Caldwell, ID Landscaping Rules: Rainwater Harvesting (2026)

Rainwater harvesting in Caldwell is governed primarily by Idaho state water law rather than a dedicated city ordinance. Idaho operates under the prior-appropriation doctrine, and all water in the state is administered under Idaho Code Title 42. The practical guidance, reflected in the Idaho Attorney General's 2008 analysis, is that a property owner may capture and use diffused surface water, including rooftop rainwater that has not yet entered a natural stream or drainage, on the property where it falls, provided the collection does not injure or interfere with senior, existing water rights. This makes typical residential rain barrels and small rooftop collection systems generally permissible across Idaho, including in Caldwell. Larger-scale capture, or capture that would intercept water otherwise flowing to a watercourse to which others hold rights, can implicate the water-right permitting system administered by the Idaho Department of Water Resources. Caldwell's published code does not appear to add a separate permit requirement for residential rain barrels, though collected water used for landscape irrigation should still be plumbed safely and kept separate from the potable system. Because Caldwell sits in an irrigation-district area, harvested rainwater is best viewed as a supplement to, not a replacement for, the pressurized irrigation supply.