Rainwater Harvesting to Fuel Irrigation

People have stored irrigation water since the beginning of organized agriculture. Rainwater harvesting is now being promoted for a number of reasons, one of which is to lower the cost of irrigation.  

Benefits of storing rainwater

  • Reduced cost: If the owner purchases irrigation water, that cost can be greatly reduced (if not altogether eliminated) by using water collected on their property.
  • Preservation of potable water: Utilizing treated potable water for landscape or agricultural irrigation really isn’t an efficient system. As more irrigation systems are removed from the potable water supply, this will reduce the burden for the water purveyor and free up more clean water for human needs.
  • Stormwater retention and sewage reduction: As more property owners collect, store, and use rainwater on their properties, our overburdened sewage systems will see a decline in volume.
  • Utility Reduction: Did you know that almost half of the energy used in a city comes from the pumping and treating of water? Harvesting rainwater cuts down on the need for that and helps you save money on your power bill (as well as your already discussed water bill).
  • Ownership: While water rights vary from State to State, generally water that falls on a person’s property belongs to them. Given their supply is large enough; they can usually irrigate all they want, even in times of drought.
  • Erosion reduction: Every drop of rainwater you collect is a drop of rainwater that won’t run off and down into a sewer drain, potentially eroding the landscape and overloaded stream banks as it does so.

In order to collect, store, and use the harvested water for irrigation, you will need to consider a number of aspects. Here a list of questions to get started:

  • How much rainfall can be collected?
  • How much water is needed?
  • How much water can be stored?
  • Will it rain when supply replenishment is required?
  • When rain supply is insufficient, what supply will fulfill the irrigation requirement?
  • What hardware will be necessary? (testing, pumping, storage, control, aeration, treatment, and transport systems, etc)

RainWater Availability

To determine supply, you will need to measure the square footage of the collection area. Rainwater is commonly harvested from roofs, paved areas, or watersheds. Because of mechanical issues, not all water delivered by rain is collectible; the general assumption is that about 50% is usable. A simple calculation can provide the estimated gallons per rain event that can be captured;

Gallons= 0.50 x Rain (inches) x Area (Ft2) x 0.623

Example: if historical average rainfall for the month of June is 2.5” and the collection surface is 1,300 Ft2, the expected water collection will be 1,012 gallons.

Gallons= 0.50 x 2.5” x 1,300 Ft2 x 0.623 = 1,012.

The important question is how much rain can be expected and when will it happen. Areas with less volume of rainfall and/or little rain during the irrigation season may need exceptionally large storage capacity.

Matching the irrigation requirement with the rainfall volume and frequency and with the storage capacity will determine the need to supplement water. Monthly historical rainfall data, the monthly irrigation requirement, and the size of the storage area are used to calculate how much supplemental water is needed and when.

Irrigation requirement

Irrigation must be calculated and will vary greatly depending on climate, soil type, plant varieties, planted area, time of year, and production expectations. Any good irrigation person will know how to estimate monthly water volume requirements for a particular irrigation system.

Some general numbers may help.

  • To apply 1” of water to an area equal to 1 acre will require 27,154 gallons at 100% efficiency.
  • A typical ornamental shrub will require 1-3 gallons per week during the mid-summer.
  • A bed of flowers or ground cover with near 100% canopy cover will need about 1 gallon per square foot per week.

Collection System

Adding a water feature provides both aeration and viewing pleasure

Collecting rainwater is typically an easy network to create. From roofs, it means routing downspout water to the storage container. From paved areas, it means collecting and or channeling water to the storage area. Lakes may be taking in run-off water from a watershed consisting of many acres.

The most difficult detail may be pre-straining the collected water before it enters the storage medium. Screening removes the trash and debris moving along with the water. Maintaining water quality in the storage is much easier without debris!

Water Storage

Water can be stored in a variety of ways. None are cheap or without issues. Ponds and lakes are by far are the most common storage facility for rainwater collection. People have created ponds and lakes to store water for centuries. Lakes are relatively easy to construct if the land is available. They can be scaled to the quantity of water storage requirement and large storage may be the only reasonable solution.

The first question to consider, is a lake appropriate for the property? Lakes require a suitable large flat surface area, which may not be present. You also need to find out if a lake requires approval from regulatory agencies. Another consideration is that a lake should never be allowed to be completely drained, so either it requires plenty of rain fall or you may need to add water when needed. In fact, to maintain acceptable aesthetics and lake water health, irrigation use is limited to 1-3’ of depth. We will also discuss the need for aeration below.

The mechanics of water tanks – either buried or on the surface – vary somewhat but are similar. There are many mechanical considerations when installing tanks. Some are not obvious but critical. Tanks have an upper storage limit. Unless assembled on-site, tank size is limited to what a truck can deliver. Work with knowledgeable people, because a botched installation is very costly to fix. Also learn about the safety, regulatory, and insurance requirements before investing.

Cisterns are similar to a buried tank but utilize a constructed storage area such as a basement or a rubber-lined cavity supported by a matrix. Cisterns can be scaled to handle extremely large volumes of water, similar to a lake. The support matrix can be stout enough to support a paved area above the cistern such as a parking lot. Again, work with knowledgeable people!

Back-Up Water Supply

If you plan to irrigate with harvested water, you want to incorporate a back-up water supply.  Supplemental water can be supplied by a purveyor, potable or another on-site water source, such as a creek or well. We recommend using a measuring device to determine when a supplemental water supply is needed. Alternate supplies can be automated or managed manually: There are automatic operating valves and pumps available that will refill your water storage.

Water Quality

Although the quality of water used for irrigation may not a primary concern, it should be tested for irrigation suitability. Any contaminants from a roof, parking lot, or watershed will enter the storage. Water stored over a long period of time will have growths of bacteria, insects, algae’s, and other life. These are contaminants that can cause plant and soil health problems.  Those issues must be checked before installation because costs will increase dramatically if you need to retrofit later on. You can order a water testing kit online, or contact your county health department, check with the Environmental Protection Agency, or your state university.

Irrigation requires large amounts of water, it usually means water is not stored for long. Tanks and cisterns will be emptied or turned before bacteria and such can become a problem. If you have had a lot of precipitation and your tanks have been full for a long time, you need to check on the water’s health.

No matter how water is stored, aeration is critical for maintaining its health. Aeration means bringing water and air in close contact; this process helps to remove dissolved gases (such as carbon dioxide) and oxidizes dissolved metals such as iron, hydrogen sulfide, and volatile organic chemicals (VOCs). Aeration happens in nature as water runs over rocks in a stream or river, but for stored water, it is necessary to add an aerator system.

In conclusion: A lake or water tank may be a valuable addition to your property and have an impact on the landscape of your area. You can reduce the problems caused by too much rain, as well as be prepared in case of drought. There are many details to consider and it is recommended to work with knowledgeable engineers or excavation companies to make sure you get the solution that is best suited.