Reservoir Evaporation Loss Calculator

Inputs

Use measured evaporation where possible. Coefficients help adjust for site conditions.

Reservoir surface area

Use average water surface area for the period.

Evaporation rate

Enter pan evaporation or local open-water estimate.

Rainfall rate

Used as direct gain on the water surface.

Period length

Converted internally using average month length.

Pan coefficient

Typical range 0.6–0.85 for Class A pan adjustments.

Floating cover reduction (%)

Covers reduce effective evaporating area.

Wind speed (m/s)

Used as a mild multiplier on evaporation.

Water temperature (°C)

Higher temperature slightly increases evaporation.

Salinity (ppm)

Included as a bounded adjustment factor.

Example Data Table

Scenario	Area (m²)	Evap (mm/day)	Rain (mm/day)	Period (days)	Kp	Cover (%)
Baseline month	250,000	6.5	0.8	30	0.75	0
Windy week	120,000	7.2	0.1	7	0.70	0
With floating cover	180,000	6.0	0.5	21	0.78	35

Use site data for best accuracy. Example values are illustrative.

Formula Used

This calculator uses a practical volume-balance approach suitable for planning and comparisons.

Convert area: A_m² from selected units.
Convert rates: E and R into mm/day.
Gross evaporation (mm/day): E_g = E × K_p × F_w × F_t × F_s
Net evaporation (mm/day): E_n = max(0, E_g − R)
Effective area: A_eff = A × (1 − Cover/100)
Total net depth: D_n = E_n × Days
Volume loss: V_n (m³) = A_eff × (D_n/1000)

The wind, temperature, and salinity factors are bounded multipliers to support scenario testing without producing extreme values. For compliance-grade studies, use locally approved hydrology methods and measured datasets.

How to Use This Calculator

Enter the reservoir surface area and select the correct unit.
Provide evaporation and rainfall rates from reliable local data.
Choose the period length that matches your analysis window.
Set the pan coefficient to align pan readings with open-water conditions.
Optional: add cover, wind, temperature, and salinity for scenario refinement.
Click Calculate to view results above the form.
Use the export buttons to download CSV or PDF reports.

Project Planning Inputs

Evaporation losses influence reservoir sizing, lining choices, and supply reliability on construction projects. Use an average wetted surface area for the operating level. Remember that 1 mm over 1 m2 equals 0.001 m3. Turning depth into volume makes it easy to compare losses with pumping capacity and expected site demand. If water level varies, repeat the estimate for several areas and sum totals.

Climate Data Selection

Choose evaporation and rainfall from the nearest station or on site measurements. Pan readings should be adjusted with a pan coefficient, often between 0.60 and 0.85, to represent open water. Enter rates as daily, weekly, or monthly values, then keep the analysis period aligned with the source record and season. For screening studies, use seasonal averages; for design, test conservative dry year cases.

Operational Scenario Testing

Run several scenarios to reflect dry months, wet months, and mitigation options. Higher wind speed can raise evaporation, while cooler water can reduce it. Floating covers reduce effective area directly, so a 35 percent cover reduces losses by about 35 percent before other factors. Use salinity as a small modifier when comparing freshwater and brackish storage.

Interpreting Outputs

The calculator reports net depth and net volume over the selected period after subtracting rainfall gain. Daily net loss supports operations planning, such as refill frequency, trigger levels, and transfer scheduling. Gross evaporation volume and rainfall volume are shown separately to make the water balance transparent for teams, clients, and regulators. Convert results to megaliters when comparing with allocation or supply contracts.

Quality Control and Reporting

Check units, decimals, and results against local benchmarks. As a quick check, a 250000 m2 reservoir with 6.5 mm per day gross evaporation can lose more than 1600 m3 per day before rainfall. Export the CSV for records, and attach the PDF report to design notes, method statements, and stakeholder updates. Record assumptions, coefficients, and data dates to support audit reviews. Update inputs whenever site conditions change.

FAQs

1) What evaporation value should I enter?

Use the best available local dataset: station estimates, on site pan readings, or project climate reports. If using pan data, apply an appropriate pan coefficient to represent open water.

2) Why is rainfall subtracted from evaporation?

Rainfall adds water directly to the reservoir surface during the same period. Subtracting rainfall from gross evaporation provides a practical net loss rate for storage and supply planning.

3) How do floating covers affect results?

Covers reduce the effective evaporating area. A cover percentage reduces losses in the same proportion before climate modifiers, making it useful for comparing mitigation options and payback cases.

4) Can I use weekly or monthly climate rates?

Yes. Select the matching unit, then set a compatible period length. The calculator converts rates to daily values internally, so keep the source data and period aligned.

5) Are wind and temperature factors engineering standards?

They are bounded scenario multipliers for planning comparisons, not a substitute for detailed hydrology models. For regulatory studies, use approved methods and measured meteorological series.

6) How should I document results for submittals?

Export the PDF for a clean summary and keep the CSV for the audit trail. Record data sources, dates, assumptions, and coefficients so reviewers can reproduce the calculation.