T E C H N I C A L R E P O R T Long-term Daily Air Temperature and Precipitation Record for the Lake Cowichan Area

Transcription

1 T E C H N I C A L R E P O R T Long-term Daily Air Temperature and Precipitation Record for the Lake Cowichan Area 217 1

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3 Long-term Daily Air Temperature and Precipitation Record for the Lake Cowichan Area David L. Spittlehouse

4 The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the Government of British Columbia of any product or service to the exclusion of any others that may also be suitable. Contents of this report are presented for discussion purposes only. Funding assistance does not imply endorsement of any statements or information contained herein by the Government of British Columbia. Uniform Resource Locators (urls), addresses, and contact information contained in this document are current at the time of printing unless otherwise noted. ISBN Print version ISBN Digital version Citation Spittlehouse, D.L Long-term daily air temperature and precipitation record for the Lake Cowichan area. Prov. B.C., Victoria, B.C. Tech. Rep Prepared by David L. Spittlehouse B.C. Ministry of Forests, Lands, Natural Resource Operations and Rural Development Climate Change and Integrated Planning Branch Victoria, BC vw 3k1 Copies of this report may be obtained, depending upon supply, from: Crown Publications, Queen s Printer 2nd Floor, 563 Superior Street Victoria, BC vw 9v For more information on other publications in this series, visit hfdcatalog/index.asp 217 Province of British Columbia When using information from this report, please cite fully and correctly.

5 ABSTRACT Daily air temperature and total precipitation data have been collected in the Cowichan River valley for more than 1 years as part of Environment and Climate Change Canada s 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 191 to December 2 for the Cowichan Lake Forestry (CLF) station ( 5' N ', 177 m). 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 R 2 ) ranged from.5 to.99 for temperature and from.5 to.99 for precipitation. Standard error of the estimate for daily temperatures ranged from.9 to 2.1 C and from 3 to 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 192 to 2 and from 1951 to 2. Trends in annual and seasonal temperature indicated a rise of.1.3 C per decade and were significant at the 99.9% level. Precipitation trends were small and most were not significant at the 9% level. EXECUTIVE SUMMARY Daily weather records of long length allow inter-annual variability and trends in the climate of an area to be evaluated. Daily weather data (maximum and minimum air temperature and total precipitation as rain or snow) have been collected in the Cowichan River valley for more than 1 years as part of Environment and Climate Change Canada s monitoring network. However, none of the weather stations in the valley operated for the whole period. This report describes the combining of station records to create a daily maximum and minimum air temperature and total precipitation record from March 191 to December 2 for a single location in the valley. The Cowichan Lake Forestry (CLF) station was used as the reference location. The extended record prior to 192 was based mainly on stations located about 5 km east of CLF (i.e., Nanaimo, Cowichan Bay, and Shawnigan Lake). Data from 192 to 199 were obtained from stations (Lake Cowichan1 and Cowichan Lake Hatchery) that were within about 7 km east of CLF. Missing data in this period were filled by using adjusted data from Cowichan Bay and Shawnigan Lake. Data from late 199 to 2 were from CLF, and missing data were based on adjusted data from Shawnigan Lake, Lake Cowichan2, and the Mesachie fire weather station adjacent to CLF. Linear regressions were fitted to daily and monthly data for records that overlapped between CLF and other stations and between other stations to generate coefficients to adjust a station to CLF. The coefficients of determination (adjusted R 2 ) ranged from.5 to.99 for temperature and from.5 to iii

6 .99 for precipitation. Standard error of the estimate for daily temperatures ranged from.9 to 2.1 C and from 3 to mm for precipitation; the larger values were for the more distant stations. The monthly record was compared to Shawnigan Lake monthly data from the Adjusted and Homogenized Canadian Climate Data base. The use of Shawnigan Lake in gap-filling the CLF record was minimal and likely had a negligible influence on this comparison. The annual and seasonal maximum and minimum air temperatures for CLF and Shawnigan Lake had similar inter-annual variability and trends; variability and trends in precipitation also show excellent agreement between the two stations. The daily data from January 192 onwards are considered suitable for analyses that are sensitive to an accurate daily record. The daily data prior to 192 are best suited to analyses where the influence of day-to-day uncertainty can average out over a week to a month. The monthly record is considered to be reliable for the whole period. The daily record was used to calculate trends for a range of variables from 192 to 2 and from 1951 to 2. Trends in annual and seasonal temperature indicated a rise of.1.3 C per decade and were significant at the 99.9% level. Precipitation trends were small and most were not significant at the 9% level. The temperature and precipitation trends were consistent with published regional trends. Changes in temperature-based variables such as degree days showed changes that were consistent with the increasing temperature trend; for example, a decrease in degrees days below C and an increase in degree days above 5 C. The evaporative demand showed a weak trend, and the climatic moisture deficit showed no trend. For most variables, trends were greater in the period than in the period. ACKNOWLEDGEMENTS Suggestions from Faron Anslow, Vanessa Foord, and Robin Pike improved the data analysis and presentation. Tracey Hooper provided an extensive language edit of the report. iv

