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Precipitation | Temperature | Heat units | Frost free period | Growing season length
Precipitation
Precipitation provides the source of all terrestrial water. It occurs in many forms, rain, snow, dew and hail; but rainfall and snow are the greatest contributors. In Alberta precipitation is not uniformly distributed. Generally, in agricultural areas, 50 - 60 per cent of annual precipitation occurs during the growing season, mostly as rain. Figure 3 shows the long term average distribution of precipitation for Medicine Hat, Edmonton and Grande Prairie for each month. Precipitation in southern Alberta peaks in June while in central Alberta and the Peace regions the peak occurs in July. The June peak in precipitation in southern Alberta accounts for the fact winter wheat normally outyields spring wheat.
Precipitation in any month is extremely variable with dry spells of a month or longer possible at any time. The duration of dry spells tends to be shorter in central Alberta and the southern Peace Regions than elsewhere. Figure 4 shows the variability of precipitation for the crop year (Sept 1 - Aug 31) and for the May 1 - July 31 period for 1950 to 1988, for Lethbridge, Edmonton and Beaverlodge. The maximum, mean and minimum recorded over the entire period of record are also shown. In general the variability of precipitation is greater in southern Alberta and the Peace River area than in central Alberta. During the growing season, precipitation variation at Lethbridge and Beaverlodge is about 50 per cent of the mean while only about 30 per cent at Edmonton.
Because crop water use generally exceeds precipitation during the growing season, precipitation between the growing seasons is significant to crop growth and production. Soil moisture reserves are built up from this precipitation either in an eight-month period between harvest and seeding as in an annual cropping system or in a 20-month period as in a crop fallow rotation. In the eight-month storage interval between annual crops, 25 to 30 per cent of the precipitation will be rain, the rest will be snow. The storage efficiency of non-growing season precipitation in the Brown and Dark Brown Soil Zone will be 40 to 45 per cent on stubble fields cultivated once. In other soil zones, storage efficiency may be less.
Temperature
Temperature affects all living things. Each plant and animal has its own specific optimum temperature for growth and a temperature range over which it thrives. Once temperatures outside this range are encountered, the animal or plant suffers and growth slows. Cool season crops such as wheat, barley and oats don't grow well at temperatures below 5° C, they grow best at temperatures of 25 - 30° C and are stressed by temperatures above 30° C. Warm season crops such as corn have temperature needs that are 5 to 10 ° C warmer.
In the winter, temperature decreases at higher latitude and with distance from the mountains and the decreasing effectiveness of the Chinook in modifying the climate. In the summer, the temperature pattern is different. The highest elevations have the lowest temperatures, but the latitude effect is reduced. The highest temperatures occur where the elevation and precipitation are the least. Upland areas will have lower temperatures than the surrounding lowland area because temperatures drop 1° C for each 100 metres rise in elevation. Also, areas with sandy soils such as Fort Vermilion, tend to have a somewhat higher air temperature than surrounding areas with heavier, wetter soils.
Heat Units
Temperature affects the rate of plant growth. The concept of growing degree-days, also called heat units, assumes that plant growth is related directly to temperature if there are no other limitations. This is generally true as long as temperatures are not high enough to cause damage to plants. There is a minimum temperature at which growth starts, which is 5° C for most of the major crops growing in Alberta. The minimum threshold temperature is 10° C for crops like beans and corn. The upper thermal threshold for most crops found in Alberta is 30° C.
To calculate the degree-days for any particular day, the average daily temperature is determined from the maximum and minimum temperature. The base temperature is subtracted from the average temperature.
| maximum temperature | 27° C |
| minimum temperature | 15° C |
| average temperature (27+15) ÷ 2 = | 21° C |
| Degree-days above 5° C = 21 - 5 = | 16 |
| Degree-days above 10° C = 21 - 10 = | 11 |
If the average daily temperature is below the base temperature, the degree-day value equals zero. Negative values are not calculated because the crop is not set back. It is important, when applying degree-days, to know the temperature scale and base temperature used for the calculation.
Degree-day normals are prepared by accumulating the degree days from each day of each month in the growing season for a thirty year period. From these values, the average number of monthly or seasonal degree-days are calculated by computing the average.
