NEW MEXICO (KRQE) – The monsoon season seems to have begun early in New Mexico this year, but how can you take advantage of the extra rainfall? Bernalillo County has released its Passive Rainwater Harvesting Guide for New Mexico homeowners.

The guide is in partnership between the county, the Cuidad Soil and Water Conservation District, and the Arid Low Impact Development Coalition; It gives step-by-step instructions on how to develop a passive rainwater harvesting system – providing New Mexico homeowners with knowledge on how to use the resource.

Summary of Rainwater Harvesting Guide

What is Rainwater Harvesting?

“In the broadest sense, rainwater harvesting is simply the collection and storage of rainwater.”

Steps to Create a Rainwater Harvesting System

  • Site Assessment
    • Locate the current path water takes, utility locations, and soil type.
      • Where does water come off your roof? Does any water come into your yard from your neighbor’s yard? Where does the water from your driveway or patio go? Are there places where puddles form that are slow to dry? Where does water leave your yard?
    • Identify possible locations for a water retention basin or swale – places that will store rainwater and fill up during a storm.
    • Make sure there are no utilities (gas, water, sewer lines) running through the selected water-storage location.
    • Consider your soil type and infiltration rate (the rate at which water enters the soil).
      • Test some water on your soil to make sure it drains within 24 hours before placing a basin or swale there. Water that drains slower than that creates a breeding ground for mosquitos and other pests.
        • To test your soil, dig a hole the size of a five-gallon bucket. Fill it with water and let it soak in. Fill it with water a second time and see how long it takes for all the water to infiltrate.
      • Figuring out your soil type tells you how often you will need to water your plants based on how long the ground will retain the water.
  • Sizing Your Basin
    • Determine the area of the surface generating the runoff you want to use (i.e. driveway, roof, patio).
      • Multiply the length and width of each area to find the number of square feet.
    • Find the runoff coefficient of that area (this is the percentage of rainwater that will become runoff and not be absorbed by the surface).
      • Metal (0.95), asphalt (0.90), concrete (0.90), PVC geomembrane (0.85-0.90), roof tiles (0.80-0.90), tar and gravel (0.80-0.85).
    • Chose a rainstorm size, in inches, and convert inches to a fraction of a foot (1 inch = 0.083 feet). The county recommends using a rainstorm size of 1 inch since that is the most frequent size of rainstorm the area gets.
    • Calculate the runoff volume by multiplying the aforementioned factors.
      • Example: 1,000 square feet (runoff surface area) X 0.90 (runoff coefficient) X 0.083 feet (rainstorm size in feet) = 73 cubic feet (in runoff volume).
    • Calculate the basin size.
      • Divide the result above by the depth of your basin to get the basin area in square feet.
        • Example: If the basin is 6 inches deep, 75 feet / 0.5 feet (6 inches) = 150 square feet. If the basin is 2 feet deep, 75 feet/2 feet = 37.5 square feet.
  • Plant Selection

For further information on swale installation, basin installation, planting techniques, irrigation installation, mulch information, watering, pruning, and more, view the full guide here.

“Catching rain in barrels for future use is beneficial, but much can be missed,” says Water Conservation Program Lead Megan Marsee. “Passive rainwater harvesting allows homeowners to capture rainwater in the ground, where it soaks into the soil and supports plants.”

Benefits of Harvesting Rainwater

  • Keeps rainwater from running into the street – protecting and improving the water quality in the Rio Grande
  • Conserves residential water for drinking, as it is not used mainly for landscaping
  • Helps foster beautiful landscapes that require little to no irrigation

To view the full Residential Rainwater Harvesting guide, click here.