7 CONTENTS Abstract... Executive Summary... Acknowledgements... 1 Introduction Methods Station Assessment and Calculation of Adjustment Coefficients Weather Stations Adjustment Coefficients and Uncertainty in the Adjusted Data Evaluation of the Long-term Record... 1 Trends and Variation in Temperature and Precipitation Summary... Literature Cited iii iii iv Appendices 1 Seasonal time series for weather stations Seasonal maximum and minimum air temperature and precipitation at Cowichan Lake Forestry and Shawnigan Lake Adjusted and Homogenized Canadian Climate Data, tables 1 Weather station name and Environment and Climate Change Canada identification number, location, elevation, record length, and period used in creating the long-term Cowichan Lake Forestry record Weather stations, location, elevation, record length, and period used in evaluating the long-term Cowichan Lake Forestry record Adjustment coefficients for daily maximum and minimum temperature and precipitation from stations to Cowichan Lake Forestry... 5 Statistics of regression equations for calculating daily maximum and minimum temperature and precipitation at Cowichan Lake Forestry from stations that overlap it... 5 Sources of data for the Cowichan Lake Forestry long-term record Summary statistics for the comparison of Cowichan Lake Forestry with Shawnigan Lake Adjusted and Homogenized Canadian Climate Data for annual and seasonal mean maximum, mean minimum, and average air temperature and total precipitation from 1913 to Decadal trends and p value of statistical significance for annual and seasonal temperature and precipitation and derived variables for Cowichan Lake Forestry, and v

8 FIGURES 1 Location of weather stations used in the analysis Scatterplot of daily and monthly maximum and minimum air temperature at Cowichan Lake Forestry as a function of Lake Cowichan2 for January 193 to December Scatterplot of daily and monthly precipitation at Cowichan Lake Forestry as a function of Lake Cowichan2 for January 193 to December Frequency of occurrence of daily maximum and minimum air temperature at Cowichan Lake Forestry and calculated from Cowichan Bay data for Frequency of occurrence of daily precipitation in winter and summer at Cowichan Lake Forestry and calculated from Cowichan Bay data for Frequency of occurrence of monthly precipitation at Cowichan Lake Forestry and calculated from Cowichan Bay data for Annual maximum and minimum air temperature for Cowichan Lake forestry from 192 to 215 and Shawnigan Lake Adjusted and Homogenized Canadian Climate Data from 191 to 2... Annual air temperature range for Cowichan Lake Forestry from 192 to 2, Shawnigan Lake Adjusted and Homogenized Canadian Climate Data from 191 to 2, and Lake Cowichan2 from to Annual precipitation and annual precipitation anomaly for Cowichan Lake Forestry from 192 to 2 and Shawnigan Lake Adjusted and Homogenized Canadian Climate Data from 19 to Annual degree days < C and > 5 C for Cowichan Lake Forestry from 192 to Annual frost-free period and annual precipitation as snow for Cowichan Lake Forestry from 192 to Annual evaporative demand calculated using the Hargreaves equation and the climatic moisture deficit for Cowichan Lake Forestry from 192 to Summer precipitation anomaly for Cowichan Lake Forestry from 192 to 2... vi

9 1 INTRODUCTION Daily weather records of long length are an important tool for resource management. They facilitate comparison of seasonal and inter-annual variability and trends in climate (Vincent and Mekis 26) with similar variability in hydrological and biological systems. Daily maximum and minimum air temperature and total precipitation as rain or snow have been recorded in the Cowichan River valley for 1 years. Data were first recorded in October 1913 in the Cowichan Bay area. Daily weather data were first measured in the Lake Cowichan area in 192 and have continued to be recorded at various sites until the present, though the separation of precipitation into rain and snow has not been continuous. Prior to 1913, the closest reliable measurements were for stations in Nanaimo and Victoria. However, none of the Cowichan River valley stations have continuous records for the whole period of interest; therefore, a long-term record must be created by joining station records. Combining records increases uncertainty in daily data, particularly when using weather stations that have a large spatial separation. However, this uncertainty tends to be reduced as the data are averaged over longer time steps; for example, monthly. Despite this, where possible, it is preferable to create a daily record to increase the use of the data. Combining records works best if the records for the various stations overlap. If this is not possible, an intermediate station that overlaps the two stations being combined is required. This further increases the uncertainty in the combined data. This report describes the generation of a daily weather record for the Lake Cowichan area starting in 191 and extending to December 2. The Cowichan Lake Forestry weather station was chosen as the reference location for the long-term record. Comparison between stations and regression equations used to adjust data are described. The Cowichan Lake Forestry record is evaluated using data from the Shawnigan Lake weather station, adjacent to the eastern end of the Cowichan River valley. 2 METHODS Data from nine weather stations were used to create the Cowichan Lake Forestry long-term record (Table 1; Figure 1), though most of the record is from three stations. Data from five stations were used to evaluate the homogeneity of the record that was created (Table 2). Daily maximum and minimum air temperature and total precipitation from 191 to 2 for Environment and Climate Change Canada (ECCC) stations were obtained from the national archive ( and the Pacific Climate Impacts Consortium ( Station history was obtained from Environment Canada (199). The British Columbia Ministry of Forests, Lands, Natural Resource Operations and Rural Development (FLNR) fire weather database provided hourly air temperature and precipitation records for Mesachie fire weather station (FWS) for 27 2 ( 1 Monthly data for 1 Note: A username and password is required to access this site. 1