Degree-day normals permit comparison of areas for their similarities or differences in growing potential. Crop requirements for heat may be matched to the number of degree-days available. For example, the two types of canola available for seeding have quite different heat unit requirements. Argentine varieties require 1040 to 1100 heat units, Polish varieties only require 860 to 920. Looking at the number of degree-days available for growth will help in selecting the type most suited to an area. In cases of delayed seeding or reseeding, heat units expected in the remaining part of the growing period will help determine which type to seed. Alfalfa will produce a cutting with 475 degree days if harvested at 10 per cent bloom. It is possible to estimate the next harvest time from the current degree day accumulation and the, weather expected.
There is a different set of heat units called corn heat units for measuring growth potential. These are calculated using separately the maximum and minimum temperatures. For further information, the publication Corn units for Corn in southern Ontario is available from the Ontario Ministry of Agriculture and Food. Corn varieties are all rated by the number of degree-days required to mature. Comparing the corn heat units available in an area to that needed by each variety permits the choice of a variety suited to the area.
Another use of the degree-day concept is heating degree-days, which indicate the amount of heating required for buildings. Heating degree-days are calculated by subtracting the daily average temperature from the base temperature of 18° C. From these values the energy needed to maintain a comfortable temperature in a building can be estimated. Heating degree-days or some other similar unit with a different base temperature could be used to estimate livestock feed requirements.
Frost Free Period
Temperature is measured at 1.2 metres above the ground surface at weather stations by thermometers located in louvered screens. A few stations, such as those at Agriculture Canada Research Stations, also record temperatures 3 or 4 cm above a short grass surface using thermometers exposed to the sky, which are more representative of the crop temperature. In late spring and early fall when frost occurs it has been shown that the grass minimum temperatures are always lower than that found at the screen height. The difference varies but is generally from 2 to 5° C.
When using screen temperature information for estimating frost dates, it must be remembered that the crop will be cooler than the screen data indicates. The important consideration is the relative comparison between areas that the data provides. Within any given area, local conditions may cause a variation in the timing of the last frost by as much as several weeks.
The actual frost date (0° C) varies each year. The variation of frost dates for Lethbridge, Calgary, Edmonton, Grande Prairie and Fort Vermilion are shown in Table 1 for spring and fall.
It can be seen that about 50 per cent of the last frosts occur in the spring in a period of a week before and after the average frost date. In the fall the first frost will occur 50 per cent of the time in a period of one week before and about one and a half weeks after the average frost date.
The frost free period is the number of days between the average last date of 0° C in the spring and the average first date of 0° C in the fall. This provides a measure of the period that plant growth should occur uninterrupted by frost; hence a method of comparing growing conditions within the province. Table 2 illustrates the variation of length of frost free periods in days at Lethbridge, Calgary, Edmonton, Grande Prairie and Fort Vermilion.
The effect of a 100-day frost free period is not equal across the province because day length varies. The longer day length in the Peace River area, compared to southern Alberta, appears to offset the shorter frost free period found in the north. Each crop reacts differently to increasing day length, so no general adjustment of frost free periods for day length is possible.
Most of the field crops produced in Alberta can withstand temperatures as low as -2° C without significant damage. Many seedlings can withstand temperatures of -4 to -6° C without harm. This permits spring seeding to start when soil temperatures reach a suitable level and not to wait until the last chance of frost disappears. The crop stage most susceptible to frost damage is flowering.
Table 1: Frost Dates For Five Alberta Locations
Table 2: Length of Frost Free Period in Days for Five Alberta Locations.
Growing Season Length
The growing season is the period of time each year during which perennial crops such as pastures and forages can grow. The growing season is longer than the frost free period. Plant growth begins whenever air and soil temperature are above freezing most of the time. When the average daily air temperatures reach 5° C for a 5-day period, daily heat unit accumulations begin. The start, end and length of the growing season will be different every year. The variability in the average start, end, and length of the growing season will be similar to that illustrated in Tables 1 and 2 for the frost free period. This information can be used to compare the growing season length across the province. As with frost free period and degree-day calculations, the length of growing season does not account for the effect of longer day length in northern Alberta compared to southern Alberta. Small areas of above average conditions will be found around Edmonton and in the vicinity of Taber and Bow Island. Other smaller pockets of longer growing season will also exist because of local conditions. |
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