Lawn Irrigation with Harvested Rainwater in Washington

Using harvested rainwater for lawn irrigation in Washington is a practical way to reduce your water bill and conserve municipal supplies. With 38.4 inches of average annual rainfall, a 1,500-square-foot asphalt shingle roof in Washington can collect approximately 27,511 gal per year. lawn irrigation requires roughly 75 gallons per day, during the growing season (April through October). With this setup, rainwater can meet approximately 100.0% of your lawn irrigation needs, saving an estimated $96.30 per year.

Annual Harvest 27,511 gal 1,500 sq ft roof
Annual Demand 16,050 gal lawn irrigation
Demand Met 100.0% of lawn irrigation
Rec. Tank 5,000 gal storage capacity

Monthly Supply vs. Demand for Lawn Irrigation in Washington

The table below shows how your monthly rainwater harvest compares to lawn irrigation demand throughout the year. Green values indicate months where your harvest exceeds demand (surplus water for tank storage), while red values show months where you will draw down your stored reserves. Since lawn irrigation is a seasonal activity, demand is zero from November through March, allowing you to build up tank reserves during the off-season.

Month Harvest (gal) Demand (gal) Balance
January 4,037 0 +4,037
February 2,841 0 +2,841
March 2,691 0 +2,691
April 1,944 2,250 -306
May 1,495 2,325 -830
June 1,196 2,250 -1,054
July 598 2,325 -1,727
August 598 2,325 -1,727
September 1,196 2,250 -1,054
October 2,392 2,325 +67
November 4,187 0 +4,187
December 4,336 0 +4,336

Harvest by Roof Size for Lawn Irrigation

A larger roof collection area directly increases your harvest and the percentage of lawn irrigation demand you can meet with rainwater. The table below shows how different roof sizes perform in Washington for lawn irrigation.

Roof Area Annual Harvest Demand Met Rec. Tank Savings/Year
1,000 sq ft 18,341 gal 100.0% 5,000 gal $96.30
1,500 sq ft 27,511 gal 100.0% 5,000 gal $96.30
2,000 sq ft 36,682 gal 100.0% 10,000 gal $96.30
2,500 sq ft 45,855 gal 100.0% 10,000 gal $96.30

Setting Up Rainwater Harvesting for Lawn Irrigation in Washington

Lawn irrigation is the most water-intensive residential use, and harvesting rainwater for this purpose in Washington can dramatically reduce your municipal water consumption. For effective lawn watering, you need a pump-and-pressure-tank system capable of running sprinklers — gravity-fed systems do not provide sufficient pressure. A 1,000-2,500 gallon tank is recommended, connected to your existing sprinkler system through a transfer pump. Many homeowners install a dual-source controller that draws from the rainwater tank first and automatically switches to municipal supply when the tank runs low. Consider reducing your lawn area or switching to drought-tolerant grasses to decrease water demand. In Washington, cool-season grasses go dormant in summer heat and may not need irrigation during the hottest months.

Metal Roof Upgrade: Lawn Irrigation in Washington

Upgrading to a metal roof increases your collection efficiency from 80% (asphalt shingle) to 95%, boosting your annual harvest to 32,670 gal from a 1,500-square-foot roof. This increases your lawn irrigation demand coverage to 100.0% and raises annual savings to $96.30. Metal roofs also last 40-70 years compared to 20-30 years for asphalt shingle, and their smooth surface sheds debris more effectively, reducing maintenance on your collection system and improving water quality.

Other Uses for Rainwater in Washington

Explore how harvested rainwater can serve other needs in Washington:

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