By the turn of the century, farmers in Oregon’s Willamette Valley will be planting earlier and will begin irrigating about two weeks sooner than they do now, according to an Oregon State University study that used computer modeling to project water availability, demand and storage in the Willamette River basin to the year 2100.
Climate change most likely will result in wetter winters, but with the snowpack severely reduced and earlier runoff. Rainy winters and springs will be followed by hotter and drier summers, but more farmers will have finished irrigating by the time water shutoffs are contemplated, the research team concluded. Although the reduced snowpack will cause the loss of an estimated 600,000 acre-feet of stored water, it won’t have a significant impact on farmers in the Willamette River basin who rely on rain-fed streams. Farmers in the more arid Eastern Oregon and Deschutes and Klamath basins, however, depend more on melting snow for irrigation water and are more likely to face shortages.
Willamette Valley cities will need more water to accommodate population growth, but other factors reduce the impact of that increased demand. Efforts to use more of the water stored in reservoirs behind 13 dams in the basin will be limited by the expense of pumping it. That is, it might not be worth the cost unless the water is going to high-value crops such as nurseries or perhaps vineyards.
The Willamette Water 2100 project involved six years of work by 30 researchers in a variety of specialties. The idea was to create a computer modeling system that could project the impact of climate, population, urban growth and other factors on water supply and demand. The result is a case study of an important and relatively large river basin – the Willamette is 186 miles long, the 19th largest river in the U.S., and flows through an area in which 70 percent of Oregon’s population lives.
The work could be useful to make projections in other river basins, “Especially ones with less water,” said Bill Jaeger, an OSU applied economics professor and a lead author on the project.
The modeling system depicts the basin in 160,000 polygons that allowed researchers to layer in variables such as existing water rights, crop choices, soil types, precipitation and temperature. Jaeger said he doesn’t know of another study in the world that’s allowed such modeling detail.
The work yielded some surprises, he said.
Researchers realized early on that the state inventory of irrigation water rights and the amount of irrigated acreage didn’t match up. In any given year, about one-third of the water rights weren’t used for various reasons, Jaeger said. In addition, some large water rights exist on paper but haven’t been exercised for years. For example, Willamette Valley lumber mills often had water rights they used to float logs in canals or ponds, but the mills have since closed or use other methods of moving timber.
Cities are major water users and the model projects that the municipal water rights they rely on may reach capacity in 30 years for the Portland area and in 60 years for Salem. “However, when the model accounts for currently underutilized water rights and those under development, urban water rights appear to be capable of meeting the overall growth in urban water demand,” the study concludes.
The computer models “reinforces the expectation that cities will continue to grow.” Jaeger said. “There will be some reduction in the availability of farmland on the outskirts.”
The impact of urban expansion and population growth in the valley is offset by a couple of other factors. Although the Willamette flows through downtown Portland, the city gets two-thirds of its water from the Bull Run Reservoir system, which is in the foothills of Mount Hood and outside the Willamette River basin. Also, only 7 percent of the water used by cities is considered “consumptive,” meaning it is used on lawns, gardens, parks and urban farms and is not available for other uses. The rest is used indoors and goes down sinks, showers and toilets, flows to municipal treatment plants and returns to the river system as treated water.
In contrast, agriculture’s consumptive use in the Willamette Basin is 25 times greater than that of cities. By far the largest use is the flow mandated for salmon and steelhead; the “regulatory minimum flows” for endangered species in the Willamette River is 200 times greater than what is consumed by cities, according to the report.
Although eyed as potential water sources, the 13 federal storage reservoirs in the Willamette River basin have “enormous social value” — estimated at more than $1 billion per year — in controlling flooding, according to the report.