As one of the oldest known gardening methods, rainwater harvesting is a great option for projects that need to reduce dependence on municipal water supplies and/or practice sustainable irrigation.
Rainwater harvesting systems typically consist of three main components:
- the capture system (roof and gutters)
- the storage unit (cistern)
- the delivery system (pipes, pumps, and valves)
Materials for the water storage unit are about two-thirds of the overall cost of the whole project, making this the most expensive component of the entire system. That is why it is important to understand how to determine the optimal size water storage unit for each individual project. If you build too large, the payback will start to decrease.
A small scale watering harvesting system for the home or small business would likely opt for a cistern that has less than a 10,000 gallon capacity. This type of system would be a viable use of collecting rain water and filtering it for reuse to water a garden or entrance area.
A large scale water harvesting system for large commercial projects is at the other end of the spectrum. The financial investment for these types of projects is going to be considerably larger. There are a number of factors that need to be considered:
- What is the size of the area to be irrigated?
- What is the area of the roof top or surface that will be allocated towards collecting and diverting water to a storage facility?
- What are the rainfall patterns for the project site?
How these factors relate to the overall design of the system will vary in every project. With so many variables to consider, it is difficult to have a standard rule of thumb for projecting costs. But generally speaking, a water system in place and installed is right around 2 dollars per gallon. So for a 150,000 gallon storage unit, you will likely be in the ballpark of $300,000 project.
Landtech’s partner, Wahaso (Water Harvesting Solutions, in Chicago) has good data about the rainfall pattern for every part of the country to help answer the questions: What was the amount of rain? How long did it rain? How frequently did the rain events occur? Answering these questions is vital to knowing what size of cistern you will need and how much you can expect to rely on harvested water for irrigation. Let’s say you need 150,000 gallons of water a week for your irrigation, and you live in a location where the amount of rainfall will not be sufficient to fill a 150,000 gallon cistern meaning the cistern would never actually be full. Therefore, spending money on a cistern that large would make a significant impact on the ROI. A cistern should only be big enough for the amount of rain to be expected.
