The BC River Forecast Centre (RFC) released their April 1 snow survey bulletin this week. April 1st snow water equivalent (the depth of water you’d have if you melted the snow instantaneously) is used as a surrogate for the total seasonal snow accumulation and for the maximum seasonal snowpack on the ground. After April 1, we expect the snowpack to begin melting rather than to continue to accumulate.
It appears that the majority of the province has an above average snowpack. Some places, like the Okanagan, are at near-record levels, at 152% of normal. This ties 1999 for the highest snowpack since 1950. The rest of the province is between 110% to 150% of normal, though the Stikine and Northwest are below normal (65% and 72%, respectively). It’s interesting that the report notes record high snowpacks at some individual measurement stations, showing the error inherent in calculating snowpack for an entire basin from a series of point measurements.
Though April 1 is used as the date of peak snowpack, given the cool conditions associated with La Niña, the RFC is expecting more snow accumulation between April and May. Given the already high snowpacks, any increase before May is expected to increase the risk of spring flooding. This is especially true in the Okanagan and Nicola regions, where the RFC is predicting April to June runoff to be between 140% and 160% of normal. Some good news for water supply in our area, as April to June runoff in the Cowichan River is expected to be about 115% of normal.
What’s also important is the role of last year’s wildfires in this spring’s potential for flooding. As I wrote in 2013, burned areas usually accumulate deeper snowpacks because there are fewer trees to intercept incoming precipitation. These deeper snowpacks then melt more rapidly in spring because there’s more incoming short- and longwave radiation to warm the snowpack.
Burned areas usually also have hydrophobic soils: soils that don’t absorb moisture. This isn’t uniform across burned areas – in many cases post-wildfire forests have a patchwork of soils that do and soils that don’t absorb water. But, with the removal of the overlying forest vegetation, the underlying soils are more susceptible to landslides and mudflows, especially on steep slopes. Riverbanks that have been burned are also subject to increased erosion.
Thus a high spring freshet in post-wildfire regions can lead to meltwater being produced faster because of the lack of forest cover. That meltwater reaches streams quickly because it isn’t absorbed by the soil, which leads to mudslides on steep slopes and highly turbid water within burned stream channels.
Usually when snow melts it goes first at lower elevations and south-facing slopes, and later at higher elevations and north-facing slopes. After a wildfire, snowmelt across a watershed can become synchronized because there’s no vegetation to shade the snow and keep it from melting, and there’s a lot of incoming radiation helping melt that snow. This can cause high runoff peaks, which are bad news for flooding.
The RFC suggests there could be post-wildfire issues in the Bonaparte River, Baezaeko River, Nazko River, Chilcotin River, Deadman River, and West Road River; including minor tributaries/creeks.
However, the extent and severity of flooding will depend heavily on the weather over the next month or two. If we have an extended period of warm temperatures, or heavy rains at high elevations, flooding could be a real possibility.
I’m surprised to see that, here on the Island, we’re sitting at pretty much the same snow level we had last year. Last year we had more snow in the yard – it fell in December and stayed until February. This year it seems we had more snow up high. The cool, damp spring has been good thus far at keeping us out of drought conditions.
Despite the cool weather, my garden is rapidly coming to life. The rhubarb and garlic are sprouting, and some leftover onions and self-seeded lettuce are growing in random garden beds. The raspberries are putting out their first mint green leaves, and the cherry tree is just coming into bloom.
The pair of Canada geese that made our backyard their home last year have returned, searching for a nesting site down by the marsh.
I am irritated and angry about my knee injury, especially after my first physiotherapy appointment, at which the therapist told me that it will be a long, slow recovery. I guess that’s what happens when you actually break a bone instead of just injuring soft tissue.
I want to be weeding, planting, mowing. Wandering the perennial gardens to see what’s coming up. Crocosmia, bleeding heart, daylilies and heather, sedums, iris, and prairie crocus. Tomorrow I’ll make my way down to the garden on crutches and see what I can see. Even if I can’t do anything, I can still absorb the sights and smells of spring.