Climate change in the semiarid prairie of southwestern Saskatchewan: Late winter early spring

Climate change in the semiarid prairie of southwestern Saskatchewan: Late winter early spring H. W. Cutforth 1, B. G. McConkey 1, R. J. Woodvine 2, D. G. Smith 2, P. G. Jefferson 1, and O. O. Akinremi 3 1 Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Box 1030, Swift Current, Saskatchewan, Canada S9H 3X2; 2 Prairie Farm Rehabilitation Administration, Agriculture and Agri-Food Canada, 603-1800 Hamilton Street, Regina, Saskatchewan, Canada S4P 4L2; 3 Lethbridge Research Centre, Agriculture and Agri-Food Canada, Box 3000, Lethbridge, Alberta, Canada T1J 4B1. Received 29 December 1998, accepted 22 March 1999. Cutforth, H. W., McConkey, B. G., Woodvine, R. J., Smith, D. G., Jefferson, P. G. and Akinremi, O. O. 1999. Climate change in the semiarid prairie of southwestern Saskatchewan: Late winter early spring. Can. J. Plant Sci. 79: 343 350. Long-term weather and hydrological data were analyzed to study climate change during late winter early spring within an approximately 15 000 km 2 area in the semiarid prairie near Swift Current, Saskatchewan. The climate has changed over the past 50 yr. Winter and spring maximum and minimum temperatures have warmed, snowfall amounts have decreased, and spring runoff has started earlier now than during past years. The percentage of precipitation as snow has decreased as temperatures have warmed. As well, even though temperatures have warmed, the date of the last spring frost has not gotten earlier with time. Key words: Climate change, semiarid prairie, winter and spring, temperature, snow, spring runoff Cutforth, H. W., McConkey, B. G., Woodvine, R. J., Smith, D. G., Jefferson, P. G. et Akinremi, O. O. 1999. Modifications climatiques observées dans la zone de la prairie semi-aride du sud-ouest de la Saskatchewan: fin d hiver et début du printemps. Can. J. Plant Sci. 79: 343 350. Nous avons scruté les données météorologiques et hydrologiques de longue durée pour étudier les changements climatiques survenus durant la période englobant la fin de l hiver et le début du printemps dans un territoire d environ 15 000 km 2 de la prairie semi-aride, situé aux abords de Swift Current en Saskatchewan. Le climat a effectivement changé au cours des dernières années. Les maximums et minimums de températures hivernales et printanières sont plus hauts, l enneigement a diminué et l écoulement des eaux de fonte printanière débute plus tôt que dans le passé. Le pourcentage de précipitations tombées sous forme de neige a diminué à mesure que la température se réchauffait. Par ailleurs, même si la température s est relevée, la date de la dernière gelée printanière n a pas évolué au cours de la période examinée. Mots clés: Changement climatique, prairie semi-aride, hiver, printemps, température, neige, écoulement des eaux de fonte de la neige Climate change is not a new phenomenon but one that has occurred throughout history. Climate change occurs over a spectrum of both time and space (Schneider 1994). Natural causes of climate change include volcanic emissions into the atmosphere resulting in global cooling, fluctuations in solar activity that may contribute to cyclical warming/cooling alterations in global temperature, and changes in the Earth s orbital path about the sun and the Earth s rotational axis. What is recent, or new, is the human influence on global climate change (Schneider 1994), especially during the past 50 yr (Idso and Balling Jr. 1991, 1992). For example, industrial and agricultural emissions of greenhouse gases (such as CO 2, N 2 O, CH 4 ) and refrigerants (such as chlorofluorocarbons [CFCs] and hydrochlorofluorocarbons [HCFCs]) may contribute to global warming whereas other gas emissions (such as SO 2 ) may contribute to global cooling. There is growing consensus among climatologists that the Earth is warming on a global scale but what is happening on a regional and seasonal scale is less clear. For example, some climatic models predict a significant increase in precipitation in the agricultural heartland of the United States due to increased temperatures caused by rapidly rising CO 2 concentrations in the Earth s atmosphere (Idso and Balling Jr. 1991). However, there are also a number of studies indicating that the opposite 343 may occur (Schlesinger and Mitchell 1987; Danard et al. 1990; Vinnikov et al. 1990). Nevertheless, climate is highly variable and continually changing, and the driving forces behind these changes have both natural and human origins (Schneider 1994). One of the main natural contributors to climate change is the sun. The output of energy from the sun has been increasing since about 1850 (Foukal and Lean 1990; Francis and Hengeveld 