Upper Thames River in Level 1 Low Water Condition (low water update, May 7, 2021)
One Month Precipitation
The Upper Thames River watershed received an average of 51 mm of precipitation in April (about 61% of normal), ranging from 73 mm in London and Mitchell, to 31 mm west of London. The long term average for April is 83 mm, measured at the London Airport and reported by Environment and Climate Change Canada (ECCC).
Three Month Precipitation
The watershed received an average of 108 mm of precipitation from February to April (about 46% of normal), ranging from 156 mm in London and Mitchell, to about 70 mm in the Woodstock area. The long term three month average total for February to April is 226 mm, measured at the London Airport and reported by ECCC. If the three month total precipitation falls below 80% of the long term average value, a Level I Low Water Condition is triggered.
18 Month Precipitation
The 18 month (November 2019 – April 2021) precipitation measured by Environment Canada at the London Airport totaled 1259 mm, or about 86% of normal. The 18 month long term average precipitation total for the end of May is 1457 mm.
Monthly Streamflow
April stream flows were generally below 10th percentile flow rates, which indicate dry conditions for this time of year.
Reservoir Status
The UTRCA maintains three large reservoirs and several smaller ones across the watershed. Two of the large reservoirs provide flow augmentation in the summer, meaning they store spring runoff and then slowly release this water to the system in the dry summer months. Pittock Reservoir in Woodstock has achieved its target summer elevation, and will hopefully be able to maintain that elevation until augmentation begins in mid-June. Wildwood Reservoir upstream of St. Marys is traditionally more difficult to fill, and this year it is about one metre below its summer target elevation.
Two smaller reservoirs which are emptied in the fall and filled in the spring are RT Orr Reservoir in Stratford and Mitchell Reservoir in Mitchell. RT Orr Reservoir was filled earlier this season and has achieved its target summer level. Mitchell Reservoir traditionally is filled a couple weeks after Orr and, at this time, has yet to be filled due to dry conditions.
Recommendation
Based on the three month precipitation being below the Level I threshold across the watershed, and below normal stream flows, it is recommended that the watershed move to a Level I Low Water Condition.