10 TABLE 1 Weather station name and Environment and Climate Change Canada identification number (ID), location, elevation, record length, and period used in creating the long-term Cowichan Lake Forestry (CLF) record. Location data are from Environment Canada (199). Station, ID Location, elevation Record Period used Comments Cowichan Bay, 2' N 3 37' W, 1 m Oct Sept. 191 Oct Sept. 191 Temperature and precipitation 11 1' N 3 36' W, 3 m Jan. 192 Dec May temperature Station homogeneity testing 3' N, 3 3' W, 1 m Jan Sept. 196 Comparison with CLF ' N, 3 35' W, 1 m Oct. 196 Apr. 19 Comparison with CLF Cowichan Lake Forestry, 1 Cowichan Lake Hatchery, Lake Cowichan1, 1193 Lake Cowichan2, 155 Mesachie fire weather station Nanaimo, 53 Shawnigan Lake, Victoria Gonzales, 11 5' N ' W, 177 m Apr. 199 Dec. 2 Apr. 199 Dec. 2 Missing some data Feb. 27 Oct. 2 5' N 7' W, 152 m Nov. Aug. 19 Nov. May Precipitation June Aug. 19 Temperature and precipitation 9' N 3' W, 6 m Jan. 192 Apr. Jan. 192 Oct. Temperature and precipitation Nov. May Temperature 5' N 5' W, m Oct Dec Precipitation only July 1991 Oct ' N 3' W, 171 m July 196 Sept. 2 Gap-filling temperature Station missing data 5' N 13' W, 1 m Feb. 27 Sept. 2 Apr. 215 Sept. 2 Gap-filling temperature and precipitation 9 1' N 3 57' W, 31 m 9 11' N 3 5' W, 7 m 3.' N 3 3' W, 139 m 3.' N ' W, 137 m 25' N 3 22' W, 26 m 25' N 3 19' W, 7 m Mar. 191 Aug Mar. 191 Dec Feb. June 196 May 1911 Nov May 1911 Sept Nov Sept. 2 Jan. 19 Apr. 191 Mar. 191 Aug. 19 May 191 Aug. 19 Missing days in the early record Gap-filling temperature and precipitation Oct June 196 Gap-filling missing temperature two ECCC stations were obtained from the Adjusted and Homogenized Canadian Climate Data (AHCCD) base ( Default.asp?lang=En&n=B1F23A-1). The ECCC stations report the maximum and minimum air temperature and total precipitation in the last 2 hours from manual readings at about : am each day. Temperature was reported to.1 C until the end of 1979, and to.5 C from 19 onward. Precipitation is reported to.1 mm, and there have been changes in how trace precipitation is recorded. Measuring instruments and procedures have changed over time. Stations in the AHCCD record have been adjusted for these changes and for inhomogeneity created by station moves or changes to their surroundings (Vincent et al. 22; Mekis and Vincent 211). Mesachie FWS reports temperature and precipitation hourly to.1 C and.1 mm, respectively. 2.1 Station Assessment and Calculation of Adjustment Coefficients The daily data used were as recorded without any adjustments for changes in the measurement procedures applied in the AHCCD record. The temperature and precipitation records were evaluated using seasonal time series for inhomogeneity that is likely due to station relocation part way through the record (Environment Canada 199) (Appendix 1). Averages were calculated for specific periods to aid in correcting for inhomogeneity. The minimum air temperature is the variable most often affected by changes in station location. Graphing and statistical analyses were done in Excel, version 1 and R, 2

11 TABLE 2 Weather stations, location, elevation, record length, and period used in evaluating the long-term Cowichan Lake Forestry record Station, ID Location, elevation Period Comments a Shawnigan Lake, ' N ' W, 137 m May 1911 Sept. 2 AHCCD monthly data Victoria Airport, 12 3.' N ' W, 2 m Jan. 199 Dec. 215 AHCCD monthly data Youbou, ' N ' W, 17 m July 1959 Dec ECCC monthly data Port Alberni, ' N ' W, 33 m July 1917 Aug. 196 ECCC monthly data Pachena Point, ' N 5 6' W, 33 m Jan. 192 Feb. 27 ECCC monthly data a ahccd: Adjusted and Homogenized Climate Data record; eccc: unadjusted record from Environment and Climate Change Canada archives. Nanaimo N Legend Highway Weather station Weather station Weather station Cowichan Lake Y C H L2 L1 Duncan S Shawnigan Lake B V Kilometres CANADA USA Victoria G FIGURE 1 Location of weather stations used in the analysis. Red triangle with C = Cowichan Lake Forestry and the Mesachie fire weather station. Yellow triangles: B = Cowichan Bay; H = Cowichan Lake Hatchery; L1 and L2 = Lake Cowichan 1 and 2, respectively; Y = Youbou; N = Nanaimo; G = Victoria Gonzales. Orange triangles: S = Shawnigan Lake; V = Victoria Airport. version (R Core Team 2) using linear model function (lm()) and the Shapiro-Wilk test for normality. All adjustments and gap-filling were done using daily data. Figure 2 illustrates the relationship typically obtained for temperature. Even stations close to each other can have differences in daily temperature record of up to 5 C. Monthly averages are much closer because the daily 3

