Drawing No. EH–ES–005 // Environmental & Safety Engineering
Pond Evaporation & Water Balance Tool
Reviewed August 2026
Estimate pond evaporation using the standard pan-coefficient method, then work through a complete water balance — precipitation, inflow, evaporation, seepage and outflow — to find whether a pond needs makeup water or is gaining volume over a period.
What problem does this solve?
A pond or reservoir's water level over a season depends on the balance between what enters (rain, inflow) and what leaves (evaporation, seepage, outflow) — and evaporation alone can be a substantial, easily underestimated loss, especially in hot, dry, or windy climates. This tool estimates evaporation using the widely used pan-coefficient method, then works through the complete water balance to determine whether a pond needs makeup water to hold its level, and how much.
Inputs
Evaporation
Pond & period
Other water balance terms
Results
Background
Epond = Epan × Kp, where Epan is measured Class A pan evaporation and Kp is the pan coefficient, typically around 0.70 as a national average, ranging roughly 0.60 (humid, sheltered sites) to 0.80 (arid, windy sites). A small pan evaporates faster than a large open pond because it heats up more from solar radiation absorbed through its sides and bottom, and has a much higher perimeter-to-surface-area ratio exposing more of its water to wind — the pan coefficient corrects for this scale difference.
ΔS = P + I − E − O − Seep, where ΔS is the change in stored volume, P is precipitation falling directly on the pond surface, I is inflow (surface runoff, streams, groundwater), E is evaporation, O is controlled outflow or discharge, and Seep is seepage loss through the pond bottom and sides. When outflow terms exceed inflow terms, ΔS is negative and the pond level falls unless makeup water is added.
Unlike evaporation (measurable with a pan) or precipitation (measurable with a rain gauge), seepage depends on the permeability of the pond's bottom and side soils or liner, and the difference between pond water level and surrounding groundwater level — both of which vary by location and can only be estimated reliably through a geotechnical investigation or by measuring an actual pond's water balance with all other terms known and solving for seepage as the residual, sometimes called the water-budget method.
Pan coefficients, evaporation rates and precipitation all vary substantially through the year — pan coefficients found in field comparisons have ranged from roughly 0.4 in low-evaporation months to over 0.9 in others, and evaporation itself is typically several times higher in summer than winter. A single annual-average calculation can significantly understate summer water loss and overstate winter loss; a proper seasonal water balance repeats this calculation month by month rather than using one annual average.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
Evaporation depends on surface area, not volume — two ponds holding the same amount of water but with different shapes (or the same pond as it draws down and its surface area shrinks) will have very different total evaporation losses, since a wider surface exposes more area to the same evaporation rate. This is part of why deep, small-surface-area storage (or floating covers, which physically block evaporation) reduce losses compared to shallow, wide impoundments.
Many national weather services, agricultural extension offices, and state or regional climatology offices publish historical pan evaporation data, often as monthly averages by location — search for '[your state/region] pan evaporation data' as a starting point, or use nearby airport/agricultural weather station records if available.
Yes, especially in permeable (sandy or gravelly) soils without an effective liner — seepage rates through unlined ponds in sandy soil can easily exceed evaporation rates, which is exactly why lining is often the more cost-effective long-term fix for chronic water loss compared to simply topping up with makeup water indefinitely.
The pan-coefficient method is a practical simplification — it implicitly captures the local combination of solar radiation, humidity, temperature and wind through the measured pan data itself, without needing all of those as separate inputs, which is exactly why it remains popular for quick water-resource screening. Full energy-balance methods like Penman-Monteith are generally more accurate and don't rely on an empirical coefficient, but need considerably more meteorological input data than most practical water-balance studies have readily available.