| | Drought indices | Precipitation | Soil moisture | Explanation of terms
Summary
Since the last Drought Report (August 28th, 2005), cool and wet weather has persisted over most of the province, delaying harvest, especially in the south. Significant storm events brought precipitation ranging from 1.2 to 158.5 mm to most parts of the south half of the province with the greatest amounts recorded in the foothills and over the west half of the Southern Region. Across the Central Region, the driest locations received 10 to 20 mm with upwards of 80 mm in the west and in Special Area 3. Precipitation totals dropped in the Northern Region where most locations reported 20 to 30 mm, with the exception of some central locations recording 10 to 20 mm. The south half of the Peace Region received amounts ranging from 30 to 60 mm, and the north half between 10 to 30 mm.
Since the start of the growing season (April 1, 2005), most of the province has seen at least Normal precipitation, with large areas in the central parts of the Northern Region, and much of the Peace Region receiving Below Normal. Across much of southern Alberta and along the eastern half of the Province, growing season precipitation has ranged from Above to Much Above Normal.
Drought areas are beginning to emerge in Sturgeon County, north of the City of Edmonton and on the border between the County of Barrhead and Lac Ste Anne County. In addition, large parts of the central portions of the Northern Region are classified as Drought Alert, as well as central parts of the Peace Region, and to a lesser extent the extreme southern tip of the Peace Region.
The 90-day trend in drought conditions across most of the province is towards Normal, with the exception of central parts of the Northern Region where four major pockets tending towards Drought Alert exist, the largest of which includes four counties north of the City of Edmonton.
Since the last report, soil moisture reserves have continued to increase over much of the southern part of the province. For the rest of the province, soil moisture levels remain largely unchanged, with most of Peace Region estimated to have less than 25 mm of soil moisture and much of the Northern Region estimated to have less than 50 mm of soil moisture. For these locations soil moisture deficits are generally Well Below Normal for this time of year, increasing drought risk for re-cropping next year, unless well above normal precipitation is received over winter and into early spring.
Current Situation
Drought Indices
Long-term drought (Figure 1) - Since the last report (August 28, 2005), Below Normal and Much Below Normal precipitation in the central parts of the Northern and Peace Regions have increased the Drought Alert areas form the 0.9 % to the 7.3 % of the of the reporting area. Also, two pockets areas of Drought constituting about 0.2 % of the reporting area have emerged in the Central Region. Currently, there are three pockets of Drought Alert in the Northern Region, the largest of which includes the City of Edmonton, the Counties of Sturgeon, Westlock, and parts of County of Anthabasca, Strathcona County, and the County of Camrose. A second, relatively large pocket of Drought Alert includes part of the Counties of St. Paul, Two Hills, Minburn and Smoky Lake, and Lamont. The third pocket lies on the border between the County of Barrhead and Lac Ste Anne County. Two other Drought Alert areas are found in the central and southern part of the Peace Region. Elsewhere, areas are showing Normal with areas in the southern half of the reporting area and along the foothills showing Above Normal to Wet conditions.
Recent (90-day) trend in long-term drought conditions (Figure 2) - Across much of the Central and the Southern Regions, precipitation events that occurred since mid-June have resulted in a 90-day trend towards Above Normal and Normal, with a number of locations reporting Wet conditions. Across the Northern Region, the general trend is towards Normal, with the exception of the a relatively large area in the center that is trending towards Drought Alert and which includes four counties north of the city of Edmonton. In addition, there are three other pockets of Drought Alert nearby.
The 90-day precipitation map depicts much of the Peace Region as having Below to Much Below Normal precipitation (Figure 4). These trends are not captured on the 90-day trend map (Figure 2) as magnitude of these precipitation deficits were not enough to bring the 90-day trend down to a Drought Alert condition. However it is important to note that precipitation deficits are occurring over much of the Peace Region, and if this pattern continues, expect to see those areas currently showing a 90-day trend towards Drought Alert to increase in size.
Precipitation
Precipitation since the August 28th, 2005 Drought Report (Figure 3) - Since the last Drought Report, storm events have delivered significant precipitation (36 to 159 mm) over much of the south portion of the reporting area, especially south of Starland County and along the foothills. In the Southern Region, precipitation was greatest in the west, with 158.5 mm recorded at Pekisko station, about 70 km SSW of the City of Calgary, grading to a low of 36.3 mm at Medicine Lodge, just south east of the City of Medicine Hat. In the Central Region, precipitation was the highest, in southwestern localities (50 to 120 mm), and also across southeast (50 to 90 mm) with the town of Ester in Special Area 3, reporting 1.2 mm, the lowest in the reporting area. In the Northern Region, precipitation dropped off sharply with the lowest amounts (10 to 20 mm) being recorded, through much of the central and northern areas of the region. Elsewhere in the Northern Region received precipitation ranged between 20-30 mm in the east and up to 30 to 40 mm in the west with the greatest amount (64.3 mm) being recorded at the Bodo station, south of Provost and 56.6 mm near the town of Breton. In the Peace Region, precipitation in general decreased form 80.1 mm in House Mountain in the southeast and 57.6 mm at Spring Creek in the south to 5.9 mm at Fort Vermillion in the north. Most of the south half of the region received precipitation in the range of 30 to 50 mm.