12 (a) CLF ( C) Maximum temperature (b) Minimum temperature Daily Monthly 5 Regression LC2 ( C) CLF ( C) Daily Monthly Regression LC2 ( C) FIGURE 2 Scatterplot of daily and monthly maximum (left panel) and minimum (right panel) air temperature at Cowichan Lake Forestry (CLF) as a function of Lake Cowichan2 (LC2) for January 193 to December 195. The black line (Regression) represents a linear regression on the daily data. differences average out. Gap-filling maximum and minimum air temperature was based on linear regression determined from daily data for a number of years of overlapping records. Daily precipitation can vary substantially over relatively short distances. Also, precipitation may be allocated to a different day, or the daily precipitation could be the accumulation over 2 3 days (Figure 3). This complicates daily comparisons; consequently, linear regression equations were developed using monthly total precipitation, which averages out many of the daily discrepancies (Figure 3). Adjustment factors between stations were developed using monthly totals grouped by season and linear relationships with the intercept forced through zero. Stations that had slopes of the regression equations for precipitation that were within 1 ±.5 were assumed to have the same precipitation as Cowichan Lake Forestry. (a) Daily precipitation (b) Monthly precipitation LC2 (mm/day) LC2 (mm/month) CLF (mm/day) CLF (mm/month) FIGURE 3 Scatterplot of daily (left panel) and monthly (right panel) precipitation at Cowichan Lake Forestry (CLF) as a function of Lake Cowichan2 (LC2) for January 193 to December 195. The black line is the 1:1 line. 2.2 Weather Stations Cowichan Lake Forestry (CLF): This station was chosen as the reference location because, unlike the other stations, it has not been moved, has long period of record, and has minimal missing data. The station was established

13 in late 199, and there is a data gap between March 215 and October 2. There is a fire weather station (Mesachie FWS) at the same location. The assessment of homogeneity indicated that the minimum temperature from 199 to was approximately 1 C too low compared to that recorded at other stations; for example, Shawnigan Lake AHCCD and Cowichan Bay (Appendix 1). The reason for this is unknown. All daily minimum temperatures from 199 to were increased by 1 C. Precipitation data from 199 to are homogeneous. Comparison with Lake Cowichan2 ( ) and Mesachie FWS (27 215) indicated that the latter part of the temperature and precipitation record was homogeneous. Mesachie fire weather station (MF): This station is about 1 m from the CLF station and is part of the FLNR fire weather network. The air temperature and precipitation instruments are different and are at different heights from those at the ECCC station. Data are recorded hourly on a data logger: temperature is reported as an average for the 5 minutes before the hour, and precipitation is reported as a total for the hour. Daily values were calculated to match the measurement procedure for the ECCC manual system; that is, : am to : am. This improved correlations with CLF, particularly for precipitation, compared to using a midnight-to-midnight day. Daily temperatures and daily precipitation were extremely well correlated between Mesachie FWS and CLF, and only a small adjustment was required for temperature, mainly to account for the different measurement heights (Table 3). Precipitation at the fire weather station is measured with a tipping bucket that will not register snow until it melts and it likely under-catches snow. This is not an issue for this analysis because the only Mesachie FWS winter precipitation measurements used were for winter 215/2, which was relatively warm with minimal precipitation as snow. Cowichan Lake Hatchery (CLH): This station was located about 1 km from CLF. Precipitation was measured from November to July 197, and temperature was measured from June to July 197. Records from CLH did not overlap those from CLF, but they did overlap those from Lake Cowichan1 and Shawnigan Lake, which facilitated testing for homogeneity and determining adjustment coefficients. Assessment of the CLH record (Appendix 1) indicated TABLE 3 Adjustment coefficients (slope and intercept) for daily maximum (Tmax) and minimum (Tmin) temperature and precipitation (PPT) from stations to Cowichan Lake Forestry. The precipitation relationships had the intercept forced through zero. Tmax Tmin PPT Slope Station Slope Intercept Slope Intercept Winter Spring Summer Fall Mesachie FWS Cowichan Lake Hatchery Lake Cowichan Lake Cowichan Shawnigan Lake Cowichan Bay Nanaimo Victoria Gonzales

14 that it was homogeneous. Records from Shawnigan Lake also overlapped those from CLF and were used to provide adjustment coefficients between CLH and CLF. A small adjustment to the temperature data was required, but none was needed for precipitation (Table 3). Lake Cowichan1 (LC1): This station had two locations about 6 km from CLF (Environment Canada 199). Assessment of the LC1 record (Appendix 1) indicated that it was homogeneous. It was used to provide daily precipitation from January 192 to October, and temperature from January 192 to April. Records from Shawnigan Lake were used to cross-reference LC1 to CLF for temperature. Data from LC1 overlapped those from CLH for part of the record, and data from LC1 s second location (precipitation only) overlapped those from CLF and Lake Cowichan2 (LC2). The second location had the same precipitation as LC2. Consequently, unadjusted precipitation data for LC1 from 192 to were used for CLF. Lake Cowichan2 (LC2): This station was close to the LC1 locations: about 6 km from CLF. Assessment of the LC2 record (Appendix 1) indicated that it was homogeneous. Records from LC2 overlapped those from CLF, which allowed direct calculation of coefficients for adjustment. Temperature adjustment was small and similar to that required for LC1. Precipitation at CLF was similar to that at LC2 in summer and was 5% greater in winter. The main use of data from LC2 was to check CLF data for homogeneity (Appendix 1). The LC2 record had substantial missing data, and days when daily precipitation appeared to be accumulated over 2 days. Data from LC2 were used to gapfill a few days of missing temperature and precipitation measurements in the CLF record. Cowichan Bay (CB): This station is about 5 km east-southeast of CLF. It has data from October 1913 to March 19, and its records overlap the early record of CLF. The CB station has been in four locations (Environment Canada 199), which complicated development of adjustment coefficients. Assessment of the record (Appendix 1) indicated that changing the station location affected the minimum air temperature during the early part of the record. Consequently, minimum air temperature was increased by 1. C before adjustment to CLF for October 1913 September 191. Shawnigan Lake (SL): This station is about 1 km southeast of CLF, outside the Cowichan River valley. Its period of operation overlaps that of CLF and all the other stations. Precipitation measurements started in March 1911, and air temperature recording began in April The station was moved in 1965 and is still operating in that location. The data from SL were used in two forms: daily data as recorded ( ), and monthly data from the Adjusted and Homogenized Canadian Climate Data base (SL_A) (1911 2). In the SL_A record, the earlier data were adjusted to make them homogeneous with the station s current location and to adjust for changes in measurement techniques. The unadjusted pre-1965 daily data were used to cross-reference between stations and CLF for some gap-filling when no other stations were available. The use of SL for gap-filling was minimized to limit its direct influence on the CLF record and thus allow the SL_A data to be used to test homogeneity of 6

