place("Loudoun County, VA"). Proposals and moratoria are tracked from public sources and checked against local documents where they can be found; each entry says when it rests on a news report.

What it estimates

For any city, town, village, township or county in Virginia, Water Ledger estimates the water a data center campus of a given size and cooling design would withdraw, consume and return on an average day and on a 1-in-10-year peak day, and compares it with the local water system, sewer plant, downstream waters and electric utility. All inputs are public data.

Data

Overall method (all states)

1. Weather. Hourly NOAA station records, 1995–2024, one station per NOAA climate division. Hybrid and assist estimates depend on the hottest hours and so on the weather source: in tests against ERA5 reanalysis at eight sites, station records gave cooling tower estimates within about 1%, and hybrid and assist estimates 4–50% higher (2–5 times higher at the coolest sites).
2. Cooling water. Hourly temperature and humidity drive the heat rejected by the campus under four cooling designs: cooling towers; hybrid, evaporative in warm weather only; dry coolers with adiabatic pre-cooling above a switch temperature (20–30 °C, assumed); and closed loop with no evaporative cooling. Evaporation is the heat load times the evaporative fraction of Rutberg et al. (2011), as used in the USGS cooling tower method. Blowdown and drift together are evaporation divided by cycles of concentration minus one (DOE FEMP). Drift is a fraction of circulating flow. Cycles are sampled from 3 to 7, above the two to four FEMP calls typical, since it says six or more is possible [A]; 2 to 4 would take about 20% more water, as blowdown. Pads run at full effectiveness above the switch temperature; a controller that wet them only as needed would evaporate 12–41% less on average and 2–8% less on the peak day, by climate [A].
3. Uncertainty. Each estimate is 200 Monte Carlo runs, sampling the cooling parameters from published or estimated ranges; estimated ranges are marked [A]. We report the 10th, 50th and 90th percentiles. The peak day is the 1-in-10-year maximum day, taken as the 90th percentile of the annual maximum daily withdrawal, at each run's average utilization, 50–80% [A]. With every computer at capacity all day, an upper case in the package, it is about 1.5 times that for the evaporating designs. The middle estimate takes water withdrawn and wastewater at their medians, and water evaporated is the remainder, so the books close.
4. Other water. Staff use, 10–30 gallons a person a day, scaled to the number of people on site; maintenance and cleaning, up to 2,000 gallons a day plus 2–20 per MW, bracketed by applicants' projections; make-up water for closed loops, which lose 1–5% of their volume a year to leaks; and water that moves with outside air, 100–1,500 cfm per MW [A]: condensate from humid air joins the wastewater, and humidifiers, sampled from none to all [A], add water in very dry air.
5. Water source. If a community water system is mapped in the place, the campus is served by it. Where several are mapped, the comparison uses one covering at least half of the place (the largest share if more than one), otherwise the largest by population among those covering at least 5%, otherwise the largest by population [A]. Where that system has under 100 connections and covers under half of the place [A], no percent is shown, and the page says a campus would need a larger supply. If no system is mapped, a closed loop connects to the nearest system serving more than 3,300 people when it is within 10 km. All other cases use new wells.
6. Wastewater. Discharge goes to the nearest sewer plant if it is within reach and, for evaporative designs, has enough unused capacity for the blowdown. Otherwise closed loops use an on-site septic system and evaporative designs discharge directly.
7. Downstream. The place's main HUC-12 watershed is traced downstream for up to 25 watersheds, across state lines where needed, with the EPA-listed impairments and TMDLs for each.
8. Not counted. The calculator covers the campus's water, electricity and runoff on the site. It does not count new water or sewer connections, power lines, access roads or other construction beyond the site.
9. Electricity use. Worked out hour by hour from the same weather and heat balance as the cooling water, after Lei and Masanet (2022): computers, power losses, fans that slow to the air the hour needs, pumps, and chillers when outside air is too warm. So it varies by place and design. Some inputs are assumptions, marked [A], including how much of a campus is liquid-cooled. The peak is every computer running in the hottest hour of a typical decade.
10. Electricity. The utilities whose territories cover the place, their sales and peak loads, and the water consumed per kWh by each state's power plants: their 2024 reports to EIA, with published factors (Macknick et al. 2012) where a report is implausible or missing, weighted for power the state imports, and updated with 2022 reports and Siddik et al. (2024) on the same basis. Power plant water is reported separately from on-site use. A reported peak below a utility's average load cannot be right and counts as missing. Utilities from the national retail territory layer are ordered by number of customers, because it draws many small utilities as whole counties.
11. State layers. A state can add its own data and rules (reported water use, domestic wells, coldwater streams, state rules) on top of this method.

Mass and energy balances

In every hour of every run, the energy and water going into a campus equal what comes out.

