Community Agriculture Alliance: Flood or sprinkler?
In Northwest Colorado, I often hear discussions about the relative benefits of flood and sprinkler irrigation for rivers and downstream water users.
Proponents of flood irrigation note that it requires an agricultural producer to divert more water from a stream than a crop can consume, and that unused irrigation water, or return flow, provides a delayed benefit to downstream users, because it tails back to a stream when flows would otherwise be low. Downstream beneficiaries of return flow could include agricultural producers, fish and others.
Meanwhile, proponents of sprinkler irrigation note that it requires a producer to divert little more from a stream than a crop demands, and that water left instream provides a benefit, because it maintains flows when water would otherwise be diverted.
But which is correct — in other words, does flood or sprinkler irrigation provide a greater benefit to rivers and downstream water users?
The short answer is, it depends.
In some cases, return flows from flood irrigation provide delayed benefits to streams. A research group from the University of Wyoming concluded, for example, that late-season return flows from flood irrigation stabilize the water level and brown trout population in the New Fork of the Green River. Moreover, the group concluded that if ranchers in the watershed improved irrigation efficiency, late-season stream flows would decline and, with them, the number of trout.
Though return flows can provide a delayed benefit, not all return flows reach streams at a time and in an amount necessary to provide a measureable benefit. An important assumption in the New Fork study was that water diverted in early-summer returned to streams later in the season. Studies on Colorado’s Western Slope suggest that lag time on return flows can range from as little as days or weeks to as much as months or more.
In short, the benefits of return flow (or lack thereof) depend on a number of factors, including return flow timing and magnitude, stream conditions and downstream needs. A 25 percent boost to stream flows in August could be quite meaningful, whereas a 5 percent boost to flows in November could go undetected.
Irrigation efficiency improvements can provide stream-related benefits. In 2016, Trout Unlimited, Natural Resources Conservation Service and local ranchers partnered to install sprinklers at sites where the lag time on return flows was minimal and the short-term benefits of leaving water instream outweighed any delayed benefits of taking it out. These projects improved both operational efficiency and stream flows during the irrigation season.
Because each agricultural operation and watershed is different, Trout Unlimited tends to avoid generalizations about the benefits of flood and sprinkler irrigation. In our thinking, ranch- and fish-related goals might be met with flood irrigation in one case and sprinkler irrigation in another.
We find that where streams are concerned, the best solutions are often those that work for both agricultural producers and fish.
Brian Hodge is a fisheries biologist and restoration coordinator for Trout Unlimited.
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