| | Drought indices | Precipitation | Soil moisture | Explanation of terms
Summary
Since the last Drought Report (March 31, 2005) precipitation totals across the province ranged from less than 10 mm throughout parts of southeastern and central Alberta, to greater than 120 mm in the southwestern Peace Region and parts of the foothills.
Since the start of the growing season (April 1, 2005), precipitation has been Much Below Normal across much of southern Alberta and parts of central Alberta. Near Normal precipitation was recorded over much of the north half of the province, with above to Much Above Normal precipitation falling across the northern tip of the Peace Region, the southwestern Peace Region and in northern Alberta along the Saskatchewan border.
Areas classified as Drought Alert since the last report have diminished significantly. Currently three small areas can be found in central and eastern Alberta (County of Starland, Minburn and Special Areas 3 and 4).
The 90-day trend across most of southern Alberta is towards Drought Alert with the exception of the County of Newell, which is trending towards Drought. Other areas trending towards Drought Alert can be found in northern Alberta with one small pocket trending towards Drought in the County of Minburn. The rest of the province is trending towards Normal, with the northern tip of the Peace River region trending towards Above Normal. If recent precipitation trends persist for the next few months, southern Alberta and parts of northern Alberta will emerge into the Drought Alert category.
In the southern half of Alberta, soil moisture levels are extremely variable, ranging between 25- 50 mm in the east and grading to 100-125 mm in the west. In the north half of the province, soil moisture levels range from 75 to > 125, mm with a few small pockets showing between 50 75 mm. Overall, soil moisture levels are better than they were at this time last year.
Current Situation
Drought Indices
Extent and severity of long term drought in the agricultural region of Alberta, as of May 18, 2005 (Figure 1). Currently 1.1 % of the reporting area is in Drought Alert, down from the 3.4% reported in the March 31, 2005 Drought Report. Drought Alert areas exist in several small pockets scattered widely across the province and include parts of the following: The M.D. of Pincher Creek; the counties of Cardston, Starland, Minburn, Two Hills, and Lamont; and Special Areas 3 and 4.
Historically, in Alberta about 45% of average annual precipitation is received through the May-July period. Since the long term drought index (SPI) is a measure of yearly precipitation deviations from Normal, precipitation totals through May-July will carry significant weight in the SPI calculations. Thus, over the next few months, the SPI will be quite responsive to precipitation departures from Normal. So, if precipitation trends remain Below Average across the current Drought Alert areas, then expect to see Drought Alert areas growing rapidly in size.
The areas previously classified as Normal have increased by almost 6% since the last report and now stand at 93.6% of the reporting area. The area classified as Above Normal is down by about 3%, to 5.3% of the reporting area.
Recent (90-day) trend in drought conditions for the agricultural region of Alberta, as of May 18, 2005 (Figure 2). Over the past 90 days, precipitation deficits in the south and central parts of Alberta have resulted in a trend toward Drought Alert, while in the north, recent rains have resulted in a trend to Normal or better conditions. East of the city of Red Deer, conditions are trending towards Exceptional Drought and in the County of Mountain View a pocket trending towards Drought can be found. If the recent trends persist over the next few months, large parts of central and southern Alberta may grade into Drought Alert with some areas of Drought emerging.
On a more positive note, much of the rest of the province, including the Peace Region, is trending to at least Normal. In addition, the northern tip of the Peace Region is now showing a trend toward Above Normal conditions. This is welcome relief, after the very dry conditions experienced through most of the 2004 growing season in this area.
Precipitation
90-day precipitation departures in the agricultural region of Alberta, as of May 18, 2005 (Figure 3). Over the past 90 days, precipitation has been highly variable across the province. Near Normal precipitation was recorded over much of northern Alberta and parts of western Alberta. Below Normal to Much Below Normal precipitation was reported throughout most of the southern half of the province, with the greatest deficits (Much Below Normal) in a large area stretching from Lacombe in the north, down to the Milk River in the south. In the Peace Region, Much Above Normal precipitation was recorded in the north, and extreme southwestern locations, and below Normal in central and south central regions
Precipitation (mm), received as of May 18, 2005, since the start of the growing season in the agricultural region of Alberta (Figure 4) and Percent of Normal Precipitation (mm), received as of May 18, 2005, since the start of the growing season in the agricultural region of Alberta (Figure 5) . To date, growing season precipitation has been highly variable across the province, ranging from less than 10 mm east of the City of Red Deer, to greater than 120 mm west of Grande Prairie (Figure 4). Growing season precipitation as a percent of normal has generally been Below Normal south of the City of Edmonton, with the greatest deficits (Much Below Normal) occurring in central and southeastern Alberta, and with a few pockets of the same being found in southwestern Alberta (Figure 5). Below Normal growing season precipitation has fallen in central and south central portions of the Peace Region and a few pockets in northern Alberta. Much Above Normal growing season precipitation has fallen in parts of the north Peace Region, west of Grande Prairie, and in northeastern Alberta along the Saskatchewan border.