1998). About half of the warming of the Earth s surface over the past century and a third of the warming since 1970 have been due to increased solar energy output. Further, natural and human emissions into the atmosphere disrupt the balance between incoming and outgoing radiant energy resulting in significant climate change. Emissions that increase the percentage of incoming radiant energy reflected back into space result in global cooling whereas emissions that decrease the amount of energy radiated through the atmosphere into outer space result in global warming. Abbreviations: ENSO, El Niño/Southern Oscillation; Tmx, average maximum air temperature; Tmn, average minimum air temperature; SPARC, Semiarid Prairie Agricultural Research Centre

344 CANADIAN JOURNAL OF PLANT SCIENCE There are natural contributors to short-term climate variability that do not result in long-term climate change. The El Niño/Southern Oscillation causes tremendous short-term climatic variability in Western Canada, but because of its sporadic and cyclical nature, the effects are temporary. For example, air temperatures in western Canada are strongly impacted by ENSO during winter, but the impact nearly disappears by spring (April, May) (Shabbar and Khandekar 1996). Over western Canada, winter temperatures during El Niño (La Niña) years tend to be warmer (colder) than normal (Ropelewski and Halpert 1986), whereas precipitation amounts tend to decrease below (increase above) normal amounts (Shabbar et al. 1997). Climatic models predict that human-induced warming should reduce the maximum minimum temperature difference, especially during winter, and that mean temperatures will increase with a majority of the increase due to increasing minimum temperatures (Smit et al. 1988; Idso and Balling, Jr. 1992; Thompson 1997). However, climate change has not and will not occur uniformly across the globe (Williams and Wheaton 1998). Some regions of the Earth have warmed faster than others, while some regions have become cooler (Environment Canada 1995). This same principle applies over large regions such as Canada. There are temporal and spatial differences in climate change across Canada. Some seasons and regions may show more pronounced temperature changes than others. For example, from 1950 to 1989, trend analyses for the prairie region indicate significant increases in both minimum and maximum temperatures during spring, with maximum temperatures increasing faster than minimum temperatures (Skinner and Gullett 1993). However, significant decreases in both maximum and minimum temperatures have occurred during winter and autumn in the Atlantic region. In contrast, Bootsma (1994) studied long term (100-yr) weather trends at five locations across Canada and found that detection of climatic warming (greenhouse-induced or otherwise) was difficult because of the extreme variability of climatic attributes. Bootsma concluded it was unlikely that the warming was caused by the greenhouse effect because the slight warming he observed was confined to the growing season, was only in western Canada, and was not apparent during winter (January). The size of the climate data set (i.e., duration of record) may have a profound influence on conclusions reached regarding climate change. For example, from 1895 to 1989, only minimum temperatures during spring and summer have increased on the prairies, whereas maximum and minimum temperatures have increased during summer in the Atlantic region with maximum temperatures increasing at a faster rate than minimum temperatures. These results are quite different from those obtained using weather data from the 1950 to 1989 time period (see above). Using the larger data set or longer time frame, average temperatures for the prairies show a trend towards greater warming during winter and spring than in summer and fall, although only spring and summer warming trends are significant (Environment Canada 1995). In the Northern Hemisphere, the carbon dioxide concentration in the atmosphere increases in winter and decreases in summer. These fluctuations in atmospheric CO 2 concentrations are mainly in response to seasonal growth patterns of forests, tundra and grasslands between 30 N and 80 N (Keeling et al. 1996). Keeling et al. (1996) found that since the early 1960s the spring decline in CO 2 concentrations now occurs about 7 d earlier than in the mid-1970s. They suggest that rising temperatures, particularly in winter and spring, have stimulated northern plants to begin growing earlier in the spring now than in past decades. Detecting and determining the direction of climate change is a challenging task. Most of the evidence provided by trend analysis of long-term weather data and by climate change