Making Every Drop Count By Jim Gerrish

MAY, Idaho: If you take a look at the USDA Drought Monitor map, you’ll see almost the entire Western half of the USA is in a serious drought situation while most of the East is at normal or above normal precipitation. It is a very stark visual contrast. I keep hearing from clients in the East who they wish it would stop raining so they can get some field work done. Almost everyone in the West is anxious for rain or snow to just get pasture growth started.

At our very local level, Patterson Creek is our irrigation water source and the creek level is at its lowest point I have ever seen it in late spring. It looks more like late August. In a “normal” spring, I can’t wade across the creek because it can be well over waist deep and a roaring torrent of ice cold snow melt tumbling down the canyon. This spring I have been walking across the creek with just my muck boots rather than the thigh-high irrigation boots I normally wear when I visit the creek to make adjustments to our irrigation diversions. It is a grim situation unlike anything we have seen in our 18 years in Idaho. The regional climatologists say it is the worst drought in the last 1200 years.

It’s times like this that make us think about how to get the most out of every drop of water that comes our direction. For those of you in the wet Eastern half of the country, some of these principles apply even with natural rainfall. If you are in a wetter environment but using supplemental irrigation to maintain pasture yield and quality for your specialty enterprises, this will be especially applicable.

In conjunction with the drought, most areas are also experiencing well above normal temperatures. Evaporative water loss is greater with higher temperatures. That means less water available for plant growth. In some parts of the world where temperatures are normally high, farmers and ranchers only run their irrigation systems at night. This practice substantially reduces evaporative water loss and greatly improves water use efficiency.

Irrigating only at the cooler part of the day does require greater water output per minute of active irrigation. This means changing the nozzles on your sprinkler system to get the desired output in the restricted time period. Upgrading your nozzle package is going to cost you some money but the potential payback can be substantial. In some situations, it may also be necessary to upgrade your pumping system to carry the increased flow demand. This is when our natural gravity pressurized center pivots system really shines. We could double our hourly output with no increased operating cost.

Monitoring soil moisture status on an ongoing basis can help you avoid over-watering as well as not allowing the soil to dry out to a point where it will take several days for the pasture to get back up to the desirable growth rate. The goal of irrigation is to maintain soil moisture levels at about 70-90% of field capacity. The shorter the return time between irrigation events, the easier it is to maintain this objective. This is where center pivots really gain their superiority over flood irrigation or side-rolls. My typical return time with the pivots is three days so we never see less than 70% of field capacity.

Watering beyond field capacity creates saturation resulting in reduced plant growth rate and wasted water. Permanent wilting point for most pasture plants is at about 50% of field capacity. The closer we allow soil moisture to get to the critical 50% level, the slower plant recovery will be after water has been applied. Like so many other things, the better grasp you have of what you are man- aging, the more effectively you can manage it. Soil moisture content is what we are trying to manage.

There are numerous hand-held, relatively cheap soil moisture monitors that work surprisingly well in the surface 8-10” of soil. The zone we are interested in, however, extends into the 12-24” depth range or even deeper on some soil types. It becomes much more expensive to get a unit that will accurately measure soil moisture at these deeper depths. If you have 10 irrigated acres, a $400 moisture sensor probably doesn’t make sense. If you have a hundred or more irrigated acres, then $400 for a monitoring tool doesn’t sound too bad. If you have thousands of irrigated acres, it would be foolish not to invest several hundred or thousands of dollars into moisture monitoring equipment that would help improve water use efficiency across all of those acres. All of the soil health advocates point out the critical importance of maintaining appropriate litter cover on the soil surface. Nowhere is this more important than in drought management. Keeping the soil covered greatly improves water use efficiency by minimizing evaporative water loss. Cooler soils also keep plants and microbes functioning more efficiently. This principle applies equally in high natural rainfall or irrigated environments.

One of the key pieces of creating and maintaining litter cover is always leaving ample post-grazing residual. It is near impossible to create litter in a pasture that is repeatedly being grazed too short. Taller, thicker residuals promote drought resiliency while short residuals cre- ate drought susceptibility.

When faced with drought or limit- ed irrigation water supply, your daily management of vegetation and soil moisture can make a big difference in how your farm or ranch weathers the drought. The key to success is to be proactive. ■

Jim Gerrish has been a practitioner, researcher, and educator in the realm of grazing management on pasture and range for over 40 years. You can learn more about implementation of and tools for Management-intensive Grazing at www.americangrazinglands.com. His books are available from the SGF Bookshelf.