15 the CLF long-term record. Analysis of the adjusted and unadjusted data showed that the pre-1965 maximum temperatures were increased by.6 C and minimum temperatures were reduced by 1.1 C in creating the SL_A record. Adjustment of precipitation data to account for changes in measurement procedures resulted in a 6% increase annually up to the late 197s and about a 3% increase since then. Such changes to precipitation data were not applied to the data in the CLF record. Nanaimo (NA): This station is about 5 km northeast of Cowichan Lake, and has data from March 193 to May 196. The primary use of data from NA was to aid extension of the record back prior to the measurements recorded at CB. As with other stations, NA has been moved occasionally, and the latter part of the record overlaps that of CLF, which facilitated development of adjustment coefficients. The homogeneity assessment of NA data (Appendix 1) indicated that the spring and summer minimum temperatures from 191 to 1913 needed to be increased by 1 C prior to generating values that were appropriate for CLF. Victoria Airport (VA): This station is about 55 km east-southeast of CLF, and has monthly data from 199 to 215. The VA station is part of the Adjusted and Homogenized Canadian Climate Data base. The record from 199 to 192 was based on data from Victoria Gonzales (VG), which were adjusted to create a record that was homogeneous with that of the airport location. The summer maximum temperatures appeared to be too high. This was likely an artifact of the adjustment procedure, and the result of moving the VG station in 191 from Victoria harbour to its current location (Environment Canada 199). Victoria Gonzales (VG): This station is about 75 km southeast of Cowichan Lake Forestry, and has data from July 19 to August 19. The VG record overlaps that of CLF and was used to fill about 5 days of missing daily temperature data prior to This produced a reasonable approximation of the CLF temperature because the temperature regime is usually regional in extent, and warm (cold) days at Victoria are warm (cold) days in the Cowichan Lake area. Youbou (YU): This station is about 9 km northeast of CLF, and it has a short precipitation record that overlaps that of CLF. YU s monthly values were within ± 2% of those recorded at CLF. Other stations: Records from Pachena Point and Port Alberni (Table 2) were used to help verify trends shown at other stations. In conjunction with data from the Victoria Airport, records from these stations verified that the low period of precipitation in the CLF record between and 1929 was realistic and not an artifact of the analysis. 2.3 Adjustment Coefficients and Uncertainty in the Adjusted Data Data from stations within 1 km of CLF required minimal or no adjustment to produce data appropriate to CLF (Table 3). The Shapiro-Wilks test showed that the data were normally distributed. The correlation coefficients of linear regressions were high for stations that were close to CLF (Table ), and standard errors were ± 1 C and ± 3 mm/day for daily temperature and precipitation, respectively. 7

16 TABLE Statistics of regression equations (Table 3) (adjusted R 2 and standard error [se_yx]) for calculating daily maximum (Tmax) and minimum (Tmin) temperature and precipitation (PPT) at Cowichan Lake Forestry from stations that overlap it. All regressions are significant at the p <.1 level. Standard error is given for daily data and daily data summed to monthly. These statistics cannot be calculated for the Lake Cowichan1 and Cowichan Lake Hatchery stations listed in Table 3 because they do not overlap Cowichan Lake Forestry. (na = not applicable). Tmax Tmin PPT a Station R 2 day, month R 2 day, month R 2 day, month se_yx C se_yx C se_yx mm Mesachie FWS.99.9, ,.3.99 M 3., 11. Lake Cowichan , ,..967 M 3., 27 Shawnigan Lake , ,.5 na 7.6, na na na na na.72 W na, 73 na na na na.769 Sp na, na na na na.71 S na, 15 na na na na.5 F na, 53 Cowichan Bay , ,. na 7.7, na na na na na.759 W na, 66 na na na na.6 Sp na, 3 na na na na.661 S na, 1 na na na na.72 F na, 57 Nanaimo , ,.5 na.1, na na na na na.693 W na, 7 na na na na.731 Sp na, 5 na na na na.59 S na, 23 na na na na.66 F na, 5 Victoria Gonzales. 2.7, ,.7 Not used na a M: all months; W: Winter = Dec., Jan., Feb.; Sp: Spring = Mar., Apr., May; S: Summer = June, July, Aug.; F: Fall = Sept., Oct., Nov. A larger adjustment was required for data from stations farther away from CLF (Table 3). Correlations between these stations and CLF were weaker and standard errors larger than those for the nearby stations. The NA, CB, and SL stations are similar distances from CLF, and the adjusted R 2 and standard errors were similar. Agreement was better for temperature than precipitation (Table ). The standard error in precipitation from the distant stations was about three times that for the stations that were close to CLF. This varied by season but was a similar percentage of the seasonal precipitation. The standard error in the calculated monthly average temperature was smaller than that for daily values (Table, second value in se_yx column). The coefficients (Table 3) were applied to the station data to create equivalent values for CLF. The previously mentioned adjustments to NA and CB minimum air temperature were applied prior to adjustment to CLF. The adjusted data were assessed to ensure that maximum temperatures were greater than the minimum temperature by >. C. This was an issue in the early record resulting from uncertainty of up to 2 C in the prediction of daily CLF temperature from distant stations (Table ) and occurred on days when the

