Rainwater Harvesting
Whether rainwater is collected in a bucket or a large capacity storage tank, it is important to properly plan the design of any new rainwater collection system to ensure it will meet future needs. Taking the time to work through the below considerations can help establish the appropriate quantity of water needed and pinpoint the primary purpose of the system.
Pre-Design Considerations
What will the water be used for?
This is an important one to start with because the use will determine the water quantity needed and tank size. Some uses would include irrigation, household water source, livestock watering, and flushing toilets.
How much water can be collected?
Generally, every inch of rainfall that falls on 1,000 square feet of a collection surface will produce approximately 600 gallons of water. A good rule of thumb is to design the collection system to hold a 2” - 3” rain event.
Space Availability
Space availability can often be a limiting factor and should be considered carefully before purchasing any equipment. Common spacing limitations can be attributed to already established landscaping, such as trees, pathways, water features, or fencing. Another consideration is the height of your house eaves when shopping for a tank. Are gutters present?
Does the local municipality or groundwater district offer rebates?
Check your local water municipality or groundwater district for any rebate programs available. Any cost savings opportunities are helpful when installing a new system. Rainwater harvesting equipment may also be eligible for a tax exemption!
Is your HOA or POA aware of the Texas Property Code Sec 202.007?
HOA’s cannot ban rainwater harvesting systems, but they may limit the location of a tank.
So now that all of the sizing and purpose considerations have been established it is time to start finalizing the design detail!
Proper Tank Sizing
Remember the general rule of thumb is to plan for a 2”-3” rain event. There are multiple rainwater collection calculators available if math just is not a strong point! If a 2,000 square foot roof will be the collection site the math would look something like this:
1” of rainfall = 600 gallons per 1,000 square feet of covered space
600 gallons x 2 = 1,200 gallons of rainwater per 1” of rainfall
A suitable tank size that fits the rule of thumb would be:
2” rainfall: 2,400-gallon tank
3” rainfall: 3,600-gallon tank
2. Inlet Size and Overflow
Overflow will happen during a rainy year or with decreased water usage, it is best to plan ahead by sizing the overflow size to the inlet size. If you are working with a tank company this is a good time to bring in your specifications to ensure that proper sizing is done ahead of time. If the tank is 1,000 gallons or less more than likely collection will be from one downspout and a 3” inlet will work great unless the gutters are 6 inches. In that case, a 4-inch inlet would be a better option. Again, consulting with a system expert might save time, money, and headaches down the road!
3. Outlet Size
When determining the outlet size, consider the method of water withdrawal to aid in this decision. If using a garden hose a ¾” bulkhead connection will allow for a screw-in spigot.
4. To Pump or Not to Pump?
Pumps are handy but not necessary. If the system will be used to pump into a house or into an irrigation system a pump is necessary in order to pressurize and move the water. Pressurized systems will require backflow protection devices, especially within the city limits. A submersible sump pump is also a suitable pumping solution.
5. Keeping the Tank Clean
Screens will be a tank’s best friend when it comes to cleanliness. When leaves clog the gutters, the leaves will also clog the tank so invest in gutter screens. An additional basket on the tank can also help.
System Installation
Choose the Base Wisely!
It is important to install the tank on a solid base. Believe it or not, water weighs about 8.3 pounds per gallon! A 2,500-gallon tank weighs over 10 tons when full! Some material options that will support the entire tank bottom are crushed granite, limestone blocks, pavers, and of course concrete. One other factor to consider is the height of the base to ensure easy access to spigots and attached hoses.
Fitting Positioning
When positioning the overflow make sure not to place it above the outlet. Be sure to also position the outlet towards the bottom of the tank with enough room to remove or install a garden hose and turn the spigot on and off.
No rainwater collection system is the same. Careful planning, in the beginning, will save a headache later. Happy collecting!