Passive water harvesting is the utilization of water-harvesting earthworks, vegetation, and/or other associated soil life to create a living sponge to capture, infiltrate, store, multi-cycle, and use these waters to grow more life, health, and resources. Rainwater is the primary water to be harvested, because if you first “plant the rain” then you have the foundation in which you can harvest all other waters (stormwater runoff, greywater, condensate, etc.) with the free power of gravity. Once in place, you don’t need to be around for things to work—a passive water harvesting system, or rain garden, works with or without you (though you learn more, and you can better enhance the system if you regularly observe and steward it).
A core aim of passive water-harvesting is to grow or enhance regenerative water-harvesting systems that get better with time, and which can repair and/or reproduce themselves because so much of these systems is alive. The soil and its vegetation is the living “tank;” and vegetation, mycorrhizal fungi, and other life are the living “filters” and “pumps” that then make that harvested and cleaned water accessible in the form of fruit, fodder, shade, shelter, beauty, habitat, erosion-control, medicinals, craft materials, lumber, and more.
The essence of passive water harvesting is the life of the passive system, the life the system supports and enables, and how well it synergizes with its unique surroundings and place. Thus, native vegetation is typically emphasized as it has evolved over millennia to be the best adapted to the site’s climate, soils, and wildlife; while also being intimately woven into the history of the site’s human cultures. (For example, a native velvet mesquite tree in my area attracts and supports over 60 species of native pollinators, while a non-native mesquite only supports 12 pollinators. The native mesquite also tells us when to plant our spring garden, as it leafs out after the last frost). So, the earth form/shape, rockwork, or mulch of a water-harvesting earthwork are in service of, not the focus of, a passive water-harvesting system’s essence.
A well-designed passive water-harvesting system does not need supplemental water once established (newly planted vegetation may need supplemental irrigation in its first one to three years to get it established). And the passive system infiltrates/gives more water back to the natural system than it takes from it.
Before. Contour berms of woodchips. Chris Meuli’s land, Edgewood, New Mexico. Credit: Chris Meuli. Reproduced with permission from Rainwater Harvesting for Drylands and Beyond, Volume 2, 2nd Edition.
After. Abundant growth of new grasses above and below woodchip berms. Chris Meuli’s land, 2006, post rains. Credit: Chris Meuli. Reproduced with permission from Rainwater Harvesting for Drylands and Beyond, Volume 2, 2nd Edition.
Free gravity—not costly mechanical pumps—powers the distribution of water to and from passive water-harvesting strategies. Utilize gravity, and you are collaborating with the free and natural. Gravity never breaks down. These passive strategies—and the water they harvest—cycle and reinvest back into the natural system, which can in turn directly recharge and enhance groundwater aquifers, springs, wetlands, creeks, and rivers.
• For more advantages of passive water harvesting, and how it compares to active water harvesting see here
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Volume 1 in its appendix 4, has info on how to figure out the water budgets of your rain gardens’ vegetation, so you can plant in a way that once your plants are established they can be irrigated solely with free on-site waters (just via passively harvested water, or also supplemented with rainwater actively harvested in a tank, and/or supplemented with passively harvested greywater). This way you sustainably put more water back into the system than you take out, thereby enhancing the resiliency of our groundwater and surface waters (springs, creeks, and rivers) for all.