17 temperature range was small (e.g., a winter day with substantial precipitation). Adjustment to meet the criterion of >. C was needed on only 3 days of the long-term record. The daily time series were also visually evaluated for outliers that may have resulted from using adjusted station data. Monthly and annual extremes for CLF may not be as well predicted from distant stations as from nearby stations. This is not surprising since they are at the tail end of the distribution of values where confidence intervals on regression equations are the widest. For example, for Cowichan Bay, between January 196 and March 19, the extreme maximum was 3 C, while the predicted value was 39 C. The measured and calculated extreme minimum temperature was.2 and 15. C, respectively. Consequently, the early data should not be used to determine record extreme values. However, the frequency distribution of temperature was well predicted (Figure ). Similar results were obtained for calculated values for the other stations. Although monthly precipitation was quite well predicted, the daily values had a standard error of up to mm/day for stations that are distant from CLF (Table ). The distant stations are farther from the mountains and closer to the coast than is CLF, which results in substantially less total precipitation and fewer days with precipitation than at CLF. For example, annually, CB has 5% less precipitation and 1% fewer days with precipitation than does CLF. Consequently, the adjusted record for CLF based on CB data does not have enough days with rain, particularly in the winter. Thus, the probability distribution of daily precipitation does not fully match the measured values, although the discrepancy is much less in summer (Figure 5). The monthly distribution (Figure 6) also shows some small discrepancies between the measured and calculated data. Stations closer to CLF (i.e., LC1, LC2, and MF) have similar total precipitation and distribution of daily precipitation as CLF. (a) Daily maximum temperature (b) Daily minimum temperature.1.1. CLF. CLF.1 Calc.1 Calc Frequency Frequency FIGURE Frequency of occurrence of daily maximum (left panel) and minimum (right panel) air temperature at Cowichan Lake Forestry (CLF) and calculated from Cowichan Bay data (Calc) for Data are in 2 C bins. 9

18 (a) Frequency Daily winter precipitation (b) Daily summer precipitation.6..5 CLF.7 CLF Calc.6 Calc FIGURE 5 mm/day Frequency of occurrence of daily precipitation in winter (left panel) and summer (right panel) at Cowichan Lake Forestry (CLF) and calculated from Cowichan Bay data (Calc) for Data are in 5 mm/day bins. Frequency Frequency Monthly precipitation mm/day CLF Calc mm/month FIGURE 6 Frequency of occurrence of monthly precipitation at Cowichan Lake Forestry (CLF) and calculated from Cowichan Bay data (Calc) for Data are in 25 mm/month bins. 3 EVALUATION OF THE LONG-TERM RECORD Data from nine weather stations were used to generate the record for Cowichan Lake Forestry, although most of the record (192 2) was based on data from three stations (Table 5). The standard error in estimating daily temperature and precipitation at CLF based on data from other stations ranges from.9 to 2.7 C and 13 to + mm/day (Table ), respectively. The homogeneity of the Cowichan Lake Forestry record was evaluated against time series of seasonal and annual data from Shawnigan Lake AHCCD from 1913 to 2. Graphical comparisons and a regression analysis of the seasonal and annual averages and totals were generated (Table 6; Figures 7 1; Appendix 2). There is excellent agreement in the inter-annual variability and trends in temperature between CLF and SL_A (Figure 7). Temperatures are similar because the stations are at similar elevations (Table 1). The minimum temperature at CLF shows greater variability, particularly for cooler values, than at SL_A. The temperature range (the difference between the maximum and minimum air temperatures) can highlight small deviations between stations. The 197s stand out; during this period, CLF had a larger temperature range 1