models suggest that climate is warming over western Canada as a whole. The objective of this study was to investigate climate change over a 15 000-km 2 tract of land, a relatively small area in southwestern Saskatchewan, within which there are several weather recording sites and at least two hydrological recording sites. Thus our study was sitespecific by design. We examined the temporal trends of hydrological and climatic parameters within the past 50 yr and compared our results to those derived from extensive weather model studies. In addition, we discuss several implications climate change may have for agriculture in southwestern Saskatchewan. METHODS AND MATERIALS For a 15 000-km 2 area west and south of Swift Current, Saskatchewan (Fig. 1), the environmental and hydrologic measurements chosen to detect changes in climate from 1950 to 1998 were: the average seasonal maximum and minimum air temperature; the date of the last spring frost ( 2.2 C); the number of days during April, May, June with Tmn < 2.2 C; the average Tmx for those days with Tmn < 2.2 C; the average seasonal soil temperature at the 10-cm depth; total snowfall during mid- to late-winter (January and February) and early spring (March and April); the Julian day for spring runoff to be 75% complete from 5-ha watersheds; and the Julian day when spring runoff caused a significant increase in streamflow rate of the Swift Current Creek (Fig. 1). Climate Data Soil temperatures at the 10-cm depth under grass have been recorded at an Environment Canada standard meteorological site located on the South Farm of the Semiarid Prairie Agricultural Research Centre (SPARC) since 1963. Soil temperatures were measured with thermistors and were recorded twice daily, at about 08:00 h and 16:00 h. Average seasonal soil temperatures were determined from the daily measurements. Maximum and minimum air temperatures and snowfall amounts were recorded at several climatological sites located west and south of Swift Current by Environment Canada (Table 1 and Fig. 1). Average seasonal air temperatures and snowfall amounts for the 15 000-km 2 land area were determined from weather data averaged across all climatological sites. The historical precipitation data are heterogeneous due to changes in measurement technique. The gauge type used to measure rainfall at all sites was changed around 1975. To

CUTFORTH ET AL. CLIMATE CHANGE AND SPRING IN SW SASKATCHEWAN 345 Fig. 1. Location of the weather recording sites and the streamflow data recorder in southwestern Saskatchewan. Circled is the approximate area over which the environmental and hydrologic measurements chosen to detect climate change apply. Included in the delineated area is that portion of the Swift Current Creek watershed from which runoff was measured by the streamflow data recorder located on the Swift Current Creek. The soil zones delineate the semiarid prairie and indicate the soil and climate variability within the region. Table 1. Location and study period for air temperature and precipitation for the various climatological stations used in this study Environment Canada Location Study period station Latitude Longitude Elevation Temperature precipitation Station number ( N) ( W) (m) Aneroid 4020160 49.7 107.3 753 1950 1996 1950 1996 Cadillac 4021040 49.7 107.8 785 1950 1959 Eastend 4032323 49.4 109.0 1080 1984 1993 Gravelbourg 4022960 49.8 106.5 698 1950 1994 1950 1994 Gull Lake 4023060 50.0 108.5 907 1959 1991 1959 1991 Maple Creek 4024920 49.9 109.5 764 1950 1967 1950 1967 Maple Creek North 4024936 50.0 109.5 764 1951 1996 1951 1996 Shaunavon 2 4027485 49.7 108.4 914 1950 1996 1950 1996 Swift Current SRL 4028060 50.3 107.7 762 1950 1972 1950 1972 Swift Current CDA 4028060 50.3 107.7 825 1959 1997 1959 1997 Tompkins 4028101 50.0 108.5 811 1986 1997 1986 1997 account for the differences in gauge type and for wetting losses, the historical rainfall data were corrected by increasing daily rainfall prior to 1975 by a factor of 1.05 in accordance with the results of Metcalfe et al. (1997). Except at Swift Current CDA where the nipher gauge was used to measure snowfall, snowfall at all sites was measured using the ruler method (with a 10:1 conversion ratio). Data were collected at both Swift Current sites concurrently between 1959 and 1972 and heterogeneity was removed from the snowfall data by correlation of Swift Current CDA data with Swift Current SRL data using regression analysis. Spring Runoff Hydrological parameters such as snowmelt runoff are excellent indicators of climatic change because they integrate the effects of several climatic variables such as air temperature, solar energy, wind and relative humidity. From 1962, spring runoff data have been collected from three watersheds of about 5 ha each located on the South Farm of SPARC near the meteorological site. We estimated that when about 75% of the spring snowmelt runoff was complete, about 90% of the field was snow-free. The remaining 10% of the field was covered with deeper drifts, which accounted for about 25% of the total snowmelt runoff. As the drifts melted, the soil in snow-free areas thawed and warmed, and in response, winter annual and perennial plants began to grow. From 1955, Environment Canada has measured streamflow rates for the Swift Current Creek at a location just below Rock Creek, which has a drainage area of 1428 km 2 (Fig. 1). Water from precipitation events in the Cypress

