How to Calculate Evaporation Loss for Cisterns?

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Understanding Evaporation Loss Evaporation loss is a critical factor in cistern design and management. It’s essential to accurately calculate this loss to ensure that your cistern has sufficient storage capacity. To do this, you’ll need to know the cistern’s surface area, which can be calculated using the following formula: Surface Area = π x Diameter^2. For example, a cistern with a diameter of 8 feet has a surface area of approximately 201 square feet.
Calculating Evaporation Loss Using the ASCE Equation The ASCE equation for evaporation loss is: Evaporation Loss (gpd) = 0.0000062 x Surface Area (sq ft) x (T - 32) x (H - 0.6), where T is the temperature in degrees Fahrenheit and H is the relative humidity as a decimal. Using this equation, let’s calculate the evaporation loss for the 8-foot diameter cistern in a climate with a temperature of 80°F (26.7°C) and a relative humidity of 60%. First, we need to convert the temperature to Fahrenheit and the relative humidity to a decimal: Evaporation Loss = 0.0000062 x 201 x (80 - 32) x (0.6 - 0.6) = 0.02 gallons per day. This is a relatively low evaporation loss, indicating that the cistern’s storage capacity can be slightly smaller than initially thought.
Practical Considerations When calculating evaporation loss, it’s essential to consider the cistern’s cover type and condition. A sealed cistern with a tight-fitting lid will experience lower evaporation loss than an open cistern or one with a damaged lid. Additionally, evaporation loss can vary significantly depending on the climate and season. In hot and dry climates, evaporation loss can be much higher, while in cool and humid climates, it may be lower. To account for this variability, you may need to use a more detailed evaporation loss equation or consult with a local expert.