Skip to main content
Municipal Free Pro Features Available

Evaporation & Makeup Water Calculator

Calculate pond evaporation, seepage loss by liner type, and required makeup water flow with 12-month projection

Free evaporation and makeup water calculator for pond owners, municipal operators, and environmental engineers who need to know how much water their pond loses and what it costs to replace. Enter the pond surface area in square feet or acres, select a climate zone preset (arid, semi-arid, temperate, humid) or enter a custom daily pan evaporation rate, and choose a liner type (unlined, compacted earth, clay, synthetic liner, or concrete). The calculator includes a wind exposure factor (sheltered, normal, exposed), a pan coefficient (default 0.7), and a rainfall offset that credits precipitation against losses. The output is a 12-month projection showing monthly evaporation, seepage, rainfall credit, net water loss, cumulative loss, and required makeup pump GPM for each month. Summary results include total annual water loss, peak makeup flow rate, average makeup flow rate, and estimated annual water cost at your local water rate. This tool answers the question every pond owner eventually asks: why is my pond level dropping and how much does it cost to keep it full?

Pro Tip: The pan coefficient of 0.7 is a national average. In arid regions with low humidity and high wind, actual lake evaporation can be 75 to 80 percent of pan evaporation (coefficient 0.75 to 0.80). In humid regions with still air, the coefficient drops to 0.60 to 0.65. If you have access to local lake evaporation data from your state climatology office, use that directly instead of applying a pan coefficient to Class A pan data. The difference between a 0.65 and 0.80 coefficient on a 1-acre pond in Texas can be over 200,000 gallons per year.

Calculate pond volume and fill/drain time

Pond Fill/Drain Calculator →

Size lagoons for wastewater treatment

Lagoon Sizing Calculator →

Size irrigation pump and pipe for makeup supply

Irrigation Pump & Pipe Calculator →

Calculate pump energy cost for makeup water delivery

Pump Energy Cost Calculator →

PREVIEW All Pro features are currently free for a limited time. No license key required.

Evaporation & Makeup Water Calculator
Pop Out

How It Works

  1. Enter Pond Surface Area

    Enter the pond surface area in square feet or acres. For irregular ponds, estimate the area as a rectangle or use a mapping tool to measure the surface. The surface area is the primary driver of evaporation loss, so accuracy here matters more than depth or volume for this calculation.

  2. Select Climate Zone or Enter Custom Rate

    Choose from climate zone presets (arid, semi-arid, temperate, humid) that load a representative daily pan evaporation rate, or select Custom and enter your own rate in inches per day. The calculator applies the pan coefficient (default 0.7) and wind exposure factor to convert pan evaporation to estimated free water surface evaporation, then distributes it across 12 months using seasonal weighting factors.

  3. Choose Liner Type

    Select the pond liner material: unlined natural soil, compacted earth, clay, synthetic liner, or concrete. Each type has a characteristic seepage rate in inches per day. Unlined ponds on permeable soils lose the most water to seepage, while synthetic-lined ponds lose almost nothing. The calculator adds seepage to evaporation and subtracts rainfall credit for net water loss.

  4. Review 12-Month Projection

    The output shows monthly evaporation loss, monthly seepage loss, total monthly water loss, cumulative annual loss, required makeup flow rate in GPM, and estimated annual water cost. Use the results to size a makeup water line, budget for water purchases, or evaluate whether upgrading the liner is cost-effective.

Built For

  • Pond owners troubleshooting unexplained water level drops and deciding whether to add a liner
  • Municipal lagoon operators budgeting annual makeup water costs and sizing supply lines
  • Environmental engineers designing water balance models for stormwater retention ponds
  • Aquaculture operators planning water supply capacity to maintain pond levels through summer
  • Golf course superintendents estimating irrigation pond losses and refill requirements

Assumptions

  • Evaporation is estimated from pan evaporation data using a configurable pan coefficient (default 0.7) and wind exposure factor.
  • Seepage rates are based on published typical values for each liner type and assume proper installation.
  • Surface area is assumed constant (no significant change in area as water level drops from losses).
  • Rainfall credit is included using either annual average (distributed by days per month) or user-entered monthly values.

Limitations

  • Does not model the effects of pond depth or water temperature on evaporation rate beyond the wind exposure factor.
  • Does not account for runoff inflow or groundwater contributions to the pond water balance.
  • Does not model seasonal groundwater table fluctuations that affect seepage rates.
  • Does not calculate pipe sizing or infrastructure costs for makeup water delivery.

