Irrigation and Watertable Management

There is a perpetual quandary that plays out during the height of the growing season in certain areas of Skagit County farmland.

It has to do with water. How to retain it. How to move it. How to manage it more efficiently.

The area we're focusing on in this edition of The Dirt lies west of Mount Vernon to LaConner, and west of Conway on Fir Island. Without question, it is some of the richest, most productive farmland in the entire country and it is—for the most part—subject to a limitation that has the potential to make much of that farmland virtually useless.

The specific Skagit County Drainage and Irrigation Improvement Districts that serve these farming acres hold junior water rights. That means that, should water in the Skagit River fall below a certain flow rate, junior water rights are interrupted, essentially shut off until the flow increases again.

With crops in the fields maturing to harvest, shutting off water is a problem.

Present consumption, anticipated demand, and allocation of the precious water resource is under review by the Washington State Joint Legislative Task Force on Water Supply, the Washington State Academy of Sciences, and the Washington Water Research Center. In due time, we will address their findings but, for the moment, we'll focus on what happens now.

Water Table Management

The hundreds of miles of drainage ditches crisscrossing the area are part of a vast network of infrastructure that is designed, first of all, to move water off the land during the heavy precipitation months of late fall, winter, and early spring.

That same network is repurposed at the planting and growing season, however, to do just the opposite. Its function then is to hold the water in place, acting like a chain of mini-reservoirs. By expert management to keep the ditches full—or at least at a fixed level—whatever water is in the soil itself tends to linger there. This controls the water table.

A Bit of Clarification

Per National Geographic, "The water table is an underground boundary between the soil surface and the area where groundwater saturates spaces between sediments and cracks in rocks. Water pressure and atmospheric pressure are equal at this boundary."

The farmland in the districts we're discussing has a naturally high water table because it is close to Skagit Bay. Saltwater, being denser than freshwater, is underlying the fresh groundwater and keeping it at a high level relative to the ground surface.

A way to think about it is as a sponge. A soil sponge. Ideally, a wet soil sponge.

Healthy soil absorbs, stores and filters water. Water needed to grow crops, of course, but also—and this is becoming increasingly important—water needed at various times throughout the growing season to adaptively manage for soil moisture and the different stages of crop growth in the face of a changing climate. By storing water, a soil sponge behaves like a damp household sponge being used to soak up a spill. It holds a lot more water than it would if it were a dry sponge.

By keeping ditches filled, the soil in the fields can better hold water. Through capillary action, that water finds its way from the high groundwater table to the root zone of the crop. It's a water management practice that meets the needs of a number of crops grown in the area, but not all. About 40% of the crops grown within these districts rely, at least some of the time, on "supplemental" irrigation.

Without water we'd lose more than the crop.

If the ditches aren't full, the structural integrity of the soil is damaged. As it dries out, it loses its ability to hold water. When rainfall does occur, much more just passes through, much like watering a hanging plant that has been allowed to dry out. Without the water, the soil loses its resiliency and cannot recover as quickly.

Management Strategies

The responsible parties for balancing both drainage and water retention strategies are the district commissioners and the farmers. The districts are responsible for the maintenance and operation of drainage and irrigation infrastructure, including pumps, ditches, check dams and culverts.

When districts need water to keep ditches full, it has to be pumped from the tidally influenced zone of the river, into district infrastructure that is woven throughout each district. This requires extensive and time-consuming permitting, maintenance activities, and capital improvements. The cost of the work is borne by the members of the district.

Farmers have additional strategies. First of all there's soil husbandry. By fostering healthy soils through best soil management practices such as crop rotation and cover cropping to increase soil organic matter, farmers set up the soil to use the water most efficiently.

Farmers also have adopted targeted watering systems and closely monitor the actual hydration needs of the crop so that water goes just where it's needed and no further.

Then there's crop selection itself. By careful rotations, there is a tradeoff between crops that must have irrigation and those that don't. It's another layer to a farmer's cropping plan, but it's every bit as vital as decisions made on the basis of economic return and market demand.

The problem is local, the solution isn't.

The complex web of factors that determine water availability throughout any given year along with the competing claims upon it, go far beyond the management practices of our specific focus area, but then so do any solutions.

The Skagit River is the largest tributary in Puget Sound. Despite an annual mean flow of 12 million acre feet of water, the Skagit is the least allocated river in Washington State with less than two percent of that mean flow allocated for consumptive uses, including domestic, industrial and agricultural consumption.

The Washington Water Research Center estimated that total existing and future consumptive demand is significantly less.

The participants of the Washington State Joint Legislative Task Force on Water Supply along with the participation of the Washington State Academy of Sciences, and the Washington Water Research Center, are identifying opportunities for better understanding, and therefore better management options on how multiple community values in the watershed can be protected, enhanced and sustained long into the future.

By Teresa Bennett: info@skagitonians.org

All photos by Gary Brown unless noted. See more of Gary’s photos here .