Calculate How Much Rainwater You Can Collect

Harvesting rainwater makes perfect sense, and we learned more about the process at a terrific workshop we attended at the NM Organic Farming Conference in Albuquerque last week. From neighboring Texas, Billy Kniffen is the water resource specialist for Texas A&M University.

Kniffen explained that catching and using rainwater is not a new concept; early settlers in the West captured water. Interest is increasing today, at least among homeowners and gardeners who live in states that allow rain harvesting. We’ve used one or more rain barrels to capture roof water for years, and hope to someday acquire a large cistern. As Kniffen said, “the first rain barrel is the ‘gateway drug’ to rainwater harvesting.”

rain barrel shed
This was actually our third rain barrel, located near our vegetable garden. I couldn’t stand to watch the water pour off that shed roof without catching at least some of it.

Getting Started Is Easy

I believe more people would harvest rainwater if they realized it’s not as complex a process as it seems. For simple gardening use, a few rain barrels or a cistern with pressure can irrigate ornamental plants. If you want to water edible plants, you have to take a few more steps; it’s recommended that you add features such as first flush equipment (which directs the first flush from your gutters away from the barrel to prevent nasty debri from entering the container) and filters. A prefilter also keeps leaves and debris out; you just have to clean it from time to time. You also can disinfect your rain water with chlorine or ultraviolet light.

filter lid on rain barrel
Inexpensive, smaller barrels have crude filters to stop debris. A larger system needs a more sophisticated filter.

Calculating Collection

A house that has a roof of 1,000 square feet can yield 600 gallons of water from just one inch of rainfall. For me, watching the small barrels fill and then overflow feels like a waste. In our case, water that runs from the house eventually refills our water table – and therefore our well. But in urban areas, water runoff fills streets and gutters, and often is wasted. Texas A&M Extension has made it easier to calculate the amount of rain you can collect from your home or shed roof.

rain collection system model
Here’s the demo system Kniffen brought to our workshop. It’s simpler than it looks!

Here’s a simple calculator for catchment area courtesy of the college. There’s also a link here to a Microsoft Excel spreadsheet (under Resources) with the formulas already included. also has a simple calculator. Texas residents, or those who can easily compare their annual rainfall to one of the included Texas towns, can input the information and receive monthly flow rates and required cistern sizes to catch as much rain as possible. Finally, the school also publishes a manual all about rainwater harvesting.

gutter on shed
We had to add a gutter to the shed roof to collect the water. It was simple and worth it.

Kniffen and his wife run their home solely off collected rainwater from their home and a shed. I don’t believe we have the money to collect all water and make it potable, but it’s always on the wish list. Any attempts to save and use rainwater can help preserve water for future years. I hope these tools make it a little easier for anyone considering rain harvesting to get started.