Pond Evaporation Loss Calculator

About the Pond Evaporation Loss Calculator

Open water surfaces lose significant volumes of water to evaporation, especially in hot, dry, and windy climates. For farm ponds, stock tanks, and irrigation reservoirs, evaporation can represent 3–7 feet of water loss per year in arid regions.

Evaporation is typically estimated from pan evaporation data (available from weather stations) multiplied by a pan coefficient (usually 0.70–0.80) to convert from the small pan to a larger open water body. Multiplying the adjusted evaporation depth by surface area gives the volume of water lost.

This page converts pan data and surface area into an expected water loss so reservoir evaporation can be budgeted like any other demand.

When This Page Helps

Evaporation is easy to ignore until storage runs short. This page makes that loss visible before the pond balance gets tight.

How to Use the Inputs

1. Enter the pond surface area in acres.
2. Enter the pan evaporation rate in inches per month (or annual total).
3. Enter the pan coefficient (default 0.70).
4. Read the evaporation depth and volume lost.
5. Compare loss to inflow and pumping to balance the water budget.

Example Calculation

Tips & Best Practices

• Pan evaporation data is available from NOAA, state climate offices, and local weather stations.
• Typical pan coefficients: 0.70 for arid regions, 0.75–0.80 for humid regions.
• Wind breaks and shade reduce evaporation by 10–30%.
• Floating covers or shade balls can reduce reservoir evaporation by 50–90%.
• Deeper ponds with less surface area lose less water proportionally than shallow ones.
• Peak evaporation occurs in June–August in the Northern Hemisphere.

Regional Evaporation Patterns

The highest evaporation rates in the U.S. occur in the desert Southwest (70–90 in/yr), followed by the southern Great Plains (55–70 in/yr). The upper Midwest and Northeast have the lowest rates (25–40 in/yr). NOAA publishes maps of mean annual lake evaporation by region.

Evaporation Reduction Strategies

Floating covers (HDPE, foam, or shade cloth) can reduce evaporation by 70–90%. Windbreaks (trees or fabric) reduce wind speed and can cut evaporation by 10–25%. Chemical monolayers (hexadecanol) can reduce evaporation by 20–40% but require regular reapplication.

Water Budget for Farm Ponds

A complete water budget: Inflow (runoff + groundwater + direct precipitation) – Outflow (evaporation + seepage + withdrawals) = Change in Storage. Balance this equation monthly to predict when the pond will be full, adequate, or critically low.

Frequently Asked Questions

• What is a pan coefficient?

  A Class A evaporation pan overestimates open-water evaporation because it's small, exposed on all sides, and heats up quickly. The pan coefficient (0.70–0.80) adjusts for this difference.

• How much water does a farm pond lose per year?

  In the central U.S., annual lake evaporation is about 40–55 inches. In the arid Southwest, it can exceed 70 inches. A 2-acre pond losing 50 in/yr loses about 8.3 ac-ft.

• Can I reduce evaporation?

  Yes. Windbreaks, shade structures, floating covers, and shade balls all reduce evaporation. Minimizing surface area relative to volume (deeper ponds) also helps.

• How does humidity affect evaporation?

  Higher humidity reduces the vapor pressure deficit, slowing evaporation. Humid regions may lose 30–40 in/yr vs 60–80 in/yr in arid areas.

• Is precipitation offset against evaporation?

  Yes. Net evaporation = gross evaporation – direct precipitation. In humid regions, net evaporation may be only 10–20 in/yr because rainfall partially offsets losses.

• What about seepage losses?

  Seepage is separate from evaporation. Clay-lined ponds may lose 0.1–0.5 in/day to seepage; sandy soils can lose 1–3 in/day without a liner. Both should be included in the water budget.