Evaporation Rate vs Rainwater Harvesting Calculator
The evaporation rate and rainwater harvesting calculators are the two outdoor water-volume tools on this site, and they work the same planning space from opposite directions. The evaporation rate calculator estimates water lost from an open pool, pond, or tank: air temperature, water temperature, humidity, wind speed, and surface area feed a vapor-pressure model that reports liters per hour, gallons per day, and an indicated depth loss. The rainwater harvesting calculator estimates water captured from a roof: roof area and monthly rainfall with a collection efficiency report monthly and annual harvest. If you are managing outdoor water — how much a pond will need topping up, or how much a rain barrel can supply — these two estimate the loss side and the supply side of the same budget. Both are weather-based estimates, not measurements, leak tests, or water audits.
What each calculator does
The evaporation rate calculator computes saturation vapor pressure at the water temperature, actual vapor pressure estimated from air temperature and relative humidity, and a wind term with a small minimum speed. Warm dry air and wind raise the rate; humid calm air can drive it to zero, and negative or tiny values are clamped to zero. The page is explicit that this is a simplified open-water model, not the FAO Penman-Monteith reference evapotranspiration method, and that covers, splash-out, solar gain, and shading are outside the estimate.
The rainwater harvesting calculator multiplies roof area by monthly rainfall by a collection efficiency, defaulting to 80 percent to reflect splash, gutter leakage, evaporation, and first-flush losses. Metric runs use one liter per square meter per millimeter of rain; imperial runs use about 0.623 gallons per square foot per inch — the exact volume of one inch of rain on one square foot (144 cubic inches at 231 cubic inches per gallon). It reports monthly and annual harvest plus a per-millimeter or per-inch yield, so a 1,000 square foot roof catches roughly 623 gallons per inch of rain at 100 percent efficiency.
Side-by-side comparison
| Feature | Evaporation rate calculator | Rainwater harvesting calculator |
|---|---|---|
| Question answered | How fast is water leaving this pool or pond? | How much rain can this roof collect? |
| Key inputs | Air temp, water temp, humidity, wind, surface area | Roof area, monthly rainfall, efficiency |
| Core relationship | Vapor pressure deficit × wind term | Area × rainfall × efficiency |
| Output units | Liters/hour, gallons/day, depth loss | Monthly and annual volume, yield per mm or inch |
| Assumptions | Named empirical model, minimum wind speed | 80% efficiency, first-flush losses |
When to use which
Use the evaporation rate calculator when your question is about an open water surface — estimating monthly topping-up needs for a pool or pond, or checking whether reported water loss is plausible before chasing a leak. Use the rainwater harvesting calculator when your question is about a roof as a catchment — sizing a rain barrel or cistern, or estimating how much of a garden’s outdoor water use collected rain could cover.
For a combined view, put both on the same unit and time basis: estimate monthly evaporation from your pool or pond surface and monthly harvest from your roof, then compare them with your outdoor water demand. Keep the uncertainty in mind — the evaporation page’s simplified model can differ substantially from FAO reference methods and measured losses, and harvest estimates ignore overflow and first-flush discards. Both pages anchor the practical context in EPA water-efficiency and green-infrastructure resources; the rainwater page points to Texas A&M AgriLife Extension’s planning publication for real system sizing.
Where to start
- Evaporation rate calculator — open-water loss from temperature, humidity, wind, and surface area.
- Rainwater harvesting calculator — roof runoff volume from area, rainfall, and collection efficiency.
Informational note: Both calculators are weather-based planning estimates, not measurements or audits. Actual evaporation depends on covers, heating, and exposure; actual harvest depends on first flush, overflow, and gutter condition. Use the results to bracket a water budget, not to size infrastructure without site-specific checks.