Impact of Land-use Change on the Diurnal Cycle Climate of the Canadian Prairies
This paper uses hourly observations from 1953-2011 of temperature, relative humidity, and opaque cloud cover from 14 climate stations across the Canadian Prairies to analyze the impact of agricultural land-use change on the diurnal cycle climate; represented by the mean temperature and relative humidity and their diurnal ranges. We show the difference between the years, 1953-1991 and 1992-2011. The land-use changes have been largest in Saskatchewan where 15-20% of the land area has been converted in the past four decades from summerfallow (where the land was left bare for one year) to annual cropping. During the growing season from May 20-August 28, relative humidity has increased by about 7%. During the first two months, May 20-July 19, maximum temperatures and the diurnal range of temperature have fallen by 1.2oC and 0.6oC respectively, cloud cover has increased by about 4%, reducing surface net radiation by 6 Wm-2, and precipitation has increased. We use the dry-downs after precipitation to separate the impact of cloud cover, and show the coupling between evapotranspiration and relative humidity. We estimate using reanalysis data from ERA-Interim that increased transpiration from the larger area of cropland has reduced the surface Bowen ratio by 0.14 – 0.2. For the month on either side of the growing season, cloud cover has fallen slightly; maximum temperatures have increased, increasing the diurnal temperature range and the diurnal range of humidity.
Plain English Discussion
Over the past 30 years farm management has changed a lot on the Canadian Prairies. In Saskatchewan, 20% of the total farm area has been converted from summer fallow (where no crop was planted) to annual cropping. This has boosted evaporation (transpiration) in the growing season so much that it has cooled the summer climate by more than a degree Celsius (2 degF), and increased the humidity by 7%. The peak of the growing season from May 20-July 19 is now also a little cloudier, as the extra evaporation condenses in clouds and falls out as extra rain.
Betts, A.K., R. Desjardins, D. Worth and D. Cerkowniak (2013), Impact of land use change on the diurnal cycle climate of the Canadian Prairies, J. Geophys. Res. Atmos., 118, 11,996–12,011, doi:10.1002/2013JD020717.