Rainwater Harvesting and Greywater Use for Idaho Landscapes

Alternative Water Sources for Idaho Landscapes

In Southwest Idaho's semi-arid climate, where average annual rainfall barely reaches 12 inches and summer months bring hot, dry conditions, water-conscious homeowners increasingly explore alternatives to traditional irrigation. Rainwater harvesting captures precipitation for landscape use, while greywater systems redirect gently-used household water to outdoor irrigation. Both approaches reduce demand on municipal or well water supplies, lower water bills, and contribute to sustainable landscape management.

For Treasure Valley homeowners committed to water conservation, understanding these alternative water sources—including their benefits, limitations, installation requirements, and legal considerations in Idaho—helps determine whether they're appropriate additions to your landscape water management strategy. While neither approach replaces conventional irrigation entirely in our climate, both can meaningfully reduce water consumption and costs.

Rainwater Harvesting in Idaho

The Potential and Limitations

Idaho's rainfall pattern presents both opportunities and challenges for rainwater harvesting:

• Seasonal distribution: Most precipitation falls October through May; summers are notably dry
• Winter collection challenges: Snow and freezing temperatures limit winter harvesting
• Peak demand mismatch: Highest irrigation needs occur when rainfall is lowest
• Storage requirements: Bridging seasonal gaps requires substantial storage capacity

Despite these challenges, rainwater harvesting makes sense for specific applications in Idaho landscapes, particularly supplementing irrigation during spring and fall shoulder seasons when some rainfall occurs and irrigation demands are moderate.

How Much Water Can You Collect?

Rainwater collection potential depends on catchment area and local rainfall:

Basic formula: Collection (gallons) = Roof area (sq ft) × Rainfall (inches) × 0.623 × Collection efficiency (typically 75-90%)

Roof Area	Per 1" of Rain	Annual Potential (12" average)
1,000 sq ft	~560 gallons	~6,700 gallons
1,500 sq ft	~840 gallons	~10,000 gallons
2,000 sq ft	~1,120 gallons	~13,400 gallons
2,500 sq ft	~1,400 gallons	~16,800 gallons

Compare these volumes to irrigation needs: a 5,000 sq ft lawn in the Treasure Valley might require 80,000-100,000 gallons annually. Rainwater alone won't meet this demand, but can meaningfully supplement other sources.

Rainwater Harvesting System Components

Basic rainwater harvesting systems include:

• Catchment surface: Typically the roof. Material affects water quality—metal and tile are ideal; asphalt shingles are acceptable for landscape irrigation
• Gutters and downspouts: Collect and direct water. Size appropriately for local rainfall intensity
• First-flush diverter: Diverts initial rainfall that washes debris and contaminants from the roof
• Filtration: Screens and filters remove leaves, debris, and sediment
• Storage tanks: Hold collected water. Size depends on collection goals and available space
• Distribution: Gravity feed for simple systems; pumps for pressurized delivery

Storage Tank Sizing

Tank sizing balances collection capacity, irrigation needs, and cost:

• Small systems (50-100 gallons): Simple rain barrels for spot watering and container gardens
• Medium systems (250-500 gallons): Useful for garden beds and supplemental irrigation
• Large systems (1,000+ gallons): Meaningful contribution to landscape irrigation, requiring significant investment
• Cistern-scale (5,000+ gallons): Can substantially supplement irrigation but represents major investment

Installation Considerations

Successful rainwater systems require:

• Adequate roof area: Larger catchment means more collection potential
• Suitable tank location: Level ground, structural support if elevated, frost protection
• Overflow management: Excess water must drain away from foundations
• Winterization: Idaho winters require tank drainage or frost protection
• Mosquito prevention: Screens on all openings prevent breeding
• Connection to landscape: Integration with existing irrigation or separate distribution

Greywater for Landscape Irrigation

What Is Greywater?

Greywater is gently-used water from bathroom sinks, showers, bathtubs, and washing machines. It's distinguished from blackwater (toilet waste) and kitchen wastewater (which contains fats and food particles). Properly managed greywater can safely irrigate ornamental landscapes.

Idaho Greywater Regulations

Idaho regulates greywater systems through the Division of Environmental Quality. Key requirements include:

• Systems must follow the International Plumbing Code provisions adopted in Idaho
• Greywater must be used for subsurface irrigation only (no surface ponding or spraying)
• Food gardens require specific separation from greywater application
• Permits may be required depending on system size and local jurisdiction
• Systems cannot discharge to surface water or groundwater
• Overflow must connect to the septic or sewer system

Local jurisdictions may have additional requirements—check with your city or county building department before installation.

