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Rainwater Irrigation System: A Guide to Planning

November 1, 2021

Are you thinking about harvesting rain to use for your garden or landscaping? Read on for our Guide to Planning a Rainwater Irrigation System.

Rainwater is especially well-suited for irrigating raised beds, drip irrigation, trees, and small lawns. Not only will your water bill benefit, but plants love the soft, untreated water and reward you with bigger blooms, tastier fruits, and veggies than watering with well or city water.

Irrigation systems come in all sizes, from smaller barrels to massive, commercial-grade tanks. Harvest Rain starts our installations at 1,000 gallons and scales up from there, depending on the area of your roof used for collection and how you’ll use the collected water. 

When designing a rainwater irrigation system, we consider a few variables to ensure the final product is sized correctly and lives up to expectations, including:

  1. How much water do you use for irrigating? 
  2. How much water can you collect? 
  3. What will you be irrigating?
  4. Do you have a good spot for the right-sized tank?

HOW MUCH WATER DO I USE? 

We have several methods to help estimate how much water you are currently using or will use in the future.

If you are on city water:

1) Call your utility company…

Your water company should be able to provide your property’s historical usage. You can tease out your irrigation numbers from this data by comparing months you water to months you don’t. 

Example using historical water bills:

  • December 2020, no watering: 4,000 gallons
  • July 2021: 10,000 gallons
  • Estimate of Gallons Used for Irrigation in July: 6,000 gallons

2) Alternatively, check your water meter

Take a look at your meter directly before running your irrigation, noting the reading. Check the meter again after your irrigation cycle is complete. The difference is how many gallons you’ve used in a single cycle.

3) If you have a sprinkler system…

Your sprinkler system may have ‘GPM’ listed per Spray Head Flow at the controller unit, indicating how many gallons of water are used per minute either by zone or actual spray head. If so, you can do some quick math to estimate your irrigation usage.

EXAMPLE:

  • Zone 1 – 2: 2 GPM / each
  • Zones 3 – 5: 8 GPM/each
  • Zones 6 – 10: 6 GPM /each
  • Total = 58 gallons of water/minute

Your water usage for (1) 20-minute cycle of irrigation using all zones= 1,160 gallons. If you are running your sprinklers 2x week = 2,320 gallons/week or 9,280 gallons/month.

If you are in the process of building a new home…

If you are working with a landscaper, you can request a projected water usage plan for their proposed design. This plan will outline how much water your landscaper anticipates you will need on a weekly, monthly, or yearly basis to establish new plants/grass as well as projected numbers for maintaining them.

On a well and unsure of how much water you use? 

Many of our customers have more than one water source on their property and are looking to augment their well usage with rainwater. But it can be challenging to know exactly how much water you’re using without a meter. We can use some rough estimates to help get you closer to your number:

Generally speaking, most Texas lawns need 1 to 1.5″ of water per week during the growing season. Let’s take two examples to explore what that means in gallons.

  • Example 1: A typical American backyard on a quarter acre property is about 8,500 square feet. Each 1,000 sq. ft. of lawn requires 624 gallons of water to receive 1″.
  • If your yard is 8,500 sq. ft = 5,322 gallons of water used to get 1″ of water into the lawn (often split up between multiple watering cycles/week).
  • Example 2: If you are looking at a more significant property and determining water usage, one acre = 43,560 sq. ft. 
  • Irrigating one acre of grass with 1″ of water = 27,137 gallons of water/cycle. 

A note about grass:

Using the math above, if you have a large lawn, it can be difficult to capture enough to rely on harvested rainwater alone for your irrigating needs. However, if you have considerable roof space and are open to a bigger storage tank, then one of our designs can likely meet your needs.

HOW MUCH WATER CAN I COLLECT?

Once we have a solid working number of how much water we need, we start to look at the square footage of your roof and how much of the roof we will need to utilize. It’s not always necessary to maximize your roof’s capacity. For example, if you are simply irrigating a small veggie garden or flower beds, a 1,000-gallon tank may suffice. In that instance, we would try to identify a nice straight run of gutter from a portion of your roof that will easily fill the tank. 

Conversely, if we are looking to meet large irrigation demands, we will likely want to collect as much water as we can. This means we will design a system that captures rain from the entirety of your roof.

SIZING A TANK + CONFIRMING A GOOD SPOT

Our goal is typically to provide for 8-12 irrigation cycles with your rainwater catchment system. Depending on your irrigating patterns, this could be anywhere from 1-2 months’ worth of water.

If we have determined you use approximately 1,740 gallons per irrigation cycle, we’d like to aim for 14,000-20,000 gallons of stored water. 

Once we have established these working numbers, our system designers will work with you to select the final tank size. The final variable we’ll want to nail down is the actual spot for tank. Below are several common rainwater irrigation tank dimensions. However, tanks come in many more capacities. 

Common Irrigation Rainwater Sizes

OUR GOAL WITH A RAINWATER IRRIGATION SYSTEM

We always want to provide our customers with a rainwater irrigation system that meets their expectations and clearly identifying how much water you need and how much water you can collect at the onset allows us to do just that. 


PLANNING A NEW HOUSE? 

Go Native! With any new landscaping, you will initially use more water to establish plants and trees. However, when you opt for native plants and grasses, after 1-2 years, you can back off on supplemental irrigating and let nature take the reins. 

Additional Resources:

Texas AgriLife Extension is an excellent resource for landscape planning. They provide comprehensive lists of trees, shrubs, perennials, ornamental/prairie grasses, and more to help you select plants that tend to be drought-tolerant and require little to no fertilizers (both of which contribute to healthy rivers, lakes, and aquifers – something we really get behind!) 

Find the full ‘Native & Adapted Landscape Plants Guide’ PDF for the City of Austin and surrounding areas here.

 

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