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How soil moisture impacts water availability in the Klamath Basin

While the Klamath Basin’s snowpack is at 85% of what it would be normally this time of year, there’s already more water stored in the mountains than last year. So why are Basin hydrologists projecting another exceptionally dry summer for Upper Klamath Lake? The answer lies beneath the snowpack—in the soil.

Chris Gebauer, a soil scientist with the Natural Resources Conservation Service, said the amount of moisture present in the Basin’s soils at the beginning of a water year has a big effect on how much of that winter’s snowpack becomes available as surface water.

“If the soil is moist already, it’s like a jumpstart,” Gebauer said.

Think of the soil as a sponge: When it’s dry, it absorbs more water released from the snowpack in the spring, but when it’s wet, it allows more water to pass through or over itself. Water not absorbed by the soil then runs off into streams and reservoirs or percolates into subsurface aquifers to recharge springs.

This year, Basin soils were parched following an extremely hot, dry summer, so Gebauer said we can expect them to retain a significant portion of this winter’s accumulated snowpack once it melts.

“Because it’s getting soaked up, you’ll have less water getting into the drainage systems,” Gebauer said.

Dry soils also take longer to absorb water, so if the snow melts quickly in the spring, most of it will run off and potentially cause erosion, and less of it will percolate into aquifers. Hydrologists are still evaluating how much of the Basin’s water comes from surface runoff versus springs, but a dry soil profile isn’t good news either way.

Scott Oviatt, snow survey supervisory hydrologist for NRCS, said the optimal situation following a dry summer is to have fall rains that wet the soil enough to saturate it before snowpack begins accumulating.

“That way, when springtime comes, we’re holding that water in the soil plus the available snowpack,” he said.

As a warming climate makes both summers and winters hotter and drier, soil moisture has become a major limiting factor for water availability in the Basin. Over the last several decades, dry falls have sent the Basin into winter with existing moisture ‘debt’ it’ll have to pay back in the spring. Oviatt said some years, like this one, have also seen abnormally warm soil temperatures to the point where the ground is above freezing during the winter. That allows the soil to soak up some melting snow or rain before spring even arrives.

It’s a vicious cycle exacerbated by a warming climate: There wasn’t enough precipitation last year to saturate soils, and a minimal snowpack meant that plants in the Basin had to suck up all the moisture they could from the soil, leaving it bone-dry at the end of the summer.

Another year with below average precipitation couldn’t replenish those soils without reducing surface water availability in the basin, setting it up for yet another dry summer. If fall rains remain elusive this year, we can expect a similar situation for Water Year 2022. Dry years beget future dry years.

“It takes multiple years to build drought scenarios, and it takes multiple years to get out of them,” Oviatt said. “You can’t just flip a switch.”

While SNOTEL sites do record soil temperature and moisture content at varying depths, Oviatt said that data isn’t being incorporated into streamflow forecast models yet. That means there’s no magic number to achieve regarding soil saturation that will keep the snowpack from being absorbed, and, by extension, streams flowing at normal rates.

But soil conditions still play a qualitative role in modeling by informing previous years’ conditions used to predict future streamflows: Dry (or not wet enough) years that follow dry years tend to exhibit lower streamflows because they accumulated moisture on top of dry soils.

“Statistics look at that based on previous years with similar conditions,” Oviatt said.

Oviatt said the Klamath Basin has seen about 75% of normal precipitation since the water year began in October. The moisture deficit accumulated by the dry soils mean that even 100% of normal precipitation will still result in drought conditions, because a portion of that precipitation won’t make it to streams or springs.

At this point, Oviatt said a best-case scenario would be an abnormally large late-winter snowpack accumulation, or a cool and wet spring that restores normal streamflows through rains. With climatologists predicting only slightly above average precipitation and slightly below average temperatures for the next few months, it’s not out of the question, but Oviatt isn’t banking on it.

“That’s the dream. It’s an unlikely dream,” Oviatt said.



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