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Long-term Daily Air Temperature and Precipitation Record for the Lake Cowichan Area

Author(s) or contact(s): D. L. Spittlehouse
Source: Ministry of Forests, Lands, Natural Resource Operations and Rural Development
Subject: Climate
Series: Technical Report
Other details:  Published 2017. Hardcopy is available.


Daily air temperature and total precipitation data have been collected in the Cowichan River valley for more than 100 years as part of Environment and Climate Change Canadas monitoring network. However, none of the weather stations have operated continuously for the whole period. This report describes combining station records to create a continuous daily maximum and minimum air temperature and total precipitation record from March 1901 to December 2016 for the Cowichan Lake Forestry (CLF) station Linear regressions were fitted to daily and monthly data for records that overlapped between CLF and other stations to generate coefficients to adjust a station to CLF. The coefficients of determination (adjusted R2) ranged from 0.85 to 0.99 for temperature and from 0.58 to 0.99 for precipitation. Standard error of the estimate for daily temperatures ranged from 0.9 to 2.1C and from 3 to 8 mm for precipitation; the larger values were for stations distant from CLF. The monthly record compared well to the Shawnigan Lake record from the Adjusted and Homogenized Canadian Climate Data base. The daily record was used to calculate trends for a range of variables from 1902 to 2016 and from 1951 to 2016. Trends in annual and seasonal temperature indicated a rise of 0.10.3C per decade and were significant at the 99.9% level. Precipitation trends were small and most were not significant at the 90% level.

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Updated January 19, 2018