BOISE, Idaho — Idaho has been in a drought for some time, but the rainy pattern we saw last week may help with our water supply in the spring.
November rain is particularly favorable because it's cool outside which limits evaporation. But, it's not cold enough for the ground to be frozen solid yet. When the ground freezes, it locks soil moisture in until the spring thaw. So if there's a soil moisture deficit, it's locked in; and if there's good soil moisture, it's also locked in.
This means the moisture we saw last week went right into the soils, even at some of the higher elevations.
Erin Whorton is a Water Supply Specialist with the National Resources Conservation Service. Whorton said a rainy fall bodes well for the water supply.
"We want our soil to get saturated with the rain, so that when the snowpack does start to melt, it is able to run off more directly into the streams rather than being soaked into dry soils," Whorton said. "So, when we have wet soils, you can think of it as a wet sponge, less moisture is being absorbed from the snowpack."
Whorton also said we're in a better position this year with reservoir storage, which is higher this year than it was last year. Whorton added this provides a buffer in case our snowpack or runoff comes up a bit short.
Right now, the storage in the Boise Reservoir System is more similar to levels we would see in March, on average (according to the U.S. Bureau of Reclamation). There are about 500,000 acre feet right now. This time last year we were near 430,000 acre feet, which is still above the average of ~375,000 acre feet.
Last year by this date, we had more than 2 inches of rain in Boise (2.05 inches). This month, we've seen 0.64 inches. Boise's average for November monthly rainfall is 1.18 inches.
But, other western Idaho spots were favored in the rounds of showers we saw. McCall stacked up 2.24 inches of rain so far this November. It's worth noting it was rain and not snow, because McCall would typically see more snow than rain this time of year.
So, it's been showery, but it hasn't been stacking up quite like we saw last year in the Treasure Valley or Magic Valley.
Even though we're off to a promising start, we may see things take a turn later this season.
"Typically in El Niño years, we do see wetter conditions early in the season. And sometimes, that does shift to drier conditions," Whorton said. "So, right now we start on a hopeful note and then hopefully we can carry those positive conditions forward."
The NWS Climate Prediction Center says the El Niño pattern is expected to stay strong until next spring. They also say "There is a 35% chance of this event becoming 'historically strong' (≥ 2.0°C) for the November-January season".
It's very early in the season. The new water year just started last month and we're just entering the time of year that's typically wetter. So, it's too soon to say with any likelihood what will happen. But, it's a good start for our water supply.
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