A combination of mild temperatures and mild precipitation through thunderstorms has given a slight respite to Yakima River Basin irrigators over the last month, resulting in a slight uptick in the amount of water available to them.

As it currently stands, junior water rights users in the basin are receiving a 69 percent supply, which is up from 67 percent in July. Bureau of Reclamation Hydrologist Chris Lynch said this number is predicted to hold for the time being.

Lynch said the 2 percent increase has to do with the bureau narrowing in on forecasted inflows and correlating that with expected usage for the remainder of the season. He said an initial estimate is locked in on June 3 when prorationing begins and slightly prior to when the system goes on storage control, which is when water begins flowing from area reservoirs for irrigation. That initial estimate focuses on the available water coming from runoff, mainly from snowpack and precipitation.

“Basically, we had to estimate what we were expecting flows to be in the summertime from June 3 to Sept. 30,” he said. “The earlier in that period we are, the less certain we are about that number. On Aug. 1, we were about halfway through that period, so half of it now we know and half it’s still ahead of us that we don’t know, but that’s more than we knew on June 3 for the runoff portion.”

Although predicting the available runoff can shift based on conditions, Lynch said one factor that is a fixed number is the amount of water available in reservoir storage. He said that number is always anchored to the first day of the forecast period.

“We don’t estimate it,” he said. “We just know it.”

Another factor that goes into forecast models is the expected amount of water that will be used by irrigators. At the beginning of the season, Lynch said it is assumed that irrigators will use all the water they are entitled to. Around the hallway point of the irrigation season, he said the bureau reevaluates the amount of water that has been used to that point and compares it to the amount the irrigators can use with their water rights.

“If there is water that doesn’t look like it will be used by some of these users and would be left in their bucket so to speak and the end of the season, then we try to take that into account to help everybody in the basin,” he said.

SUPPLY FACTORS

Looking forward towards the second half of the irrigation season, Lynch said there are a few factors that will impact whether the 69 percent water supply for junior rights holders will hold steady. One factor is whether stream flows stay consistent and within the range they are expected to be in.

“If the flows are dropping off more than we expect, then we may have to back off on the proration rate,” he said. “If the inflows hold or go up, then we would hold and possibly go up, but I’m not sure we have much more room to go up. We’ll keep an eye on things.”

Another factor Lynch said plays into holding the forecast at its current number is the precipitation the area receives. He said they must be careful when taking precipitation into account, however because short incidents of rain may not be able to be captured and used and cannot be factored into available water supply as a result.

“If a precipitation event actually materializes into saved water, now people aren’t diverting as much,” he said. “It could stretch the supply a little bit for later.”

Lynch said care must be taken while making the estimates of available water supply, because although some water rights holders may not have used their expected amounts to a certain date, they may have plans on using more later in the season.

“They might just use it differently,” he said. “We can’t necessarily give it away because they might have it budgeted for later and we’re thinking we have more and we don’t. We’ve got to be careful about that.”

Another factor that Lynch said has been beneficial this summer to irrigators is temperature. When temperatures rise, demand for water rises, whereas a mild forecast aids in reducing evapotranspiration, where water evaporates from the soil and transpires faster from plants.

“I think temperatures have helped us in maintaining and improving a little bit,” he said. “We haven’t had any really long stretches of heat. The temperatures have been very good to us this summer I’d say.”

As he begins to look at the second half of the season, Lynch said he is pleased and somewhat relieved that the forecast has held steady and looks to stay on track.

“It’s maintaining and actually has some signs that it is on the healthy side of our forecast range,” he said. “It seems as though things will hold and will be fairly stable through the rest of the season.”

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