346 CANADIAN JOURNAL OF PLANT SCIENCE Fig. 2. The seasonal average soil temperature at the 10 cm depth for mid- to late-winter (WST) (January and February) and for early spring (ESST) (March and April) from 1963 to 1998. Soil temperatures were measured at the meteorological site located near Swift Current on the South Farm of SPARC. Hills southwest of the Swift Current Creek streamflow gauge recharge ground water that within a short time may contribute substantially to the flow rate of the Swift Current Creek. For this reason, we chose as an environmental integrator of snowmelt runoff the date when spring runoff first started and resulted in a substantial increase in streamflow rates for the Swift Current Creek. At this date, runoff flowed over frozen ground and therefore we assumed snowmelt would not have affected ground water flow into the Swift Current Creek. Statistical Analysis Linear regression analysis was used to determine relationships between environmental parameters and time. Unless otherwise stated, we chose P = 0.10 as the probability level for statistical significance. Statistical analyses were performed with the JMP software package (SAS Institute, Inc. 1995). As well, we examined the distribution of data for normality. Fig. 3. The relationships of seasonal mean maximum and minimum air temperatures for mid- to late-winter (January and February) (top) and for early spring (March and April) (bottom) as a function of year from 1950 to 1998. Temperatures were averaged across several locations within an approximately 15 000-km 2 area west and south of Swift Current. The data for 1998 (triangles) are from Swift Current only and were not been included in the analysis. RESULTS AND DISCUSSION Soil and Air Temperature At Swift Current, from 1963 to 1998, the average soil temperature at the 10-cm depth for winter and for early spring increased significantly with time (Fig. 2). Winter and early spring 10-cm depth soil temperatures increased at an average rate of 0.084 and 0.062 C yr 1, respectively. Therefore, to 1998, the average soil temperature at 10 cm for winter and early spring have increased by about 2.9 C and 2.2 C, respectively, since 1963, and by about 1.9 C and 1.4 C, respectively, since 1975. Since 1950, across the land area included in this study, the Tmx and Tmn have increased during late winter (P < 0.05) and during early spring (P < 0.001) (Fig. 3). Although the differences were not significant, Tmx tended to increase at a higher rate during spring compared to winter whereas rates of increase for Tmn were approximately equal during winter and spring, and when averaged across the two seasons, both Tmx and Tmn tended to increase at about the same rate (0.0958 C yr 1 ). Thus, to 1997, Tmx and Tmn for both winter and spring have increased, on average, approximately 4.5 C since 1950 and by approximately 2.1 C since 1975. In contrast, Skinner and Gullet (1993) found that only spring Tmx and Tmn have increased from 1950 to 1989 in the Prairie region. As well, the increase was greater for Tmx compared to Tmn. Further, Berry (1991) found the seasonal temperature increase had been highest for winter and spring and for western versus eastern Canada from 1950 to 1989. In particular, Berry (1991) found the temperature increase in southern Saskatchewan was the highest increase of any region in Canada. Mean minimum temperatures during winter and spring have increased steadily during the past 50 yr but we did not find any relationship between the date of the last spring frost ( 2.2 C) and year (P > 0.6) (Fig. 4, top left). In contrast, Bootsma (1994) found that the dates of the last spring frost have become significantly earlier at several locations in western Canada (such as Brandon and Indian Head). However, we did find evidence of climatic warming within the frost data. The number of days during April, May and June with Tmn < 2.2 C has decreased since 1950 (P = 0.051) (Fig. 4, top right), and the average Tmx for those days when Tmn < 2.2 C has increased since 1950 (P = 0.006) (Fig. 4, bottom left).

