| | Current situation | Explanation of terms
Summary
Since the last Drought Report (February 28, 2005) precipitation totals across the province ranged from less than 10mm throughout parts of south central and east central Alberta, to greater than 50mm in the Swan Hills, Cypress Hills and extreme southwestern Alberta. The rest of the reporting area received between 10-40mm.
Normal March precipitation for the reporting area ranges between 10-30mm. During March 2005 across southern Alberta precipitation was variable with large areas receiving below normal precipitation and some parts of southern and south central Alberta, receiving much below normal precipitation. Across the north half of the reporting area, precipitation was highly variable, ranging from much above normal around the city of Edmonton, Swan Hills and the northern tip of the Peace Region, to below normal in several widely scattered locations, the largest of which can be found in the central Peace Region.
Areas classified as Drought Alert since the last report have diminished slightly. These areas are in east-central Alberta, parts of Special Areas 3 and 4, and few locations in southwestern Alberta.
The 90-day trend across central parts of southern Alberta and parts of north central Alberta is towards Drought Alert. Two small areas are currently trending towards Drought, one in the County of Minburn centered on the town of Vegreville and one east of Calgary in the County of Newell. On a more positive note, much of the rest of the province, including the Peace Region, is trending to at least normal or low risk of drought. Of particular interest, the northern tip of the Peace Region, which experienced very dry conditions throughout the 2004-growing season, currently has a 90-day trend toward Above Normal.
Current Situation
Drought indices
Extent and severity of long term drought in the agricultural region of Alberta, as of March 31, 2005 (Figure 1) - Currently 3.4% of the reporting area is in Drought Alert, down from the 5.1% reported in the February 28, 2005 Drought Report. The largest area in Drought Alertt extends from the County of Smoky Lake in the north, down to the County of Starland in the south. Other areas of Drought Alert can be found straddling the border between Special Areas 3 and 4 and in two small pockets in southwestern Alberta, one in the M.D. of Pincher Creek and one in the County of Cardston.
The areas previously classified as Normal have increased by more than 4% since the last report and now stand at 87.7% of the reporting area. The area classified as Above Normal is down by about 2%, to 8.3% of the reporting area, while the area classified as Wet, in the Swan Hills, has all but disappeared (approximately 0.2% of the reporting area).
Recent (90-day) trend in drought conditions for the agricultural region of Alberta, as of March 31st, 2005 (Figure 2) - Over the past 90 days, precipitation deficits in the south have resulted a trend toward Drought Alertt over most of the central parts of southern Alberta, while in the north a mid March snowstorm significantly decreased the size of the areas trending toward Drought Alert. If the recent trends persist over the next few months, some areas in northern Alberta and central parts of southern Alberta may move into Drought Alert.
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 north 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
Precipitation departures for the past 90 days (December 1 to February 28) (Figure 3) - Over the last 90-days, precipitation was highly variable across the province. Near normal precipitation was recorded over much of western Alberta, parts of eastern Alberta and much of the central and eastern parts of the Peace Region. Below normal precipitation was reported throughout most of the east half of the province as well as across most of the southern Alberta. In southern Alberta a number of areas are reporting much below normal precipitation, as are a few small pockets in the north.
The period represented by this 90-day time frame typically accounts for less than 15% of the total annual precipitation, so deficits during this period are not as severe as they would be during the wetter months of the year (typically May through to August). For the northern tip of the Peace Region, the 90-day precipitation totals continue to be much above normal and it remains to be seen whether or not this is sufficient to alleviate the dry conditions experienced during the 2004 growing season.
Precipitation (mm), since the February 28, 2005 Drought Report, in the agricultural region of Alberta as of March 31, 2005 (Figure 4) - Since the February 28th, 2005 Drought Report, a mid March snow storm brought winter-like conditions back to much of northern Alberta and the southeastern Peace Region. Accumulations of precipitation ranged between 20-30mm in west central Alberta and parts of the northeast. The Swan Hills region reported up to 50mm of precipitation with the greatest amounts, more than 60mm, recorded in the extreme parts of southwestern Alberta and the Cypress Hills. Less than 10mm of precipitation was recorded in south central Alberta, increasing to between 10-20mm for much of central Alberta. Most of the Peace Country received between 10-20mm with the north tip of the region receiving between 30-40mm.
March is a relatively dry month, falling on the heels of February, which is historically the driest month of the year. Precipitation received in March typically accounts for about 3-5% of the total annual precipitation with the southwest regions of the province receiving the most precipitation (typically between 20-40mm). As such, deviations from normal for this month do not account for a significant proportion of the yearly precipitation totals.
Precipitation departures for March 2005 (Figure 5) - The map shown in Figure 5 indicates that below to much below normal precipitation fell over most of southern Alberta, with near normal or better falling in the northern parts of the province. Exceptions are the central Peace Region and a few isolated pockets in northern Alberta, which recorded below normal precipitation.
Average (1971-2000) precipitation for March in the agricultural region of Alberta (Figure 6) - Typically in March precipitation ranges from 40-50mm in the southwest to a little as 10-20mm in the east and over most of the Peace Country.
Average (1971-2000) precipitation for April in the agricultural region of Alberta (Figure 7) - April generally marks the end of the dry season in Alberta with precipitation normally increasing to 30-40mm over much of the province with the exception of the southwest where 50-60mm is normal.
At this time of year in the south, precipitation received in April generally accounts for about 9% of the annual precipitation, whereas in the north, April precipitation generally accounts for less than 5% of the annual precipitation. As a result, precipitation deficits in the south during the next month can have a greater impact on cropping success than will similar deficits in the north.
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 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
This report was created on March 31, 2005.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of February 28, 2005. |
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