My last post outlined some of the water issues we’re having here in the Cowichan Valley. Coincident with the fall equinox, we were in for some wet, stormy weather – and there was a lot of hope that the rain would address some of our low river flow concerns.
So what do the data tell us? Not what we were hoping for, I’m afraid.
The plot below shows 20 minute river discharge data, as well as the daily average calculated from those data. That line at the top of the plot? That’s the long term daily average streamflow based on data from 1912-2014 – and we’re well below it this year. You can see that daily river flows have been much lower than average since the beginning of September (the date I selected for the beginning of my plot).
The plot below shows daily average streamflow for September 2014, as well as total daily rainfall during the same time period. You can see that, while the rain event around 23 September increased flows, the flow peak lasted for only a short time before returning to low flows. This suggests that much of the rain went to surface runoff, rather than filling of depleted water storage locations (soil, groundwater, etc.). While the current low flows are a bit higher than before the rain event, the <0.5 m3/s increase is well within the margin of measurement error!
I also discovered that the Water Survey of Canada installed a stream temperature sensor on the Cowichan River at Duncan on 3 September. These data are important for salmon spawning conditions – if the river is too warm (which often happens when flows are low), salmon survival is compromised.
The plot below shows hourly river temperature, and the calculated daily average, from 3 September to today. The daily temperature cycle is very pronounced – earlier in September we were seeing temperature swings of up to 2.5°C within a 24 hour period. While the amplitude of the daily cycle has decreased to ~1.5°C since the rain event, the average daily river temperature is still around 15.5°C. A study out of northern California tried to put some brackets around ideal water temperatures for spawning steelhead, coho, and chinook salmon – looks like our temperatures are within the ‘okay’ range, but our flows are definitely too low.
In good news, there was enough rain to dampen the ground around our house. Things are greening up (well, the weeds are poking up), and the Scotch broom (very invasive around here) is easier to pull up by hand. Trouble is, there’s no rain in the forecast for the next week. Let’s hope the models are wrong and that low pressure system that’s hammering the North Coast will move south to bring us some more moisture.