19 TABLE 5 Sources of data for the Cowichan Lake Forestry long-term record Date start Temperature Precipitation Comments a Mar. 1, 191 Nanaimo Nanaimo Gap-fill temperature from Victoria Gonzales Nanaimo Tmin +1 C in spring, summer May 2, 1911 Nanaimo Shawnigan Lake Gap-fill temperature from Victoria Gonzales Nanaimo Tmin +1 C in spring, summer Apr. 1, 1913 Shawnigan Lake Shawnigan Lake Oct. 1, 1913 Cowichan Bay Cowichan Bay Gap-fill temperature from Shawnigan Lake Cowichan Bay Tmin +1.2 C Oct. 1, 191 Nanaimo Nanaimo Dec. 1, 191 Shawnigan Lake Shawnigan Lake Jan. 1, 192 Lake Cowichan1 Lake Cowichan1 Nov., Lake Cowichan1 Cowichan Lake Hatchery Account for shift in Tmax and Tmin in 1935 May 1, Cowichan Bay Cowichan Lake Hatchery June 15, Cowichan Lake Hatchery Cowichan Lake Hatchery Gap-fill from Shawnigan Lake July 19, 197 Shawnigan Lake Shawnigan Lake Oct. 1, 197 Cowichan Lake Hatchery Cowichan Lake Hatchery Gap-fill from Shawnigan Lake Sept. 1, 19 Shawnigan Lake Shawnigan Lake Apr. 1, 199 Cowichan Lake Forestry Cowichan Lake Forestry Gap-fill from Shawnigan Lake Cowichan Lake Forestry Tmin adjusted by +1 C Aug. 1, 196 Cowichan Lake Forestry Cowichan Lake Forestry Gap-fill from Lake Cowichan2 Oct. 1, 27 Cowichan Lake Forestry Cowichan Lake Forestry Gap-fill from Mesachie Lake fire weather station Apr. 1, 215 Mesachie Lake fire weather station Mesachie Lake fire weather station Nov. 11, 2 Cowichan Lake Forestry Cowichan Lake Forestry a Tmin = minimum temperature; Tmax = maximum temperature. TABLE 6 Summary statistics (adjusted R 2, se_yx ) for the comparison of Cowichan Lake Forestry with Shawnigan Lake Adjusted and Homogenized Canadian Climate Data for annual and seasonal mean maximum (Tmax), mean minimum (Tmin), and average (Tave) air temperature and total precipitation (PPT) from 1913 to 2. All relationships are significant at p <.1. Period Tmax Tmin Tave PPT Annual.736,.5 C.777,.3 C.53,.3 C.9, 7 mm Winter.5,.5 C.7,.5 C.927,. C.77, 13 mm Spring.21,.6 C.77,. C.,. C.76, 6 mm Summer.676,.9 C.621,.5 C.777,.5 C.769, 27 mm Fall.72,.6 C.66,.5 C.75,. C.6, 7 mm than did SL_A (Figure ). This could indicate that there has been a change at one of the stations, perhaps a change in surrounding conditions. The larger range at CLF is a result of higher maximums and lower minimums than at SL_A (Figure 7). The record for Lake Cowichan2 is contemporary with CLF and shows the same pattern as CLF (Figure ), which suggests that the difference between CLF and SL_A is not due to a measurement error in the CLF record. The seasonal temperature data show similar patterns as, and agreement with, the annual data. The difference in the temperature range between CLF and SL_A is most noticeable in the summer (data not shown) and corresponds 11

20 (a) FIGURE Annual maximum temperature Annual maximum (left panel) and minimum (right panel) air temperature for Cowichan Lake forestry (CLF) from 192 to 215 and Shawnigan Lake Adjusted and Homogenized Canadian Climate Data (SL_A) from 191 to FIGURE CLF SL_A (b) Annual temperature range Annual minimum temperature Annual air temperature range (maximum minus minimum) for Cowichan Lake Forestry (CLF) from 192 to 2, Shawnigan Lake Adjusted and Homogenized Canadian Climate Data (SL_A) from 191 to 2, and Lake Cowichan2 (LC2) from to CLF SL_A CLF SL_A LC (a) 35 Precipitation (mm/yr) Annual precipitation CLF SL_A (b) FIGURE 9 Annual precipitation (left panel) and annual precipitation anomaly (annual value minus the period average) (right panel) for Cowichan Lake Forestry (CLF) from 192 to 2 and Shawnigan Lake Adjusted and Homogenized Canadian Climate Data (SL_A) from 19 to 2. Anomaly (mm/yr) Annual precipitation anomaly CLF SL_A

21 (a) Degree days/yr Degree days < (b) Degree days > Degree days/yr FIGURE 1 Annual degree days < C (left panel) and > 5 C (right panel) for Cowichan Lake Forestry from 192 to 2. The red line indicates the trend of 3 and +3 degree days per decade for < C and > 5 C thresholds, respectively. to about a.5 C shift in the temperatures. Correlation coefficients and standard errors (Table 6) are similar to those reported in Table where the measured CLF record and SL overlap. Trends and inter-annual variation in annual precipitation are similar between CLF and SL_A (Figure 9). Annual precipitation at CLF averages about 7 mm more than at SL_A. This accounts for the greater variation in the annual anomalies (annual value minus the average of the whole record). The pattern in the inter-annual variation in anomalies shows excellent agreement between CLF and SL_A. The period of low precipitation from 1923 to 1929 is also apparent in the Victoria Airport (Appendix 1), Pachena Point, and Port Alberni records (not shown). As with temperature, correlation coefficients and standard errors for precipitation (Table 6) are similar to those reported in Table where the daily CLF and SL records overlap. TRENDS AND VARIATION IN TEMPERATURE AND PRECIPITATION Trends in seasonal and annual variables were calculated for the and periods in R using the zyp() package (Bronaugh and Werner 213). This package assesses the data for autocorrelation in the time series. Trends and their significance were calculated using the Zhang and Yue-Pilon methods as well as a custom package (Schwartz method) (R. Pike, ENV, pers. comm., 217). Trends are reported if the significance was 95%. The annual derived variables were calculated from daily data except precipitation as snow, evaporative demand, climatic moisture deficit, and heat/moisture index, which were calculated from monthly data following Wang et al. (2). Seasonal values were calculated using seasonal averages or totals. The record is based on daily data from Cowichan Lake Forestry station with minor gapfilling and is not subject to the issue of uncertainty in the early daily record. The periods correspond to the analysis of regional trends in temperature and precipitation in PCIC (213). The temperature record can visually be divided into three periods: is relatively constant, 1925 to the mid 195s shows a shift to warmer and relatively constant temperatures, and the late 195s to 2 shows rising 13

