Harvested Rainwater Guidelines

2.0 Rainwater for Irrigation

• Since the largest need for irrigation water in our area occurs during the time of lowest rainfall and highest temperature, a rainwater system designed to meet this need will have to capture water prior to the summer.

• The size of the storage system may be prohibitive for using rainfall for the sole source of irrigation water in large or water-intensive landscapes. A low water demanding landscape is required.

• The average rainfall will not indicate the actual amount that will fall in any particular year.

• Table 3.0 shows the amount of gallons of rainwater that can be captured from rain for various roof areas in smaller increments than Table 1.0 (these are termed rainfall "events") and the gallons of water it takes to irrigate various landscape areas to equal a certain amount of rainfall.

• Table 3.0 and the average rainfall amounts are useful in calculating the storage size and roof area associated with various irrigation requirements.

• 2.1 Example of Irrigation Requirement Estimation

  The landscape to be irrigated for this example consists of 2,500 square feet. It is determined through consultation with landscape specialists that the plants should receive a minimum of one inch of rain per week to be healthy from June through September. The roof area for collection in this example will be 1,500 square feet.

  • 1. Table 3.0 shows that 2,500 square feet of landscape area requires a little over 1400 gallons of water to equal one inch of rain. (Find 2,500 in the landscape/roof size column and follow across to the one inch rainfall column.)

  • 2. In 16 weeks (June - September), the water requirement is 22,400 gallons. (16 weeks x 1400 gallons per week)

  • 3. Choose if you wish to assume average rainfall or a lesser amount will fall during this period. For this example, we will estimate that only half of the average summer rainfall will occur. (June through September rainfall totals 10.79 inches. We will assume therefore only 5.25 inches will fall.)

  • 4. Select the amount of gallons from Table 3.0 that this amount of rainfall will equal. The five inch column for 2500 feet of area equals 7,023 gallons. The 0.25 inch column for 2,500 feet gives 351 gallons. (The total is 7, 374 gallons. This is the amount of natural rainfall the landscape will receive.)

  • 5. Subtract the natural rainfall (7,374) from the required amount (22,400) for the net need of the landscape. This amount equals 15,026 gallons. This is the amount of water that will need to be collected for irrigating the landscape when rainfall is half the average amount.

  • 6. The roof area during this period will similarly receive 5.25 inches of rain which can be collected for irrigation purposes. Locate the 5 inch column and the 0.25 inch column totals for 1,500 square feet of roof/landscape area. (The 5 inch total is 4,214 gallons and the 0.25 inch column gives 211 gallons for a total of 4,425 gallons.)

  • 7. Subtract the amount the roof will collect in step #6 (4,425 gallons) from the required amount in step #5 (15,026 gallons). (15,026 minus 4,425 equals 10,600 gallons. This is the amount of rainwater that must be in storage prior to June for use as irrigating water for the landscape if rainfall is one half the average amount.)

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