Rain Water Harvesting for Outdoor Irrigation

Rainwater harvesting captures free water falling on your roof, storing it for landscape irrigation and outdoor use. A single inch of rain on a 1,000 square foot roof yields 600 gallons. In climates with 30+ inches annual rainfall, roofs can harvest 18,000+ gallons yearly.

The economics depend on rainfall patterns, outdoor water use, and local regulations. Simple rain barrel systems cost under $100 and pay back within a year. Complex cistern systems cost thousands and may never recoup installation costs through water savings alone.

How Rainwater Harvesting Works

Rain falls on your roof, flows to gutters, and directs to downspouts. Instead of routing this water to storm drains or the ground, harvesting systems capture and store it.

Basic systems use barrels or tanks connected directly to downspouts. Water fills the containers during rain events, and you use it later for watering plants.

Advanced systems include filters to remove debris, first-flush diverters that discard initial dirty water, and distribution systems with pumps and hoses.

System Types and Costs

Rain Barrels

The simplest harvesting method places 50-100 gallon barrels beneath downspouts. A spigot near the bottom allows you to fill watering cans or attach hoses.

Cost: $50-$200 per barrel installed

Most homes have 4-8 downspouts, so multiple barrels capture more water. Linking barrels in series or parallel increases capacity without adding many components.

Benefits: Low cost, simple installation, immediate use Drawbacks: Limited capacity, manual use, seasonal mosquito habitat if not sealed

Cisterns and Large Tanks

Cisterns are large storage tanks holding 200-5,000+ gallons. They install above or below ground and require more complex plumbing.

Above-ground tanks cost $200-$2,000 depending on capacity. Below-ground cisterns cost $1,500-$5,000+ including excavation and installation.

Larger capacity means you can capture significant rainfall from major storms and use it throughout dry periods.

Benefits: Large capacity, automated distribution possible, less maintenance than multiple barrels Drawbacks: High initial cost, space requirements, installation complexity

Complete Harvesting Systems

Full systems include gutters, filters, first-flush diverters, storage tanks, pumps, and distribution networks.

Cost: $2,000-$10,000+ depending on capacity and automation level

These systems can irrigate entire landscapes automatically, mimicking municipal water convenience.

Benefits: Maximum capacity, automation, filtration, professional appearance Drawbacks: High cost, maintenance requirements, potential permit requirements

Calculating Harvestable Rainwater

Roof area determines collection capacity. Measure your roof's footprint (the ground area it covers), not the actual sloped surface area.

Formula: Roof area (sq ft) x Rainfall (inches) x 0.623 = Gallons harvested

Example: A 1,500 sq ft roof in a climate with 25 inches annual rainfall: 1,500 x 25 x 0.623 = 23,363 gallons potential annual harvest

This assumes 100% capture efficiency. Real-world efficiency is 75-90% due to evaporation, spillage, and system losses.

Realistic harvest: 23,363 x 0.85 = 19,858 gallons annually

Storage Needs

You don't need capacity to store every drop. Storage should handle typical storm volumes while providing enough water between rain events.

For most climates, storage equal to 1-2 times your largest monthly outdoor water use works well.

If you use 500 gallons monthly for irrigation, plan for 500-1,000 gallons storage capacity. More storage helps in dry climates with long intervals between rains.

Water Quality Considerations

Rainwater quality depends on your roof material, local air quality, and storage practices.

Suitable Roof Materials

Metal, tile, slate, and composite shingles all work for harvesting. These materials don't leach chemicals that harm plants.

Treated wood shingles can release preservatives. Older homes might have asbestos shingles that shouldn't be used for harvesting.

Lead-based roof paints or flashing can contaminate water. If your roof predates 1978, test for lead before using harvested water on food crops.

Debris and Contaminants

Roofs collect leaves, bird droppings, dust, pollen, and atmospheric pollutants. The first portion of rainfall during each storm washes this material off.

First-flush diverters automatically discard the first 10-20 gallons from each storm, sending cleaner subsequent water to storage.

Filters remove leaves and larger debris before water enters storage. Mesh screens over gutter openings and downspout filters provide basic protection.

Storage Water Quality

Water sitting in warm barrels or tanks can develop algae, bacteria, and mosquito larvae.

Dark-colored or opaque containers limit light penetration, reducing algae growth.

Sealed lids prevent mosquito breeding while allowing air exchange.

Using water regularly prevents stagnation. Water that sits for months deteriorates in quality.

Uses and Limitations

Harvested rainwater works perfectly for:

• Landscape irrigation
• Lawn watering
• Garden beds and trees
• Washing vehicles
• Non-potable outdoor uses

Most jurisdictions prohibit or restrict using rainwater for:

• Drinking water (without treatment)
• Food crop irrigation (in some areas)
• Indoor use

Check local health department regulations before using harvested water for anything beyond basic irrigation.

Economics and Payback

Water Savings

Calculate potential savings using your water utility rate and outdoor water consumption.

Example calculation:

• Outdoor water use: 5,000 gallons/month during summer (6 months)
• Total annual outdoor use: 30,000 gallons
• Water/sewer rate: $0.015 per gallon
• Annual cost: 30,000 x $0.015 = $450

If rainwater harvesting displaces 50% of outdoor water purchases: Annual savings: $450 x 0.50 = $225

Simple Rain Barrel System

Cost: $200 for two 50-gallon barrels installed Annual savings: $50-$100 (conservative estimate) Payback period: 2-4 years

This assumes DIY installation and modest water rate. Higher water rates improve payback.

Cistern System

Cost: $3,000 for 500-gallon above-ground cistern with basic filtration Annual savings: $150-$300 Payback period: 10-20 years

Professional installation adds to costs, extending payback further.