Average (1971-2000) precipitation for May in the agricultural region of Alberta (Figure 6) and Average (1971-2000) precipitation for June in the agricultural region of Alberta (Figure 7). May marks the beginning of a significantly wetter period over most of the province, accounting for on average about 10% of the average annual precipitation, with significant amounts (> 60 mm) falling in the western half of the province and along the southern US border. June is typically the wettest month in Alberta, accounting for on average about 17% of the average annual precipitation.
Soil Moisture
Soil moisture in the agricultural regions of Alberta as of May 18, 2005 (Figure 8). The spring soil moisture-sampling program started on April 4, 2005 in southern Alberta, and concluded on May 3, 2005 in the Peace Region. The soil moisture map shown in Figure 8 represents assessed soil moisture reserves gathered from 275 locations from across the province during this time.
Amounts of soil moisture range from between 25-50 mm in south central and southeastern Alberta, to greater than 125mm over much of the northwest. Other areas where soil moisture reserves of greater than 125 mm exist, including parts of the northern Peace Region, and several small pockets east of Edmonton, such as a large area centered around the town of Lloydminster.
Over most of the Peace Region soil moisture is well over 75 mm with large areas showing between 100 and 125 mm. The driest (50-75 mm) location in the Peace Region can be found in one small pocket just north of the town of Fairview.
In northwestern Alberta, the situation is similar, with most of the areas showing soil moisture reserves ranging from 75-100 mm and there is a significant amount of land in the 100-125 mm category. With several small pockets of 50-75 mm, it appears that a significant portion of the 75-125 mm category has soil moisture reserves on the low side of this range.
Across southern Alberta, soil moisture reserves are variable with much of the east half showing between 25-50 mm, grading to a mixture of 100-125 mm and 75-100 mm along the foothills. Those areas showing between 25 and 50 mm of soil moisture will need significant precipitation in the next two weeks in order to prevent crop moisture stress.
Soil moisture departure from the 30-year average (1971-2000) modeled soil moisture in the agricultural region of Alberta for May 18, 2005 (Figure 9). Soil moisture deficits are computed relative to normal (which is the 30 year statistical average taken from the years 1971-2000). In over the north half of the province, including the entire Peace Region, soil moisture ranges from Near Normal to Extreme Surplus.
For the south half of the province, soil moisture reserves are highly variable with many areas showing Near Normal reserves, particularly along the Alberta/ Saskatchewan border and along Highway 2 between Calgary and Fort Macleod.
In contrast, southwestern Alberta, in the County of Cardston, is showing Extreme Deficits in soil moisture. However, soil moisture reserves here are still between 50-75 mm and will provide a week or so of reserve against the prevailing dry conditions. Precipitation is needed soon in this area to prevent crop moisture stress.
Well Below Normal to Below Normal soil moisture reserves are found in south central Alberta and in a broad band through central Alberta beginning just north of the city of Lethbridge and extending up to the towns of Stettler, Castor, and Coronation.
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. It also 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, precipitation is considered to be snow when it occurs during a mean daily temperature that is below 2°C When the precipitation is estimated to be snowfall, drifting is simulated. Only 70% of the total precipitation is considered to have accumulated, with 30% of the snow considered to have been lost 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)
A 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, Isabel Simons-Everett and Kris Sabourin
Conservation & Development Branch & Program
Alberta Agriculture, Food and Rural Development
Edmonton, AB T6R 5T6
Contact: Ralph Wright; ph 780-427-3556; toll-free in Alberta at 310-0000
This report was created on May 18, 2005.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of March 31, 2005. |
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