Climatic change, grass pasture and potential evapotranspiration

Impact of warm summers on the actual evapotranspiration from spring wheat grown on the eastern Canadian prairies

“…This question has been addressed by Morison and Gifford (1984), Brklacich et al (1992), Parton et al (1994), Kimball et al (1994) and Grant et al (1995), among others. Recently, Lockwood (1993, 1994 concluded that climate warming will decrease actual evapotranspiration rates while Lindroth (1996) advanced the opposite view.…”

“…This question has been addressed by Morison and Gifford (1984), Brklacich et al (1992), Parton et al (1994), Kimball et al (1994) and Grant et al (1995), among others. Recently, Lockwood (1993, 1994 concluded that climate warming will decrease actual evapotranspiration rates while Lindroth (1996) advanced the opposite view.The seasonal trends in the temperature records for the eastern portion of the Canadian Prairies are broadly consistent with the global picture (Environment Canada 1995). This raises the question of whether climate warming will increase or decrease the actual evapotranspiration from the cultivated lands in this semi-arid region.…”

The suppression of evapotranspiration by rising levels of atmospheric CO2

“…Enhanced temperatures associated with global warming may slightly increase evaporation from the land surfaces, but in contrast higher levels of atmospheric CO, may actually suppress vegetation transpiration (Lockwood 1994). At the present time, in regions like north-west Europe, it is not clear whether land surface evaporation is being enhanced or suppressed.…”

“…Thus the observed recent 25 per cent increase in atmospheric CO, above pre-industrial levels could well produce (assuming a linear relationship) a transpiration reduction of 2.5-5 per cent for common C3 plants and up to 6 per cent for C4 plants. For typical British daytime summer temperatures, the increase in transpiration loss per degree Celsius temperature rise is around 8 per cent for moist forest covers and 2 per cent for moist grass pasture (Lockwood 1994). This suggests a recent longterm increase in vegetation transpiration, due to changes in maximum temperatures because of global warming, of 2.3 per cent for forest and 0.6 per cent for grass pasture.…”

Greenhouse hydrology

“…By contrast, Bultot et al (1988a) suggest that plant physiological responses would be more sensitive to temperature/rainfall pertubations and nutrient-limiting factors than to the direct effects of a C02-enriched atmosphere. Lockwood (1994) used a simple numerical soil -vegetationatmosphere model to determine that the potential evapotranspiration of grass pasture could increase by +2% per degree C rise in temperature and that the rate of change is nonlinear with a maximum rate for mid-day temperatures of 10°C.…”