Calculating expected savings before procurement gives you a defensible ROI case for the cover. The arithmetic is straightforward; the inputs do all the work.
The formula
Annual water saved (m³) = Surface area (m²) × Evaporation rate (m/year) × Reduction factor × Coverage fraction
Where:
- Surface area is the open water area of the reservoir in m².
- Evaporation rate is the local annual pan evaporation in metres (mm/year ÷ 1,000).
- Reduction factor is the cover’s evaporation-suppression efficiency. AWTT publishes up to 95% (0.95) for the Hexprotect® AQUA hexagonal element at full tessellation [AWTT — Hexprotect® AQUA hexagonal cover] .
- Coverage fraction is the fraction of the surface actually covered (typically 1.0 if you cover the whole reservoir).
Annual cost saved (€/year) = Annual water saved (m³) × Water cost (€/m³)
Payback (years) = Capex (€) ÷ Annual cost saved (€/year)
That’s the entire model. Everything else is sourcing accurate inputs.
A worked example
Take a Spanish agricultural reservoir in Andalucía:
- Surface area: 10,000 m²
- Local pan evaporation: 1,600 mm/year (typical southern Spain [FAO Paper 56] )
- Reduction factor: 0.95 (95% — hexagonal cover, full tessellation)
- Coverage fraction: 1.0 (cover whole reservoir)
- Water cost: €1.20/m³ (regional irrigation tariff)
- Cover capex (estimate): €280,000 (for 10,000 m² at €28/m²)
Annual water saved = 10,000 × 1.6 × 0.95 × 1.0 = 15,200 m³/year
Annual cost saved = 15,200 × 1.20 = €18,240/year
Payback = 280,000 ÷ 18,240 = 15.3 years
That payback looks long because we used a low water cost. Re-running with municipal water cost (€2.50/m³) gives:
Annual cost saved = 15,200 × 2.50 = €38,000/year Payback = 280,000 ÷ 38,000 = 7.4 years
For a mining tailings application in the same climate with operator-confirmed make-up water cost at €5.00/m³ (typical for arid mining):
Annual cost saved = 15,200 × 5.00 = €76,000/year Payback = 280,000 ÷ 76,000 = 3.7 years
This is why the answer “how long is payback?” depends entirely on water cost and climate — not on the cover itself.
Sourcing accurate inputs
Surface area — measure from a recent aerial photo (Google Earth Pro is free and works for most public-facing reservoirs). Round down to the nearest 100 m² to be conservative.
Evaporation rate — use measured pan evaporation if your site has a class A pan or equivalent. Otherwise, use national meteorological-service open data. Pan evaporation is consistently higher than reference evapotranspiration (ET0); avoid using ET0 directly without an adjustment factor.
Water cost — use the marginal cost of replacement water at the reservoir inflow. For agriculture this is typically the irrigation tariff; for mining it’s the make-up water cost or the avoided abstraction cost; for utilities it’s the embedded treatment cost.
Reduction factor — 0.95 is conservative for hexagonal modular at full tessellation. For partial coverage (e.g. exclusion zones around pumps), reduce proportionally.
Why two evaporation figures appear in this catalogue
Two related-but-distinct evaporation figures appear across this site, and the difference is intentional:
- Up to 95% is the manufacturer-published specification for the AWTT Hexprotect® AQUA hexagonal element at full tessellation [AWTT — Hexprotect® AQUA hexagonal cover] . Use this when citing the product specification.
- 90–97% is the range reported by independent field studies on hexagonal modular covers under operating conditions [USDA Bureau of Reclamation evaporation field trials] [FAO Irrigation and Drainage Paper 56] . Use this when citing field-verified performance across multiple deployments.
Both are accurate in their respective context. The manufacturer figure is a controlled-deployment ceiling; the field range reports the spread that real installations have produced, with the AWTT 0.95 sitting inside that band. For planning arithmetic, use 0.95 as the conservative default.
Including secondary savings
The calculation above is water savings only. Real installations deliver three more revenue streams:
- Algae/chemical reduction — reservoirs covered for evaporation also stop algae growth at the source, reducing chemical dosing by 70–90% on potable applications. This often saves another €5,000–€50,000/year on a 10,000 m² reservoir.
- Abstraction permit headroom — under the EU Water Framework Directive, reducing abstraction by 10–20,000 m³/year may unlock additional operational flexibility worth more than the direct cost saving.
- Carbon and ESG reporting — water-savings are reportable under CSRD/CDP Water and may contribute to investor-facing sustainability metrics.
For a complete cost-benefit analysis, include all four streams. The interactive calculators on this site let you adjust the headline assumption and see savings projected over the cover lifetime.
Use the calculator
The evaporation savings calculator on the homepage runs this arithmetic interactively. Plug in your numbers; we’ll come back within one working day with a project-specific sizing and ROI breakdown that includes secondary savings.
Sources
Sources cited above are reproduced in the footer. For project-specific climate data, your national meteorological service open data is the authoritative source.