When Economics Favor Harvesting

High water and sewer rates: Areas with rates exceeding $0.02 per gallon see faster payback Large irrigation demands: Properties needing extensive landscape watering benefit most Tiered water pricing: Utilities charging premium rates for high usage make harvesting attractive Water restrictions: Drought-prone areas with use restrictions or penalties

When Economics Don't Work

Low rainfall: Areas with less than 20 inches annual rainfall collect insufficient water Minimal outdoor use: Properties with small yards or drought-tolerant landscaping see little benefit Cheap water: Municipal rates below $0.01 per gallon create long payback periods Rental properties: Short-term occupancy prevents recouping installation costs

Regulatory Landscape

States Encouraging Harvesting

Arizona, Colorado, Nevada, New Mexico, Texas, Utah: Active promotion and incentives California, Oregon, Washington: Permitted and encouraged with specific guidelines Georgia, North Carolina, Virginia: Permitted with varying requirements

These states often provide tax credits, rebates, or streamlined permitting.

States with Restrictions

Some states historically restricted rainwater harvesting due to water rights doctrine. Most restrictions have been removed or reduced, but check current laws.

Colorado now allows residential harvesting with specific barrel limits after years of restriction.

Municipal Regulations

Even in states allowing harvesting, local ordinances may:

Require permits for large systems Specify approved storage materials and locations Mandate overflow management to prevent drainage problems Include backflow prevention if connected to any municipal water Restrict or prohibit indoor use

Check with local building departments before installing systems requiring permits.

HOA Restrictions

Homeowner associations may prohibit visible rain barrels or cisterns regardless of state law.

Review your HOA covenants before purchasing equipment. Some HOAs approve systems if they're screened from view or installed in specific locations.

Installation Considerations

Gutter Requirements

Rainwater harvesting requires functional gutters in good condition. Homes without gutters need them installed first, adding $500-$2,000 to project costs.

Clean gutters before harvesting. Accumulated debris creates dirty water and clogs filtration.

Foundation Drainage

Disconnecting downspouts from foundation drainage to fill barrels can create foundation problems if overflow isn't managed properly.

All systems need overflow mechanisms that direct excess water away from foundations.

Mosquito Control

Open barrels become mosquito breeding sites within days. Sealed systems with fine mesh screens prevent entry.

Some harvesting systems add mosquito dunks (larvicide tablets) that kill larvae without harming plants. These cost $10-$20 annually.

Freeze Protection

Water expands when frozen, cracking barrels and cisterns. Cold-climate systems need winterization.

Drain barrels completely before freezing weather. Below-ground cisterns below frost depth don't freeze, but above-ground tanks must be emptied or placed indoors.

Pump Requirements

Gravity feed works if barrels are elevated above irrigation points. Otherwise, pumps are necessary.

Simple hose-end pumps cost $30-$100. Permanent pump installations cost $200-$600.

Pumps require electricity, adding to operational costs and complexity.

Maintenance Requirements

Monthly Tasks

Inspect gutters and remove debris Check screens and filters for clogs Verify overflow systems direct water properly Use stored water to prevent stagnation

Seasonal Tasks

Clean gutters thoroughly before rainy seasons Inspect barrels or tanks for cracks or leaks Test pumps and distribution systems Winterize systems in cold climates

Annual Tasks

Deep clean storage containers Replace filters or screens showing wear Inspect and repair gutters and downspouts Check all connections for leaks

Time investment: 1-2 hours monthly during active use, 4-6 hours annually for deep maintenance

Maximizing Benefits

Right-Sized Systems

Match storage capacity to actual irrigation needs and rainfall patterns. Oversized systems cost more without additional benefit.

Start small with one or two rain barrels. Expand if you consistently empty them and need more capacity.

Efficient Irrigation

Combine harvesting with efficient irrigation methods. Drip systems and soaker hoses use 30-50% less water than sprinklers, making harvested water go further.

Water during early morning or evening to minimize evaporation.

Mulch garden beds to retain moisture, reducing overall irrigation needs.

Drought-Tolerant Landscaping

Native plants and drought-tolerant species need minimal supplemental water. This reduces irrigation demand and makes harvested water sufficient.

Replacing high-water lawns with lower-water alternatives maximizes rainwater harvesting effectiveness.

Alternative Water Conservation

Before investing in rainwater harvesting, consider simpler conservation methods:

Low-flow fixtures reduce indoor consumption Efficient irrigation systems cut outdoor waste by 30-50% Drought-tolerant landscaping eliminates much outdoor water demand Mulching and soil improvement retain moisture naturally

These methods may provide better return on investment than harvesting systems in some situations.

The Bottom Line

Rainwater harvesting makes environmental and economic sense in specific situations. Properties with high outdoor water use, expensive water rates, and adequate rainfall benefit most.

Simple rain barrel systems costing under $200 often pay back within 2-4 years through water savings and provide environmental benefits.

Expensive cistern systems rarely make purely economic sense unless water rates are very high or property value increases from the system justify costs.

Before installing systems, calculate your actual outdoor water consumption, research local water rates including tiered pricing, and verify local regulations permit harvesting.

Start small if you're unsure. Two rain barrels cost little and demonstrate whether harvesting works for your situation. You can always expand successful systems.

Rainwater harvesting won't solve all outdoor water needs in most climates, but it supplements municipal water, reduces demand during droughts, and provides free irrigation during growing seasons.

The best systems combine rainwater harvesting with efficient irrigation and appropriate landscaping. This integrated approach maximizes water conservation while minimizing costs.

Evaluate your specific situation before investing. Rainwater harvesting is a valuable tool, but it's not the right solution for every property or climate.