Water Evaporation Rate & Loss Calculator
Understanding Water Evaporation Rates and Environmental Loss
Evaporation is the process by which water transitions from liquid to vapor, returning to the atmosphere. It occurs continuously from water surfaces (pools, lakes, soil, oceans) and is driven by solar energy, air temperature, relative humidity, wind speed, and water surface temperature. In many regions, evaporation is the largest component of water loss—more water is lost to evaporation than to runoff or groundwater recharge. Understanding evaporation rates is critical for water resource management, irrigation planning, pool maintenance, and agricultural water budgeting. In arid and semi-arid climates, evaporation rates can exceed 5–10 inches per year, representing substantial water loss.
The evaporation rate depends on multiple environmental factors. Higher air temperature increases evaporation because molecules have more kinetic energy. Higher water temperature increases evaporation similarly. Lower relative humidity increases evaporation because the air can "hold" more water vapor. Higher wind speed increases evaporation by continuously removing saturated air adjacent to the water surface and replacing it with drier air from elsewhere. These factors interact; calm, humid, cool conditions minimize evaporation, while hot, dry, windy conditions maximize it.
Different surface types evaporate at different rates. Open water surfaces (pools, ponds) evaporate at the "potential evaporation" rate determined by the environmental factors above. Vegetated soil evaporates via "evapotranspiration"—evaporation from soil plus transpiration from plants. Wet bare soil evaporates faster than vegetated soil (plants have cuticles and stomata that control water loss). Wet mulched soil evaporates more slowly than bare soil because mulch insulates the soil and reduces direct solar heating.
The Penman-Monteith equation is the standard scientific model for evapotranspiration, accounting for solar radiation, air temperature, humidity, wind speed, and atmospheric pressure. A simplified version often used is: Evaporation (inches/day) = 0.0001 × [(Saturation Vapor Pressure - Actual Vapor Pressure) × (0.26 × Wind Speed + 0.5)]. This requires calculating vapor pressure from temperature and humidity—a multi-step process. For practical estimation, empirical models often use simpler factors based on temperature, humidity, wind, and surface type.
MathML Formula for Simplified Evaporation Rate:
Where: Base Rate (0.1 inches/day at 70°F, 50% RH, 5 mph wind), Temperature Factor (increases with temperature), Humidity Factor (decreases with humidity), Wind Factor (increases with wind speed)
Worked Example: A swimming pool with 400 square feet surface area in Arizona (95°F air, 25% humidity, 8 mph wind, 85°F water). Base evaporation in these hot, dry, windy conditions is approximately 0.3–0.5 inches per day. At 400 square feet, that is 400 sq ft × 0.4 inches/day = 133 cubic inches/day = approximately 58 gallons/day. Over a month (30 days), that is 1,740 gallons—a significant loss. A pool cover can reduce this by 50–80%, saving 900–1,400 gallons monthly. In humid Florida (80°F, 75% humidity, 3 mph wind), the same pool might lose only 0.1–0.15 inches/day, or approximately 15–20 gallons/day, a much lower rate due to higher humidity.
Comparison table of evaporation rates by environment:
| Location/Conditions | Temp/Humidity/Wind | Daily Rate (inches) | Annual (inches) |
|---|---|---|---|
| Hot, Dry, Windy (Arizona) | 95°F / 20% / 8 mph | 0.4–0.6 | 150–220 |
| Moderate, Humid (Florida) | 80°F / 70% / 3 mph | 0.1–0.15 | 40–55 |
| Cool, Humid (Pacific NW) | 65°F / 75% / 2 mph | 0.05–0.08 | 20–30 |
Limitations and Assumptions: This calculator uses simplified evaporation models and does not account for solar radiation (latitude, season, cloud cover), atmospheric pressure, or the Penman-Monteith equation's full complexity. Actual evaporation varies by these factors significantly; sunny days evaporate faster than cloudy days. The calculator assumes steady conditions; in reality, conditions vary hourly and daily. Surface roughness, vegetation, and soil type affect evaporation (not fully modeled here). High altitude locations have different atmospheric pressure and accelerated evaporation. The model assumes no external water inputs (rain, irrigation); real evaporation loss is net loss. Pool covers, soil mulch, and vegetation can reduce measured evaporation by 50–80%, but these are not accounted for. For detailed planning (irrigation design, water budgeting), consult site-specific evapotranspiration data from local agricultural extension offices.