References

  • NOAA Technical Report NWS 33 - Evaporation Atlas for the Contiguous 48 United States
  • USDA NRCS - Ponds: Planning, Design, Construction (Agriculture Handbook 590)
  • EPA - Design Manual for Municipal Wastewater Stabilization Ponds (EPA 625/1-83-015)
  • Farnsworth, R.K. and Thompson, E.S. - Mean Monthly, Seasonal, and Annual Pan Evaporation for the United States (NOAA Technical Report NWS 34)

Frequently Asked Questions

A Class A evaporation pan is a standardized 4-foot diameter, 10-inch deep metal pan used by weather stations to measure evaporation. Because the pan is small, shallow, and made of metal, it heats up more than a large water body and evaporates faster. The pan coefficient corrects for this difference. The standard coefficient of 0.7 means that a real pond evaporates about 70 percent as much as the pan measurement. This value was established by decades of comparative studies between pan data and actual lake evaporation measurements. It is a reasonable national average, but local values range from 0.60 in humid coastal areas to 0.80 in dry windy regions.
Seepage rates depend heavily on soil type. Sandy soils can lose 2 to 6 inches per day, silty soils lose 0.5 to 2 inches per day, and clay soils lose 0.1 to 0.5 inches per day. A 1-acre unlined pond on sandy loam losing 1 inch per day loses about 27,154 gallons per day, or nearly 10 million gallons per year. This is why most constructed ponds in permeable soils require a liner. Compacted clay liners reduce seepage to 0.01 to 0.1 inches per day, and HDPE geomembrane liners reduce it to essentially zero.
Evaporation is water loss from the surface into the atmosphere, driven by temperature, humidity, wind, and solar radiation. It varies seasonally, peaking in summer and dropping in winter. Seepage is water loss through the pond bottom and sides into the surrounding soil, driven by the head differential between the pond water level and the local groundwater table. Seepage is relatively constant year-round unless the groundwater table rises seasonally. Both contribute to total water loss, and a water balance must account for each separately because they respond to different factors and have different mitigation strategies.
The easiest method is to use a satellite mapping tool like Google Earth, which lets you trace the pond outline and calculates the enclosed area automatically. Alternatively, you can break the pond shape into simple geometric pieces: rectangles, triangles, and half-circles. Measure each piece and add the areas. For a rough field estimate, measure the longest length and widest width and multiply by 0.8 to approximate an oval shape. For permit-level accuracy, a professional survey or drone mapping with photogrammetry software provides the best results.
This depends on seepage rate, pond size, liner cost, and local water price. For a typical 1-acre pond on sandy soil losing 1 inch per day, makeup water at $5 per 1,000 gallons costs roughly $50,000 per year. An HDPE liner for the same pond costs $30,000 to $60,000 installed and lasts 20 to 30 years. In this case, the liner pays for itself within one to two years. For a pond on clay soil losing only 0.1 inches per day, the annual makeup cost is closer to $5,000, making a liner harder to justify on economics alone. The calculator helps you compare these scenarios by showing annual water cost for each liner type.
Disclaimer: This calculator provides estimates based on published evaporation and seepage data. Actual water loss depends on local climate, soil conditions, groundwater levels, wind exposure, and pond geometry. Permit-level water balance studies should be performed by a licensed engineer using site-specific data. ToolGrit is not responsible for water supply or liner selection decisions based on these estimates.

Learn More

Municipal

Pond Evaporation & Water Loss: Calculating Makeup Water Needs

How to estimate pond evaporation, seepage by liner type, seasonal water budgets, and required makeup water flow rate and annual cost.

Read Guide →

Related Tools

Municipal Live

Chemical Dosing Calculator

Calculate gallons per day or pounds per day of chemical feed for any water or wastewater treatment process. Supports chlorine, alum, ferric chloride, polymer, lime, caustic, permanganate, and fluoride with automatic unit conversions and metering pump settings.

Launch Tool →
Municipal Live

Detention Time Calculator

Calculate hydraulic detention time for any basin, tank, or lagoon and check against regulatory minimums. Supports rectangular and circular tanks with dead zone correction for actual vs theoretical retention time.

Launch Tool →
Municipal Live

Disinfection CT Value Calculator

Calculate CT values for chlorine disinfection and verify EPA Surface Water Treatment Rule compliance. Check Giardia and virus log inactivation credits based on residual, contact time, temperature, and pH.

Launch Tool →