By Brian Hodge In Trout Unlimited’s work with ranchers and farmers of the Upper Colorado River Basin, we often hear arguments about the relative benefits of flood and sprinkler irrigation for stream flows and fish.

Proponents of flood irrigation note that it requires a producer to divert more water from a stream than the crop can consume, and that unused irrigation water provides a benefit to fish because it returns to the stream when flows are otherwise low.

Meanwhile, proponents of sprinkler irrigation note that it requires a producer to divert little more from a stream than the crop demands, and that water left instream provides a benefit to fish 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 fish in the Upper Colorado River Basin?

The short answer is, it depends.

In some cases, return flows from flood irrigation provide delayed benefits to streams and fish.  A research group from the University of Wyoming recently concluded, for example, that late-summer return flows from flood irrigation are critical to the brown trout fishery in the New Fork of the Green River (Blevins et al. 2016).  Moreover, the authors posited that if ranchers in the watershed improve efficiency by converting from flood to sprinkler irrigation, late-season streams flows will drop, and with them, the density (pounds/mile) of brown trout.  The authors also suggested that perpetuating flood irrigation in the Upper Green River Basin might be an effective way to maintain its healthy trout streams.

Although flood irrigation return flow can provide delayed benefits to fish, not all return flows reach streams at a time and in an amount necessary to confer a benefit.  An important assumption in the New Fork study was that water diverted in early-summer returned to streams later that season.  Our own analyses suggest that lag time on the western slope of Colorado can range from as little as days or weeks to as much as months or more.  Lag time on surface return flow (e.g., water flowing across a saturated field) is typically very short, whereas lag time on sub-surface return flow is highly variable and largely dependent upon the groundwater flow rate and distance between field and stream.  In some cases, return flows are intercepted and used by other irrigators.

In our thinking, fish- and flow-related benefits are intimately tied and dependent on a number of variables.  A 25% boost to flows in August, for instance, when flows are typically low and temperatures warm, could be quite meaningful for fish.  In contrast, a 5% increase in discharge in mid-fall or early-spring, when flows are low to moderate and temperatures cool but not cold, could go undetected by fish.  In short, the fish-related benefits of return flow (or lack thereof) depend on a number of factors, including the timing and amount of return flow, stream conditions, and the biology of the fishes in the stream.

Irrigation efficiency improvements, such as conversions from flood to sprinkler irrigation, can provide fish-related benefits where the short-term effects of leaving water instream outweigh the long-term benefits of taking it out.  Consider an example where the current regime is flood irrigation and the lag time on return flows is on the order of days or weeks.  In this case, there is little if any delayed benefit: water leaves from and returns to the stream in relatively short order.  In the meantime, more water is diverted from the stream than is required to meet crop demand, and consequently, stream flows and fish habitat are reduced between the point of diversion and point of return.  Also, return flow water—especially that on the surface—is likely to be warmer and more nutrient-rich than the source stream.  Here, a simple efficiency improvement could improve flows and water quality below the point of diversion.

Because each agricultural operation and watershed is different, TU tends to avoid generalizations about flood and sprinkler irrigation. The truth is, fish-related goals might be attained with flood irrigation in one watershed, and with center pivot sprinklers in another.  At the end of the day, the best solution is the one that works for both agricultural producers and fish.

Brian Hodge is a fisheries biologist and restoration coordinator for Trout Unlimited in the Upper Colorado River Basin.