The amount of time water will last in a cistern depends on several factors, including the size of the cistern, the amount of rainfall, and the water usage rate. Here are some quick answers to key questions about how long water will last in a cistern:
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How big is the cistern?
The larger the storage capacity of the cistern, the longer the stored water will last. Cisterns can range in size from a few hundred gallons to over 20,000 gallons. A larger cistern can store more water to last through extended periods of low rainfall.
How much rainfall is collected by the cistern?
The amount of rainwater that can be collected depends on the size of the catchment area, the amount of rainfall, and the efficiency of the collection system. Areas with high annual rainfall will typically collect more rainwater for the cistern versus drier areas. Maximizing roof and surface runoff capture into the cistern increases water collection.
What is the daily water usage rate?
The daily water demand from the cistern determines how quickly the stored water gets used up. Low-flow fixtures and water-efficient practices will extend the length of time water in a cistern will last. Tracking usage and consumption can help identify trends and usages to optimize the water supply in the cistern.
Putting it all together
We can use an example to illustrate how these factors impact cistern water storage duration:
| Cistern size | 10,000 gallons |
| Annual rainfall | 40 inches |
| Catchment area | 1,500 sq ft roof |
| Daily usage | 500 gallons |
For this example, the 10,000 gallon cistern filled by 1,500 sq ft of roof catchment area with 40 inches of annual rainfall can expect to collect about 20,000 gallons of rainwater per year. With a daily usage of 500 gallons, the 10,000 gallons of stored water would last approximately 20 days without any rainfall.
Actual storage duration depends on the real-world rainfall patterns and daily usage profile. Cistern size could be increased to store more water if longer storage is desired. Implementing rainwater collection from additional surfaces beyond just roof catchment could also increase yearly water collection. Daily usage could be reduced with conservation practices.
Maintenance
Proper maintenance is crucial for ensuring the cistern reliably provides water and safely stores it. Key maintenance practices include:
- Inspecting gutters and filters – clean out debris and sediment regularly
- Checking for leaks – fix any cracked pipes or seals
- Cleaning the cistern – remove settled sediment and biofilm buildup
- Inspecting the water quality – test for bacteria and treat if necessary
- Checking the pumps – maintain, repair, or replace pumps as needed
Conclusion
A few key factors determine how long water will realistically last in a cistern system. The size of the cistern, amount of local rainfall, catchment efficiency, and daily water demands all impact storage duration. Proper maintenance and usage habits also play a role in optimizing useful life of the water. Considering all these variables with an application-specific example provides a reasonable estimate for how long water can be reliably stored in a cistern.