Each year heading into spring melt, B.C. tries its best to predict how much water will runoff into Okanagan Lake and other water bodies in the province.
It’s important information as the provincial government manages the lake level using a dam in Penticton and has to strike the right balance to ensure sufficient water in the lake to last through the hot and dry summer, but not so much it causes flooding.
Partially in reaction to the 2017 flood, the province developed a new model for making those predictions and its latest forecast is very different from the old method.
The River Forecast Centre’s Feb. 1 update, says the old model is predicting “Okanagan Lake inflows to be near normal…while the updated model forecasts well above normal (>140 per cent) seasonal inflows.”
Jonathan Boyd a hydrologist with the River Forecast Centre explained the old forecasting method only uses data from up to 2010 while the new method includes more recent data.
“That model also incorporates seasonal forecasting, but it is a little more skillful. It actually will take the Environment Canada seasonal forecast for the temperature and precipitation for the upcoming few months and use that to come up with a forecast.”
Shaun Reimer, the provincial employee who is in charge of managing the lake level, said in early February, he is putting more emphasis on the newer forecast model that is predicting a larger volume of runoff.
The province has been letting more water out of the lake to compensate for the expected larger-than-normal runoff. Reimer said the outflow was “more than doubled” last week and will be increased again this week.
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Reimer said the province is emphasizing the larger inflow prediction to “be more conservative in our approach.”
“But it should still allow us to potentially change our approach as we get our March forecast or our April forecast as well,” Reimer, the section head for public safety and protection with the Ministry of Forests, said.
Meaning, if the higher-than-normal local snowpack dips later in the season there is still time to pull back the water release and allow for more water to build up in the lake.
“That is the approach that we take now. It doesn’t always work perfectly if we get more extreme conditions,” said Reimer.
A report done after the 2017 floods noted officials were initially expecting runoff into the lake at or below normal but then intense spring precipitation (which is harder to predict) caught them off guard, leaving the province without the capacity to draw down the lake fast enough to prevent flooding.
Since 2021, the province says it has been running both the old and new runoff modeling each year “to examine the effects of the updates.”
“The challenge is with models that, by nature, they are inherently always wrong they are just models. And the more models that you have the more confusing that it can get because you don’t know which one to use and it is a challenge,” Boyd said, in a Feb. 8 interview.
“It really just revolves around coming up with a prediction that seems to make sense. Certainly, I do think this is the type of year where it does make sense that the volume forecast is going to be a little bit higher than normal because the snowpack is above normal.”
Okanagan Lake is one of only a handful of B.C. waterways where the province has updated the inflow modeling. Boyd said it was prioritized after the 2017 flood.
He said the plan is to ultimately transition all of the volume forecasts to an updated model, but that will take time.