CUTFORTH ET AL. CLIMATE CHANGE AND SPRING IN SW SASKATCHEWAN 347 Snowfall Snowfall totals have decreased for winter (January and February P < 0.01) (data not shown), and for winter plus early spring (January, February, March, April P = 0.028) since 1950 (Fig. 5, top). Therefore, to 1997, winter plus spring snowfall amounts have decreased on average by about 29.9 cm since 1950 and by about 14.0 cm since 1975. Snowfall in the zone 45 N to 55 N (which includes the southern Prairies) is negatively correlated with the annual mean surface air temperature over the Northern Hemisphere (Groisman and Easterling 1994). Taking into account this relationship and the coherence of annual snowfall totals with regional maximum temperatures, Groisman and Easterling (1994) foresee a future decrease in snowfall totals for southern Canada if global warming occurs. Our findings support this statement: we observed that winter (January, February) and spring (March, April) average maximum air temperatures have increased (Fig. 3) and winter plus spring snowfall totals have decreased (Fig. 5, top) since 1950. As well, winter plus spring snowfall totals have decreased as Tmx increased (P < 0.0001) (Fig. 5, bottom). Further, total precipitation (rain plus snow) during winter plus spring has decreased as Tmx increased (P < 0.0001) and has decreased with year (P < 0.08) (data not shown). As temperatures have warmed during winter and spring, the proportion of precipitation as snow has decreased. For example, the proportion of winter plus spring precipitation as snow has decreased with year (P < 0.096) (Fig. 6, top) and has decreased as temperatures have increased, especially Tmn (P < 0.063) (Fig. 6, bottom). Spring Runoff A further indication of climate change at Swift Current was provided by spring runoff data. From 1962 to 1998, the Fig. 4. The relationships of the date of the last 2.2 C spring frost (top left), the number of days the Tmn was < 2.2 C during April, May and June (top right), and the Tmx for those days when Tmn < 2.2 C (bottom left) to year from 1950 to 1997. Temperatures were averaged across several locations within a 15 000-km 2 area west and south of Swift Current. Fig. 5. The relationships of winter plus spring snowfall totals (cm) (January, February, March and April) as a function of year (top) and average maximum temperature (Tmx) (bottom) from 1950 to 1997. Snowfall amounts were averaged across several locations within a 15 000-km 2 area west and south of Swift Current. The data for 1998 (triangles) are from Swift Current only and were not included in the analysis.