Energy, per kW of IT load, each hour

Water, per kWh of IT load, each hour

The equations

• Energy: facility power + outside air heat = heat through the water or coil + heat rejection fans
• Heat through the water or coil: H = q + air handler fans + outside air heat + chillers + pumps, with q = 1/UPS + 1/(1 − distribution loss) − 1 + lighting per kW of IT
• Chillers: load × (1 − economizer share) × (w / COP water-cooled + (1 − w) / COP air-cooled)
• Tower fans: design fan power × (air needed / design air)³, from 10% of design air to all of it, where air needed goes with the tower's water over the enthalpy the air can pick up. Design is the 78 °F rating wet-bulb [A]. Dry cooler fans follow the same law.
• Outside air heat: air flow × (enthalpy outside − enthalpy indoors). A dehumidifying coil carries the latent part.
• Evaporation: H × w × evaporative ratio / latent heat. Assist evaporates at pads instead, from the air's own energy balance: H × depression / (switch point + air rise − air temperature + depression) / latent heat, where depression is pad effectiveness × (air temperature − wet-bulb).
• Drift: H × w / (8.34 × range) × drift share
• Blowdown: evaporation / (cycles − 1) − drift, from the dissolved solids balance: makeup × solids = (blowdown + drift) × cycles × solids
• Water: withdrawn + condensate = consumed + returned. The middle estimate takes withdrawn and returned at their medians, and consumed is the remainder.
• Lake-source: flow × density × specific heat × rise = heat = electricity, since the lake water is colder than the loops and needs no chillers or heat rejection fans [A]. Withdrawn equals returned at the pipe. The added heat then drives evaporation from the lake surface, so that balance is the lake's.
• Power plants, off site: generation = campus electricity / (1 − line loss). Plant water is generation × water per MWh generated, reported separately and never added to the campus's own water.

Terms

IT load, IT capacity	The power the computers draw, and the most they can draw. Utilization is load over capacity.
Facility power, PUE	All the electricity the campus uses. Power usage effectiveness (PUE) is facility power over IT load.
Sensible and latent heat	Sensible heat changes the temperature of air or water. Latent heat evaporates or condenses water without changing its temperature, about 1,050 BTU per pound.
Enthalpy	The heat air carries, sensible and latent together.
Economizer	Cooling without running a chiller, when outside conditions are cold enough.
Chiller, COP	A machine that moves heat from cool water to warmer surroundings using electricity. Its coefficient of performance (COP) is heat moved per unit of electricity.
Wet share (w)	The share of the heat going through evaporation: 1 for towers, 0 for closed loop and assist coils, and rising with air temperature for hybrid.
Evaporative ratio	The share of a tower's heat that leaves by evaporation, the rest warming the air. It rises with air temperature (Rutberg et al. 2011, as in USGS SIR 2013-5188).
Range	How much a tower cools its water, °F.
Drift	Droplets carried out of a tower in the air.
Blowdown	Water drained from a tower to keep dissolved minerals from building up. Pads drain a similar bleed.
Cycles of concentration	How many times more concentrated the tower's water is than the water coming in.
Loop top-up	Water replacing small leaks from a closed loop.
Outside air	Air brought in to pressurize and ventilate the building. It is a small flow, 100–1,500 cfm per MW of IT capacity [A], not used for cooling.
Dew point, condensate	Air cooled to its dew point starts to condense. Above the 15 °C indoor limit, a coil removes the extra moisture as condensate, water that comes from the air and not from the water system.
Humidifier	Adds moisture when the air's dew point is below −9 °C. How many campuses do this is not published, so the share is sampled from none to all [A].
Withdrawn, consumed, returned	The model's names for water taken; water evaporated or lost as droplets and leaks; and wastewater, sent to a sewer, septic system or stream.
Line loss	Electricity lost between power plants and the campus, about 5% (EIA, U.S. average 2018–2022). The national figure is used in every state, and a campus on the transmission system may lose less [A].

Model version 1.6 is used in every state. The Michigan methods page has the full parameter table with sources and compares the estimates with published and reported use: Michigan methods and sources.

What is specific to Virginia

Virginia follows the overall method with national data throughout. What is particular to it:

Places. 227 cities, towns and villages; 95 counties. Counties are included so that unincorporated land is covered.

Water quality. Impairments and TMDLs from Virginia DEQ's 2024 integrated report, through EPA ATTAINS.

Not yet added: Virginia's reported water use by sector, domestic wells, coldwater streams, lake-source cooling and state rules; pages show "state rules not yet reviewed".

Weather. 6 NOAA climate divisions, with the stations below.

Climate divisions and their stations

Each NOAA climate division uses the ISD station nearest the centroid of its counties with at least 85% of hourly observations for 1995–2024. Gaps of 6 hours or less are filled by linear interpolation; longer gaps use the station's mean for that month and hour.

Average-day withdrawal per 100 MW of IT capacity, million gallons per day (median); cooling and loop water only, excluding staff use.

Example: 300 MW of IT capacity in Loudoun County

Million gallons per day, median with the 10th–90th percentile range. Consumed water is evaporated or otherwise not returned. Source and wastewater are the defaults described above (muni: community water system; sewer: nearest sewer plant).

Checks on this snapshot

Coverage of Virginia's places: water system 89%, watersheds 100%, electric utility 98%, climate division 99.7%, a sewer plant within 40 km 99.7%. 50 randomly selected places were run through every step and cooling design with 0 errors. Most places without a mapped water system are rural areas on private wells.

Limits

These are screening estimates from public data, intended for comparing designs and sizes before an applicant's figures are available. They are not a substitute for facility design data, a utility capacity analysis or a permit review. Downstream paths are traced between HUC-12 watersheds, not along individual stream reaches. EPA assessments are from each state's most recent integrated report. Weather covers 1995–2024.

Planning-level hydrologic assessment. Not engineering design. Built with the open waterledger Python packages (waterledger 3.8.1). Northline Hydrology.