Passive and Active Rainwater Harvesting - What's The Difference

Here in a dryland environment (San Diego County), rainwater harvesting is critical in weaning ourselves off of foreign water and creating a sustainable and local water supply. What a lot of people don't realize is even in a place that doesn't get a lot of rain, like coastal San Diego County - approximate average of 10 inches of rain per year - you can still catch a HUGE amount of water! For example, the average home in San Diego County has a 2000 square foot roof. With JUST one inch of rain this size roof would catch 1200 gallons of water - this is active rainwater harvesting. This pales in comparison to the amount of rainwater that can be caught in healthy soil - passive rainwater harvesting. Lets explore these two types of systems, active and passive.

Passive rainwater harvesting at our education and demonstration site

Passive rainwater harvesting freely works without the need of a person, switch or valve. The more passive a system, the less energy it needs.  Earthworks, simple structures and strategies that change the topography and surface of the soil, are a great example of this system. They can speed the infiltration of water, augment soil moisture and nutrient availability and provide large quantities of high quality rainwater to gardens and landscapes. Water can be collected in huge amounts from roofs, streets, vegetation, bare dirt or greywater drains. This water is only available for limited periods after rainfall depending on soil type, mulch, climate and plant uptake but it can recharge groundwater.

Something as simple as applying a layer of mulch around your plants can reduce water usage by 25%, creating a passive system.

Active rainwater harvesting system - one of our ferrocement tanks

Active rainwater harvesting requires a person, switch or valve to actively turn it on/open or off/closed. The more active a system, the more energy it requires. The best example of this are storage tanks or cisterns that collect rainwater from roofs. This allows you to store the water for when you need it, like summer, to be used to water plants or anything else. The downside is these systems tend to cost more than earthworks and limit your storage capacity to the size of the tank.

So which is better? Lets look to nature for the answer since natural systems are the most efficient as they don't need external inputs. For this reason passive rainwater harvesting systems are the best! You only need to build them once, and if done properly, you will never need to maintain them or add anything to them - like healthy soil in your yard. However, active rainwater harvesting systems are still a critical tool for creating water independence. For example, here is San Diego County we can use rainwater harvesting tanks, an active system, to significantly reduce our need to import water from distant places (Click for our past blog on where San Diego's water comes from).

The best is to use both active and passive systems. What's important is to build and install them properly to ensure they work efficiently, creating a sustainable and even regenerative future for us all!

Resources
Brad Lancaster's rainwater harvesting website - full of useful information. His books on rainwater harvesting are the best out there!
https://www.harvestingrainwater.com/

Build healthy soil - a passive system

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