348 CANADIAN JOURNAL OF PLANT SCIENCE Fig. 6. The proportion of winter plus spring (January, February, March and April) precipitation as snow plotted as a function of year (top) and average minimum temperature (Tmn) (bottom) from 1950 to 1997. Snowfall and precipitation amounts were averaged across several locations within a 15 000-km 2 area west and south of Swift Current. The data for 1998 (triangles) are from Swift Current only and were not included in the analysis. Julian day when spring runoff from the field scale watershed was 75% complete decreased (i.e., occurred earlier) with time (P = 0.057) (Fig. 7, top). The Julian day when 75% of the spring runoff was complete decreased at an average rate of 0.483 d yr 1. On average, spring runoff from the field in 1998 was 75% complete 18 d earlier than in 1962, and 11 d earlier than in 1975. Further, from 1955 to 1998, the date when spring runoff first started and resulted in a substantial increase in streamflow rates for the Swift Current Creek decreased with time (P = 0.0007) (Fig. 7, bottom). The day when spring runoff started decreased at an average rate of 0.547 d yr 1. On average, in 1998, spring runoff from the drainage basin started 24 d earlier than in 1955 and 13 d earlier than in 1975. Implications for Agriculture For a 15 000-km 2 agricultural area west and south of Swift Current, in the semiarid prairie of southwestern Saskatchewan, the climate during the past 50 yr has changed. Measurable changes were obtained in all the parameters that were used as indicators of climate change. The region has become warmer: soil temperature, and minimum and maximum air temperatures have increased during late winter and early Fig. 7. The relationships of runoff date for small (5-ha fields) (top) and large (1428 km 2 ) (bottom) watersheds with year. For the small field-scale watershed, plotted are the dates on which 75% of the spring runoff had been completed for the years 1962 to 1998. For the larger watershed, plotted are the runoff start dates when spring runoff caused the first substantial increase in flow rates of the Swift Current Creek for the years 1955 to 1998. The portion of the Swift Current Creek watershed referred to in this study is southwest of Swift Current, SK. spring. As well, even though we found no relationship between the last day of spring frost ( 2.2 C) and time, the number of days during April, May and June with Tmn < 2.2 C has decreased, and the average Tmx for those days when Tmn < 2.2 C has increased since 1950. The amount of snow received during winter and spring in the region has decreased during the past 50 yr. The warming trend has probably caused an increasingly larger proportion of annual precipitation to fall as rain rather than snow. Several other researchers (Berry 1991; Skinner and Gullett 1993; Bootsma 1994; Groisman and Easterling 1994) have found warming trends during winter and spring, particularly in southern Saskatchewan, and reduced snowfall amounts over the past 50 yr. For example, Akinremi et al. (1999) found snowfall across the prairies has declined over the past 35 yr. As well, Saunders and Byrne (1994) analyzed the simulation experiment of the CCC GCM for a 2 CO 2 scenario and concluded that less snow will fall on the prairies under a changed climate. Further, there are positive feedbacks from an early spring. For example, with less snow, lower amounts of solar energy are needed to melt the snow and expose the ground. Once the ground is exposed, less solar energy is reflected back into the atmosphere resulting

CUTFORTH ET AL. CLIMATE CHANGE AND SPRING IN SW SASKATCHEWAN 349 in a larger proportion of incoming energy being used to warm the soil surface and then the air. As the air is warmed, there is less chance of additional snowfall. Snow ablation and snowmelt runoff during spring from two independent indicators, small watersheds and the Swift Current Creek, have occurred increasingly earlier (by about 0.5 d yr 1 ) with time a direct effect of a warmer climate. Earlier springs have several implications for agriculture in this region. A decrease in the amount of snow will affect the spring soil water levels as a significant proportion of the overwinter soil water recharge in this region of the prairie comes from snowmelt (Steppuhn 1981). As well, snow pack buffers soil temperature in the winter increasing the chances for survival of winter crops (Steppuhn 1981). Thus, a reduction in snowfall could increase the susceptibility of winter crops to injury from cold temperatures. On the other hand, overwinter survival of insect pests and diseases may be reduced without the buffering influence of a snow pack. Further, for plants to respond to winter and spring warming and begin growing earlier, they need to tolerate, or become hardened to, spring frosts occurring at later growth stages. Hardened plants can tolerate severe frosts. For example, fall-seeded canola that begins growing early the following spring can survive up to 8 degrees of frost (Byer 1997; Johnson et al. 