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S. M. McGinn and H. D. J. McLean

Abstract

The automation of weather stations necessitates an alternative approach to the traditional manual measure of free-water evaporation made using a class A pan. This study compared commercially available water-level sensing transducers mounted on class A pans to manual measurements using a class A pan. Measurements of free-water evaporation with two automated transducers over a 24-h period resulted in mean differences of 0.23 and 0.98 mm. Hourly measurements for free-water evaporation allowed examination of the correlation between principal weather elements and evaporation. Evaporation from the pan was highly correlated with wind speed at night (r = 0.86) and with air temperature during the day (r = 0.75). In addition, it was found that during the summer some 33% of the daily free-water evaporation occurred at night. For a 24-h period, accumulated free-water evaporation was highly correlated with air temperature (r = 0.85), net radiation (r = 0.81), incoming solar radiation (r = 0.80), and wind speed (r = 0.69).

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S. M. McGinn, K. M. King, and G. W. Thurtell

Abstract

The objective of this study was to improve the accuracy of net radiation (R n) measurements under conditions conducive to dew or frost deposition. Two nonventilated net pyrradiometers were mounted over grass during November and December 1986. A heating element was located on the supporting arm of each radiometer so that heal would be conducted to the sensing head. Heat was applied to one radiometer for a few days, followed by a period during which no heat was applied. The procedure was repeated, alternating between radiometers throughout the experiment. Heating the radiometers successfully averted the deposition of dew and frost on the domes, which produced errors in R n as high as 54 W m−2. The effect of heating alone was slightly asymmetric and resulted in a significant decrease in the mean R n of 8 W m−2 relative to the unheated radiometer. This effect can be compensated for in the calibration of the radiometer.

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O. O. Akinremi, S. M. McGinn, and H. W. Cutforth

Abstract

Regression analysis was used to establish linear trends of rainfall amounts and number of events at 140 stations with 40 years of record across the Canadian prairies. Annual rainfall was further split into three 4-month seasonal groups of amounts and events, and similar analysis was performed on these variables. There has been a significant increase in the amounts and number of rainfall events during the most recent 40-yr period (1956–95). Increase in annual rainfall was 51 mm, or about 16% of the 40-yr mean, while the number of rainfall events increased by 17, or about 29%. Spring (January–April) experienced proportionately the largest increase, with amount and number increasing by 46% and 64%, respectively, during the 40-yr period. This result may be related to the conversion of snow to rain as a result of warming during this period. The increases in rainfall amount and number of events during summer (May–August) were similar to the annual patterns. There was no significant increase in amount and number of rainfall events during the autumn season (September–December). The increases in rainfall amount and number of events were not uniform across the prairies, with the least increase in number and amounts of rainfall in southern Manitoba, Canada, and the largest increase in Alberta and Saskatchewan, Canada. Little or no change in amounts was obtained in the northern portion of the prairie provinces. The results confirmed that the prairies are not getting drier; however, there are seasonal and spatial differences in rainfall trends on the prairie.

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O. O. Akinremi, S. M. McGinn, and A. G. Barr

Abstract

Drought on the Canadian prairies is the single most limiting factor to crop yield. Several indices have been developed that indicate the onset, severity, and persistence of drought. This study was conducted to assess the validity of the Palmer Drought index for characterizing drought on the Canadian prairies. When the empirical relationship used by Palmer for calculating the weighting factor K was applied to historical weather data, the relationship appeared inappropriate. There was only a weak relationship between K and the moisture balance variables from which it is usually calculated. The regional correction factor was calculated to be 14.2, which is lower than the generally accepted value of 17.67. A soil water model, the Versatile Soil Moisture Budget (VMB), was coupled with the Palmer model to improve the modeling of soil water. The drought index obtained with the VMB explained 49% of the variation in wheat yield, while the original Palmer index explained 33%. In addition, a new drought index, which does not rely on the weighting factor K explained 57% of the variation in wheat yield, which is almost twice the variation explained by the original Palmer index.

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O. O. Akinremi, S. M. McGinn, and H. W. Cutforth

Abstract

The Canadian prairies are a major producer of grain, much of which is produced under rain-fed agriculture. The amount and timing of precipitation are critical to grain production. Information on the precipitation trend is therefore vital to this region. Regression analysis was used to establish linear trends of precipitation amounts, number of precipitation events, and variance of precipitation at 37 stations with 75 yr of record across the Canadian prairies. The precipitation was further split into rainfall and snowfall, and similar analysis was performed on these variables. The analysis showed that there has been a significant increase in the number of precipitation events mainly due to an increase in the number of low-intensity events. As such, precipitation events are not getting more intense on the Canadian prairies. The number of precipitation events (excluding events that are 0.5 mm or less) has increased by 16 events during the last 75 yr. Precipitation and rainfall amounts have increased significantly by 0.62 and 0.60 mm yr−1, respectively, on the Canadian prairies during the last 75 yr. During the period from 1921 to 1960 the trends in precipitation, rainfall, and snowfall were not statistically different from zero. However, from 1961 to 1995, snowfall has declined significantly by 0.95 mm yr−1. The trends in the most recent period (1961–95) were also significantly different from those in the 1921–60 period for snowfall. The difference in trends between the two periods for snowfall, combined with the inverse relationship in the rainfall–snowfall trends, suggest that these trends may be related to climate change.

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