Expanding Reusable Water Supplies Through Lawn Irrigation Return Flows

It takes a lot of water to have lush green lawn in the semi-arid desert climate of Colorado, and not surprisingly, home irrigation is not 100% efficient. Somewhere around 15-25% of the water applied to grass lawns seeps past the grass roots and is lost to the groundwater table below the lawn. This can be a lot of water. In the southwest, as much as 60% of a home’s total water use is outdoors. What may surprise you, is that this water can work its way through the shallow groundwater aquifers and make it back to nearby streams. Since this water was not consumed but ultimately returned to the stream, the municipal water provider that provided the water in the first place has the right to claim credit for the water that returns to the stream and can re-divert that water, with the one caveat being that the water used for irrigation has to be from a legally-reusable source. The inefficient water from lawn irrigation that makes it back to nearby streams is called a lawn irrigation return flow, or LIRF for short. Some municipalities have been claiming LIRF credits for over 30 years in Colorado, but many municipalities are not claiming this water to which they have a right.

Lawn Irrigation Return Flow Schematic

How to Estimate the Amount of LIRF

LIRFs are estimated by water resources engineers using a surprisingly simple amount of data. The only data required from the municipality are total monthly residential water use volumes, the number of homes being served, and a parcel map of the area from which the LIRFs are going to be claimed. The engineer uses these data to estimate the indoor versus outdoor water use. The outdoor water use, after accounting for non-irrigation uses like washing cars, is then compared to the amount of water needed to grow grass in that area for an average sized yard. The excess water not consumed by the grass is what returns to the groundwater system.

Once the excess irrigation water gets to the groundwater table, it still must flow back to a nearby stream. The engineer will analyze the local geology, and will also need to install some shallow monitoring wells to demonstrate that there is a shallow groundwater table, and that the groundwater table is sloping toward a nearby stream. Using these data and empirical equations, the engineer can estimate how long it takes for the LIRF to make it back to the stream, where it can be re-diverted by the municipal water provider.

Once you establish the time, place, and amount of water that returns to the stream, you have asserted legal “dominion and control” over your previously lost water and it may be reclaimed. The LIRF credits can then be used as a supplemental source of water for a variety of purposes, such as:

  • Augmentation credit

  • Exchanges for direct use through alluvial wells or surface diversions

  • Exchanges into storage for later use

Augmentation credit is one of the more common uses of reusable LIRFs as it can help offset out-of-priority depletions from other tributary water supplies available to a municipality.

The Cost and the Catch

In these current times of generally limited available water supplies, LIRF water is often some of the lowest cost water accessible to a municipality. There is an economy of scale at play in favor of the larger water providers, but a LIRF case is nearly always a vastly less expensive way to obtain additional water supplies. For larger municipalities, the cost per acre foot of water gained in a LIRF case can be as low as $100 per acre foot. Compare that to the $25,000 - $30,000 per acre-foot for municipal water sales in the Denver metro area, and you can see the massive appeal in claiming water in a LIRF case.

There are two catches to getting this water in your portfolio. The first is that the water used on residential/commercial irrigated areas must be decreed as reusable water to qualify for claiming LIRF credits. Reusable water is non-native water that would not have been in the watershed had you not brought it to the watershed. Great examples of reusable water are nontributary groundwater like Denver Basin wells. Many local water providers rely heavily on Denver Basin wells, so it is a common supply of reusable water. Water obtained from change of use cases related to senior irrigation water rights (see LWS Making Municipal Water Supplies Available From Irrigation Rights blog) is nearly always reusable as well, and is a common municipal supply. Transbasin water, which is water that was originally diverted from a different watershed, is also fully reusable.  If you are unsure of the balance between non-reusable and reusable water in your portfolio, an engineer can look through your water rights and let you know. Don’t be discouraged if your portfolio isn’t 100 percent reusable, very few municipal water supplies are fully reusable. If you use 40 percent reusable water, then you simply multiply the overall LIRFs by 40 percent to determine your reusable credit.

The second catch is that it may take a couple of years to work a LIRF case through the courts, as this requires a change of use application. However, if you plan ahead, LIRF credits are an excellent supplemental water supply. Just don’t plan on calling an engineer and having ready-to-use LIRF credits 6 months later.

If you think LIRF credits would make a great additional water supply in your water portfolio, or if you have any other water resources questions, you can always give Lytle Water Solutions a call.