1998). However, once canola dehardens it is very sensitive to frost. If earlier springs result in early seeding dates, the early seeding will allow crops to take advantage of the wetter spring while avoiding the drier summer period that is typical of this semiarid prairie region. Thus, early seeding promotes a better synchrony between water availability and water demand by crops. Earlier seeding may benefit some but not other crops. For example, for the past several decades, maximum yields in the Brown soil zone were obtained when spring wheat was seeded anytime from about mid-april to mid-may (Cutforth et al. 1990). Thus, earlier springs (i.e., earlier seeding dates) may mean relatively little with regards to maximizing yields of spring wheat. However, for crops where May planting is not optimal in the semiarid prairie the situation changes. For example, the recommended seeding date for maximizing yield of field pea in the Brown soil zone is about 20 April (Miller et al. 1998). There is usually a substantial yield penalty for delaying seeding beyond this date. Similar to field pea, research is showing that canola should also be seeded early to obtain maximum yields (Byer 1997; Johnson et al. 1998). The earlier springs we now experience relative to the past decades allow for earlier seeding and thus higher yields of crops such as field pea and canola. However, other computer simulations of various warm climate scenarios suggest warming over the next few decades could be expected to reduce the total biomass productvity of the agricultural zone of the Canadian prairies, although it would increase in the Canadian Shield areas (Williams and Wheaton 1998). As well, throughout Saskatchewan the peak growth season would occur earlier. The simulation of earlier peak growth is supported by the observations of Keeling et al. (1996) who postulated that observed spring declines in atmospheric CO 2 concentrations over North America have occurred earlier now than in the past in response to climatic warming that has resulted in earlier seasonal vegetative growth, the majority of which are forests, grasslands and tundra of the Canadian north. Estimating the impacts of climate change is still a new science, and because simulated results are not easily confirmed by experiment or by historical analysis, caution must be exercised when interpreting results (Williams and Wheaton 1998). The need for judicious interpretation is emphasized by the marked differences in outcome from one scenario, impact model, or location to another. Nevertheless, we postulate that because of the warmer temperatures during spring, crops have the opportunity to begin growing earlier now than in the 1950s and 1960s. SUMMARY For a 15 000-km 2 area west and south of Swift Current, Saskatchewan, the climate has changed over the past 50 yr. Winter and spring temperatures have warmed, snowfall and precipitation amounts have decreased, and spring runoff has started earlier now than in the past. As well, the percentage of precipitation as snow has decreased as temperatures have warmed. However, because precipitation totals are negatively related to temperature, especially Tmx, winter plus spring precipitation totals within the semiarid prairie (which already show a strong trend to decreasing with time P < 0.08) are likely to further decrease if climatic warming continues. Further, the climate has warmed over the past 50 yr and will likely continue to warm for the next century because of the effects of increased atmospheric levels of carbon dioxide and other gases (Williams and Wheaton 1998). ACKNOWLEDGEMENTS We gratefully appreciate the dedication and perseverance of the several volunteer observers who recorded daily weather observations at several of the climatological stations used in this study. We thank Doug Judiesch for his technical help throughout this study. Akinremi, O. O., McGinn, S. M. and Cutforth, H. 1999. Precipitation trends on the Canadian Prairies. J. Climate (in press). Berry, M. O. 1991. Recent temperature trends in Canada. Operational Geographer 9: 9 13. Bootsma, A. 1994. Long term (100 yr) climatic trends for agriculture at selected locations in Canada. Clim. Change 26: 65 88. Byer, J. 1997. Alternative seeding dates for spring canola. Agrifax, Publishing Branch, Alberta Agriculture, Food and Rural Development, Edmonton, AB. pp. 4. Cutforth, H. W., Campbell, C. A., Brandt, S. A., Hunter, J., Judiesch, D., DePauw, R. M. and Clarke, J. M. 1990. Development and yield of Canadian Western Red Spring and Canada Prairie Spring wheats as affected by delayed seeding in the Brown and Dark Brown soil zones of Saskatchewan. Can. J. Plant Sci. 70: 639 660. Danard, M. B., El-Sabh, M. I. and Murty, T. S. 1990. Recent trends in precipitation in eastern Canada. Atmos.-Ocean 28: 140 145. Environment Canada. 1995. The state of Canada s climate: monitoring variability and change. State of the environment report:soe Report 95-1. Environment Canada, Downsview, ON.

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