22 temperature (Figure 7). This was confirmed using the R package changepoint (Killick et al. 2). The first period is based on data calculated from stations that are distant from CLF and so is subject to greater uncertainty than the rest of the record. However, this cooler period can also be seen in the Shawnigan Lake AHCCD data. The three trend analysis methods gave essentially the same results. Trends in the raw data and the significance (p value) for the pre-whitened data from the Yue-Pilon method are reported in Table 7. Trends ( C/decade) in maximum and minimum temperature are consistent with those in PCIC (213). TABLE 7 Decadal trends and p value of statistical significance for annual and seasonal temperature and precipitation and derived variables for Cowichan Lake Forestry, and Seasons follow the meteorological definition. The p values are ***<.1, **<.1, *<.5, and no trend. Period Variable Change/decade, p Change/decade, p Annual Maximum temperature.15 C, ***.21 C, *** Minimum temperature.11 C, ***.1 C, *** Extreme maximum temp..2 C, ** No trend Extreme minimum temp. No trend No trend Average temperature.13 C, ***.1 C, *** Precipitation: annual 3 mm, ** No trend Precipitation: as snow 6 mm, *** 1 mm, ** Precipitation: May Sept No trend No trend Precipitation: 1-day max No trend No trend Precipitation: 3-day max No trend No trend Degree days < C 3 days, ** days, * Degree days > C days, *** 61 days, *** Degree days > 5 C 3 days, *** 6 days, *** Degree days < 1 C 2 days, *** 6 days, *** Degree days > 1 C 5 days, *** 6 days, * Frost-free period No trend No trend No. frost-free days No trend 3 days, * Evaporative demand mm, *** 5 mm, ** Climatic moisture deficit No trend No trend Heat/moisture index No trend No trend Winter Maximum temperature.22 C, ***.29 C, ** Minimum temperature.9 C, ** No trend Average temperature. C, ***.19 C, * Precipitation No trend No trend Spring Maximum temperature.9 C, *. C, * Minimum temperature.6 C, **.25 C, *** Average temperature. C, **.21 C, ** Precipitation 15 mm *** No trend Summer Maximum temperature.13 C, *.19 C, * Minimum temperature. C, ***.17 C, *** Average temperature.1 C, ***.1 C, ** Precipitation No trend No trend Fall Maximum temperature.17 C, ***.2 C, ** Minimum temperature.13 C, *** No trend Average temperature.15 C, ***.13 C, * Precipitation No trend No trend 1

23 There are small differences in the temperature trend between seasons (Table 7). The greatest warming occurred in the winter. The period has a greater trend than the period and likely includes a signal of anthropologically induced global warming (PCIC 213). The positive air temperature trends are reflected in trends for variables derived from the temperature record; for example, a decrease in degree days below C, and an increase in degree days above 5 C (Figure 1). However, even though minimum temperatures have increased, there is no trend in the frost-free period (Figure 11) or the number of frost-free days, except for a small increase in the period. There is a small increasing trend in the evaporative demand calculated using the Hargreaves equation (Figure ). The extreme maximum and minimum temperatures show a weak or no trend. Trends in precipitation can be masked by large inter-annual variability. Only the annual and spring precipitation trends for show a significant change (increase) at 95%. This is consistent with trends at the regional level (PCIC 213). Precipitation as snow shows a decreasing trend (Figure (a) 3 Frost-free period (b) 5 Precipitation as snow 25 Days/yr 2 mm/yr FIGURE 11 Annual frost-free period (left panel) and annual precipitation as snow (right panel) for Cowichan Lake Forestry from 192 to 2. The black line in the left panel is the period mean of 17 frostfree days per year; the red line in the right panel is the trend of 6 mm/decade. (a) 5 Evaporative demand (b) 5 Climatic moisture deficit mm/yr mm/yr FIGURE Annual evaporative demand calculated using the Hargreaves equation (left panel) and the climatic moisture deficit (right panel) for Cowichan Lake Forestry from 192 to 2. The red line in the left panel is the trend of mm/decade; the black line in the right panel is the period mean of 277 mm/yr. 15

24 11) for both periods, consistent with the winter warming trend. The climatic moisture deficit (a measure of the difference between the evaporative demand in a month and precipitation) shows no trend (Figure 11). The increase in rainfall in spring is likely offsetting the increase in demand. One-day and 3-day annual maximum precipitation show no trend. As noted previously, daily extreme values in the first two decades may not be reliable and could affect the analysis for the longer period. The large inter-annual variability in precipitation is accompanied by periods of consecutive years with above- and below-average precipitation. For example, during the 192s, precipitation averaged ~ mm below the longterm mean (Figure 9), while it was ~2 mm below the mean during the 195 period. Other decades have dry years but no extended dry periods. From 29 to 2, summer precipitation averaged ~3 mm (25%) below the long-term mean, similar to conditions in the mid-19s, late 193s through early 19s, and 192s (Figure 13). Anomaly (mm/yr) Summer precipitation anomaly FIGURE 13 Summer precipitation anomaly (annual summer value minus the period average) for Cowichan Lake Forestry from 192 to 2. The average precipitation for the period is mm. Summer is June, July, and August. 5 SUMMARY A daily maximum and minimum air temperature and precipitation record from March 191 to December 2 was created for the Cowichan Lake Forestry station. The record for the period was based on data from two stations that are approximately 5 km from Cowichan Lake Forestry. These data had greater uncertainty than those from later periods, and extreme values need to be used with care. The adjusted monthly data likely well-represent conditions at Cowichan Lake Forestry. From January 192 to 199, almost all of the daily temperature and precipitation data were from two stations that are within 7 km of Cowichan Lake Forestry. Only small adjustments to station data were necessary to make them equivalent to Cowichan Lake Forestry. Gap-filling of missing