Quick Answer
Evaporation affects water retention in cisterns by reducing the available water for use, typically by 1-3% per day, depending on temperature, humidity, and wind speed.
Factors Affecting Evaporation
Water retention in cisterns is influenced by several factors, including climate, cistern design, and location. In hot and dry environments, evaporation rates can be as high as 5% per day. To mitigate this effect, cisterns should be designed with a wide surface area and a low water level to reduce the exposed water surface. A cistern with an exposed surface area of 10 square meters can lose up to 50 liters of water per day due to evaporation in a hot climate.
Cistern Sizing Considerations
When sizing a cistern, it’s essential to consider the effects of evaporation and rainfall variability. A general rule of thumb is to size the cistern for 100-150% of the average daily water demand. However, this may not be sufficient in areas with high evaporation rates or seasonal rainfall fluctuations. To account for these factors, cisterns should be sized for a minimum of 6 months of average water demand, with a buffer for unexpected losses due to evaporation and other factors.
Strategies for Reducing Evaporation
To reduce the impact of evaporation on cistern water retention, several strategies can be employed. These include using a floating cover or a cistern with a built-in cover, which can reduce evaporation by up to 90%. Applying a thin layer of oil or a specialized coating to the water surface can also help reduce evaporation. Additionally, cisterns can be designed with a siphon or a drain system to periodically drain and refill the cistern, which can help minimize the effects of evaporation and maintain water quality.
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