90-day precipitation departures (Figure 4) - Over the past 90-days, precipitation across most of the Southern Region was Much Above Normal, with the exception of one large pocket of Below Normal precipitation reported in the southeast corner of the region. In the Central Region, precipitation graded from Much Above Normal in southeast to Near Normal in northwest. Most of the central portion of the Northern Region recorded Below Normal precipitation grading to Much Above Normal in the east, around the town of Lloydminster and in addition to one small pocket of Much Above Normal in the west (M.D. of Brazeau). Across the Peace Region precipitation over the past 90-days was Below Normal in the south, central and northern portions of the region and Near Normal in the western and eastern parts of the region.
Growing season precipitation (GSP) to date (Figure 5 and Figure 6) - To date measured GSP in the reporting area varied widely, ranging from 898.1 mm at Spionkop Creek in the Foothills areas in the Southern Region to a low of 165.7 mm at Manning in the Peace Region
In Southern Region, the GSP ranged from 898.1 mm at Spionkop Creek and 890 mm at Pincher Creek in the Foothills to 270.5 mm at Masinasin in the southeast corner. The GSP across most of the western half of the region ranged from 500 to 700 mm and in the eastern half, from 275 to 500 mm. Across the Central Region, GSP was lowest in central areas near the town of Drumheller (229.8 mm) and in the east near the town of Esther (250.2 mm) with the greatest amounts (575. 9 mm), in the west, near Water Valley. In the Northern Region an extended area in the central portion of the region centered on the county of Sturgeon, received a less GSP, ranging from 200 to 300 mm compared to the rest of the Region, which recorded between 300 to 500 mm. The Peace Region received the least amount of GSP relative to the other three regions. Here, the GSP ranged from 470 mm at House Mountain in the west to 165.70 at Manning in the central north portion of the Region.
In the Peace Region, GSP expressed as a percent of Normal (Figure 6), ranged from Below Normal throughout most of the central areas to Near Normal in the west and east. Similarly, in the Northern Region the central portion recorded Below Normal GSP with most of the rest of the region reporting Near Normal. In the Central Region the GSP expressed as a percent of Normal grades from Near Normal in the north to Much Above Normal in the south. Most of the northern and western part of the Southern Region is reporting Much Below Normal grading to Near Normal in the southeast.
Precipitation departures for the month of August (Figure 7) - August precipitation was generally Well Above Normal with the exception of the extreme southeast,where precipitation was Above Normal. Across the north half of Alberta, the situation was quite different. Throughout most of the Peace Region, August precipitation was Below to Much Below Normal, with some pockets of Normal in the extreme northwest and northern portions of the region. Similarly, in the Northern Region, precipitation was Below to Much Below Normal in central and the western locations, grading sharply to Much Above Normal in extreme south and eastern areas.
Normal precipitation for the month September (Figure 8) - September marks the transition between the Wet season (May - August) and the drier winter months (October - March). During this month precipitation totals typically decline significantly with total accumulations ranging from just over 20 mm in the Special Areas to less than 60 mm in the Swan Hills and Foothills. Provincially, on average about 9% of the annual precipitation falls in September.
Soil Moisture
Soil moisture in the agricultural regions of Alberta (Figure 9) - Soil moisture reserves across most of the south half of the province continued to improve since the last report, a trend that is normal for this time of year. This was due to Above Normal precipitation during the month of August (Figure 7), as well as significant rainfall received since the last report (August 28, 2005), as shown in Figure 3. Across the Peace Region, soil moisture reserves improved slightly in most of the southern half of the region, but much of the rest of the region is estimated to have less than 25 mm of reserves. Soil moisture reserves are also low throughout much of the central and western parts of the Northern Region, but do improve eastward towards the Saskatchewan border. Those areas with less than 50 mm of soil moisture in both the Peace and Northern Regions have diminished capacity to resist drought and require at least Above Normal precipitation over the fall, winter and early spring in order to establish soil moisture reserves that are sufficient to resist short-term dry spells during the early part of the next growing season.
Typically soil moisture reserves begin to increase slightly throughout September and into October. This is due to significant reduction in evaporative demands, brought on by killing frosts, colder temperatures and shorter day lengths.
Current soil moisture departures (Figure 10 and Figure 11) - Soil moisture deficits are computed relative to Normal, which is the 30-year statistical average taken from the years 1971-2000 (Figure 10).
In the Peace Region, most areas are reporting Well Below Normal soil moisture reserves, except for areas in the extreme western and eastern regions, where Near Normal reserves are found and the extreme northwest and south corners where Extreme Deficit are estimated to occur (Figure 11). Throughout most of the west half and central portions of the Northern Region, soil moisture reserves have continued to decline, particularly north and north east of the City of Edmonton. Elsewhere in the Northern Region, soil moisture reserves range from Normal in the south to Extreme Surplus in the east, along the Saskatchewan Boarder. Across most of the Central and Southern Region, soil moisture reserves range from Near Normal in the central parts of the Central Region to Extreme Surplus over much of the Southern Region and eastern and western portions of the Central Region.