Greywater Volume Potential

A typical household generates substantial greywater:

• Showers and baths: 10-25 gallons per use
• Bathroom sinks: 1-2 gallons per use
• Washing machines: 15-40 gallons per load

A family of four might generate 40-80 gallons of usable greywater daily—potentially 12,000-24,000 gallons during the irrigation season. This represents meaningful irrigation supplementation.

Greywater System Types

Systems range from simple to sophisticated:

• Laundry-to-landscape: Simplest system, redirects washing machine water directly to subsurface irrigation through mulch basins. Often doesn't require permit in Idaho.
• Branched drain systems: Gravity-fed distribution through branching pipes to multiple landscape areas. Requires careful design for proper flow splitting.
• Pumped systems: Collect greywater in tanks and pump to irrigation areas. Allows greater flexibility in distribution but requires more equipment.
• Treatment systems: Filter and treat greywater before distribution. Required for some applications and provides better water quality.

Safe Greywater Practices

Proper greywater use requires attention to safety:

• Subsurface only: Greywater must soak into soil, not pond on surface or contact people
• Avoid food gardens: Don't use greywater on vegetables or fruits eaten raw
• Use "greywater-safe" products: Avoid bleach, boron, sodium-heavy detergents
• Rotate application areas: Prevent salt and residue buildup in one location
• Don't store: Use greywater within 24 hours to prevent bacterial growth
• Divert when inappropriate: Divert to sewer during illness, when washing diapers, or using harsh chemicals

Combining Alternative Water Sources

Integrated Water Strategy

The most effective approach combines multiple water sources:

• Municipal/well water: Primary irrigation source during peak summer demand
• Rainwater: Supplements irrigation during shoulder seasons when some rain falls
• Greywater: Provides consistent daily volume for designated landscape areas
• Smart irrigation: Optimized sprinkler systems maximize efficiency of all water sources

Landscape Design for Alternative Water

Landscapes can be designed to maximize alternative water use:

• Group plants by water needs, directing limited alternative water to appropriate areas
• Create greywater-specific zones with tolerant plants and mulch basins
• Position rainwater collection near garden beds for easy distribution
• Use mulch extensively to reduce overall water needs
• Select drought-tolerant plants that can survive on reduced irrigation

Practical Considerations for Idaho Homeowners

Cost-Benefit Analysis

Evaluate alternative water systems realistically:

• Simple rain barrels: Low cost ($50-150), limited volume, good for containers and small gardens
• Larger rainwater systems: Moderate to high cost ($500-5,000+), useful supplementation but limited by Idaho's rainfall
• Laundry-to-landscape greywater: Low cost ($200-500), consistent volume, relatively simple installation
• Complete greywater systems: Higher cost ($1,500-5,000+), more complex but greater capacity

Payback periods depend on local water costs and system complexity. In the Treasure Valley, where water costs remain relatively low, environmental motivation often outweighs financial return.

Winter Considerations

Idaho winters require seasonal management:

• Rainwater tanks must be drained or protected from freezing
• Greywater systems may need winterization or diversion to sewer
• Exposed pipes require freeze protection
• Spring activation should include system inspection and cleaning

Maintenance Requirements

Alternative water systems need regular attention:

• Clean gutters and screens before and during rain season
• Inspect tanks and connections for leaks and damage
• Clear any debris from distribution systems
• Monitor soil in greywater areas for salt or pH changes
• Service pumps and filters as required

Getting Started

Start Small

Beginning with simple systems allows learning without major investment:

• Install one or two rain barrels for container watering
• Try a laundry-to-landscape greywater setup for one landscape area
• Evaluate results before expanding systems

Professional Consultation

Complex installations benefit from professional guidance:

• Plumbers for greywater system installation
• Irrigation specialists for integration with existing systems
• Local code officials for permit requirements
• Landscape professionals for plant selection and layout

Lawn Care Kuna provides comprehensive landscape and irrigation services throughout Kuna, Meridian, Boise, Eagle, Star, and the Treasure Valley. While we don't install rainwater or greywater systems, our irrigation services help maximize efficiency of all water sources, and our landscape expertise can help design water-wise landscapes that work with alternative water systems. Request a free consultation or contact us to discuss sustainable landscape management for your property.