Trend of Ontario Precipitation

Trends of Precipitation in Ontario

Ziwang Deng, Xin Qiu, Xiaolan Zhou, Huaiping Zhu*
LAMPS,Department of Mathematics and Statistics,York University
*Project Leader: Professor Huaiping Zhu

Abstract: From 1979 to 2016, Ontario’s annual total precipitation increased by 7.6%(~2.0%/10Y), summer precipitation increased by 4.3%(~1.2%/10Y) and winter precipitation increased by 24%(~6.6%/10Y). The most significant increases occurred at south of the Nelson River Basin and west of the Hudson Bay Basin.


Global average of over-land annual total precipitation has increased by ~2.5%/10Y although changes vary significantly from region to region. What has happened in Ontario in the last four decades? and what is the spatial pattern of the trends for seasonal and annual precipitation in Ontario? Our previous analysis has revealed that annual precipitation averaged over the Province of Ontario has been increasing since 1901 and is expected to continue the increase trend in the future[1]; this report will focus on the spatial distribution of precipitation trends in the recent four decades based on most up-to-date historical data. This brief report includes data description, trend analysis methodology, results and a summary.


Trend of precipitation anomaly percentage change over the world for the period 1979-2016. The data used for trend calculation is the CRU 4.01 (unit in the map:%/10Y). This figure was generated by LAMPS, York University.


The Climatic Research Unit (CRU) TS v. 4.01 data[2] was used in this study.
  • Data Source:The gridded time-series used in this study can be found at DOI:;
  • Temporal Coverage:This version, released 20 September 2017, covers the period 1901-2016
  • Spatial Coverage: All land areas (excluding Antarctica) at 0.5° resolution
  • Variables: pre, tmp, tmx, tmn, dtr, vap, cld, wet, frs, pet
  • Reference: Harris et al. (2014) doi:10.1002/joc.3711

In this study we only used the precipitation data over Ontario from 1979 to 2016. The main reason for starting from 1979 is that, significant amount of satellite data became available since 1979, which is supposed to improve reanalysis datasets, including the CRU data used in this study.


Definition of precipitation anomaly percentage(PAP)

Precipitation Anomaly Percentage (PAP) is a simple drought indicator. In this report, PAPs of seasonal (summer and winter) and annual total precipitation are calculated using the following equation:

Where,Pr is the precipitation of a certain period (a season or a year), mPr is the long-term (25 years for 1981-2005) average precipitation of the period.

Trend analysis

PAP time series are fitted with linear regression[3]:

Where, a and b are regression coefficients; t=1979,1980,...,2016 is time (year);the last term is error. we used unit percent per decade (%/10Y) by multiplying the regression coefficient (b) by 10:

The calculation was carried out for all of the grid cells in Ontario. Finally, the spatial distribution maps of the PAP trends for the period 1979-2016 were generated with GIS software.


Statistics of the trends (%/10Y)

Values in the following table are based on trends at all the 8964 grid cells: average (Mean), standard deviation (Std), minimum (Min) and Maximum (Max) over Ontario.

Annual Summer Winter
Mean 2.0 1.2 6.6
Std 1.8 2.0 3.0
Min -1.0 -1.6 1.0
Max 5.6 7.2 14.8

Spatial distribution of the trends (%/10Y)

The spatial distributions of annual and seasonal (Summer and Winter) PAP trends are shown as following.

Fig. 1 PAP trends for annual total precipitation (%/10Y)
Fig. 2 Same as Fig.1 but for summer total precipitation
Fig. 3 Same as Fig.1 but for winter total precipitation


Seasonal and annual total precipitation amounts were calculated using the most updated high-resolution monthly climatic data in Ontario. A simple drought indicator, precipitation anomaly percentages (PAPs), were derived and their linear trends were estimated. Results show that averaged over the province, precipitation has increased at both annual and seasonal scales. The most significant increases happened in winter with maximum trends at south of the Hudson Bay Basin (~13%/10Y) and south of the Nelson River Basin (11%/10Y) (basin definition in [4]). Decreasing trends are rarely found in the province, for example, summer precipitation slightly decreased in a couple of locations in the central and northeastern Ontario.


Map of the three primary watersheds in Ontario: Hudson Bay (yellow), Nelson River (blue), and Great Lakes (purple) basins, with the locations of cities shown for reference and a call out map of the five sub-basins on the Great Lakes Basin.[4]


[1]OCDP,2017,Historical and Projected climate Trends over Ontario click here

[2]I. Harris, P. D. Jones, T. J. Osborn, D. H. Lister, Updated high-resolution grids of monthly climatic observations—the CRU TS3.10 dataset. Int. J. Climatol. 34, 623–642 (2014). doi:10.1002/joc.3711

[3]Madsen, H., Lawrence, D., Lang, M., Martinkova, M., & Kjeldsen, T. R. (2014). Review of trend analysis and climate change projections of extreme precipitation and floods in Europe. Journal of Hydrology, 519, 3634-3650.

[4]McDermid, J., Fera, S., & Hogg, A. (2015). Climate change projections for Ontario: an updated synthesis for policymakers and planners. Climate Change Research Report-Ontario Ministry of Natural Resources and Forestry, (CCRR-44).

Prepared by LAMPS York University