Probability of returning to normal fall and spring conditions (Figure 12 and Figure 13) - The amount of precipitation recharge needed to bring current soil moisture conditions to 30-year average fall (Nov 1) or spring (May 1) levels are compared to recharge rates that have occurred in the past (1971-2000) for similar periods. The probability of returning to Normal conditions is then computed from the number of years, between 1971-2000, that soil moisture recharge rates have equaled or exceeded those currently required to bring soil moisture levels to average.
Across The Peace Region and most of the Northern Region, the computed probability of achieving normal fall soil moisture conditions is less than 10%, grading sharply to well over 70% for most of the eastern parts of the Northern Region, and most of the Central and Southern Regions. The probability of achieving normal spring soil moisture is less than 20% for the Peace Region and most of the North Central parts of the Northern Region. In these areas Much Above Normal precipitation will be required between now and spring to bring soil moisture reserves to a point where short duration dry spells can be tolerated without undue crop moisture stress.
Explanation of Terms
Long term (hydrologic) drought
Long term, or hydrologic, Drought is a result of the cumulative effect of several dry months. It primarily impacts livestock feed and water supplies and may affect annual crops. Hydrologic Drought is determined from precipitation totals over a 365-day period using the Standardized Precipitation Index (SPI). Long term Drought is rated as either Wet, Above Normal, Normal, Drought Alert, Drought or Exceptional Drought. The United States National Drought Mitigation Centre recommends the SPI for drought identification. Long term drought conditions are reported year-round.
The trend in long term drought is determined by comparing the 365-day SPI with the 90-day SPI. Where the 90-day SPI value is -1 to +1, then a trend toward moderating conditions is occurring, potentially resulting in Normal status. If the 365-day SPI values for that area are already Normal, then the trend is toward no change. If the 90-day SPI value is -1 to -2, then the area is trending toward Drought Alert status. This could be a deteriorating condition if the current 365-day value is Normal, however it could represent a continuing condition if the area is already in Drought Alert, or an improving condition if the area is already in Drought. Values of the 90-day SPI that are between of -2 to -3 and lower than -3 indicate a trend toward Drought and Extreme Drought respectively. Values of the 90-day SPI that are between +1 and +2, and greater than +2 represent a trend toward Above Average and Wet respectively.
Snow pack (reported during the winter season only)
Snow pack snow water equivalents (SPWE) are modeled for stubble fields. SPWE is defined as the equivalent depth of water (mm) that the snow pack contains if it were to be melted. SPWE is computed from precipitation and subsequent losses due to blowing, sublimation and snow melt processes.
In the model, if precipitation falls when the mean daily temperature is below 2 °C that precipitation is estimated to be in the form of snow. If precipitation if estimated to fall as snow then to simulate drifting, only 70% of the total precipitation is allowed to accumulate resulting in a 30% loss due to snow "blow off". If precipitation occurs as rain on an existing snow pack, it is added directly to the snow pack as SPWE.
Soil moisture (reported during the growing season months only)
The crop gets the moisture it requires from the reserve of soil moisture, which in turn is replenished by precipitation. Soil moisture is a valuable indicator of drought potential because it indicates the reserve of water available to the crop at a given point in time. During peak growing periods, soil moisture reserves are consumed quickly and must be replenished frequently by rainfall. Low soil moisture reserves during peak water use indicate a high risk of immediate crop stress. Prolonged stress becomes drought and results in significant unrecoverable yield loss.
Because the climate varies across Alberta, comparing current moisture levels to normal levels provides a valuable indicator of drought risk that can be applied to all localities during the frost-free season. Below average soil moisture levels, at any time, indicate a need for more precipitation to restore reserves.
Soil moisture is measured as millimeters (mm) of plant available water. Plant available water is approximately half of the total water that can be measured in the soil. Soil moisture is monitored from May through October.
Soil moisture needed to return to average spring or fall conditions
Soil moisture needed to return to normal spring or fall conditions is computed by subtracting normal soil moisture (spring or fall), computed using the 1971-2000 period, from current soil moisture conditions. This yields the amount of recharge needed to bring current soil moisture levels to average. Historic model runs are then analyzed to determine how many years during the 1971-2000 period that soil moisture recharge was similar to or greater than that currently needed. The number of years that this occurred is then used to compute the probability of returning to normal. However, the process is currently unable to account for snow currently existing on the ground and as such is not accurate where snow packs exist.
Report prepared by the Drought Reporting Team
Ralph Wright, Daniel Itenfisu and Isabel Simons-Everett
Conservation & Development Branch & Program
Alberta Agriculture, Food and Rural Development
Edmonton, AB T6R 5T6
Contact: Ralph Wright; ph (780) 427-3556
This report was created on September 18, 2005.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of August 28, 2005. |
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