--- url: https://www.eurocovers.eu/knowledge/how-to-calculate-evaporation-savings title: How to calculate evaporation savings on your reservoir description: Step-by-step formula, worked example, and ROI calculator for estimating water and cost savings from a floating cover on an industrial reservoir. updated: 2026-05-25 --- # How to calculate evaporation savings on your reservoir Step-by-step formula, worked example, and ROI calculator for estimating water and cost savings from a floating cover on an industrial reservoir. import Citation from '@eurocovers/ui/astro/Citation'; 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. - **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) - **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. Use this when citing the product specification. - **90–97%** is the range reported by independent field studies on hexagonal modular covers under operating conditions. 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](/quote#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.