| | Summary | Current situation | Explanation of terms
Summary
Since the January 31, 2004 Drought Report, precipitation totals across the province ranged from less than 10mm in the east to generally less than 20 mm in the west. However, average precipitation in February does not normally exceed 20 mm for most of the province.
Compared to the average precipitation for February for the period 1971-2000: Much Below Normal was received in much of northern and southeastern Alberta; near to Above Normal was received in the west, between Edmonton and Calgary; and Normal to Below Normal precipitation was received in all other areas.
Areas classified as Drought Alert since the last report remained largely the same. Drought Alert areas currently persist in east-central Alberta, parts of Special Areas 3 and 4 and few locations in southwestern Alberta.
In February, Below to Much Below Normal precipitation was recorded throughout most of Alberta, with the exception of the parts of the Peace Region and central parts of western Alberta. As result, the 90-day trend across much of northern Alberta, and most of the west half of southern Alberta is towards toward Drought Alert. Within these areas, a few isolated locations indicate a progression towards Drought conditions. If the current 90-day trend persists, Drought and/or Drought Alert conditions will begin to increase in size, particularly in the north and southwest portions of the province. 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.
The snow pack in stubble fields is considered to be Much Below Average to Below Average through most of Alberta. Notable exceptions occur across the northern tip of the Peace region and extreme southeastern portions of the province were Much Above Normal snow packs are predicted to occur. Elsewhere in the Peace Region, many areas are estimated to have Near to Above Normal snow cover for this time of year.
Current Situation
Drought indices
Long-term drought (Figure 1) - Currently 5.1% of the reporting area is in Drought Alert, down slightly from the 6.2% reported in the January 31st, 2005 Drought Report. The largest area in Drought Alert 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 less than 1% since the last report and now stands at 83.6% of the reporting area. Similarly, the area classified as Above Normal is up marginally to 10.3% while the area classified as wet has decreased slightly to 1.0% of the reporting area.
Recent trends (Figure 2) - Over the past 90 days, precipitation deficits have resulted in trends toward: Drought Alert in northeast and southern Alberta; and Drought in the counties of Westlock, Minburn, and Warner. If these trends persist over the next few months, those areas currently classified as Normal will trend toward Drought Alert and those areas classified as Drought Alert will trend toward Drought.
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 towards Above Normal conditions, with one small area trending towards Wet 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) - Precipitation was variable for most of the province: Near Normal precipitation was recorded over much of western, central, and parts of eastern Alberta; Much Below Normal precipitation was recorded in the central parts of northern and southern Alberta; and Below Normal precipitation was reported throughout the rest of the province with several areas reporting Well Below Normal precipitation.
The period represented by this 90-day time frame typically accounts for less than 15% of the total annual precipitation in the Peace Region and it remains to be seen whether or not this precipitation is sufficient to alleviate dry conditions resulting from extreme precipitation deficits experienced during the 2004 growing season. For the northern tip of the Peace Region, the 90 day precipitation totals have now shifted from Normal to Much Above Normal. For the remainder of the Peace Region, the western areas have received Much Above Normal precipitation, and central and eastern areas from Near Normal to Above Normal precipitation. Two exceptions exist where Below Normal precipitation has been received, one centered on Manning and one centered on Valleyview.
Precipitation received during the past 28 days (Figure 4) - Since the January 31, 2004 Drought Report, accumulations of precipitation were at less than 10 mm throughout most of eastern and northern Alberta and the southern Peace Region. In western Alberta and the northern and central parts of the Peace Region, accumulations of less than 20 mm were recorded with a few locations receiving between 20-30 mm.
February is historically the driest month of the year, accounting for less than 4% of the total annual precipitation. As such, deviations from Normal for this month do not account for a significant proportion of the yearly precipitation totals. As a result, the map shown in Figure 5 tends to lead the reader into thinking that significant shortages or surpluses exist in several areas of the province when in fact total deviations from Normal may be only several mm of total precipitation.
As the dry season progresses (October through to March) the categories on the monthly precipitation departures maps become very sensitive to small deviations in total precipitation amounts. These seemingly large monthly deviations tend to have a relatively small impact on total yearly precipitation deviations (Figure 1). However, it is desirable to see Normal or better monthly precipitation patterns across the province given the serious deficits consistently experienced in many areas of the province over the past few years.
February precipitation, expressed as a percent of Normal, was variable across the province with Much Below Normal being recorded in the southeast; in one small pocket south of Red Deer; in the southeast and west Peace Region; and over most of northern Alberta. Above Normal precipitation was received in most of western Alberta between Edmonton and Calgary; and centered on the town of Peace River. Near Normal precipitation was received in the north Peace Region.
Average precipitation for February (Figure 6) - February is the driest month of the year in Alberta with total accumulations of less than 20 mm for most areas, excluding the foothills, where Normal precipitation totals were between 20-30 mm.
Snow pack conditions
Modeled snow pack conditions are shown in Figure 7 - This map represents the current snow pack water equivalent (SPWE) in stubble fields; it reflects a 30% precipitation loss due to blowing, in addition to losses due to sublimation and snow melt processes. It is important to note that only a portion of the snow water is expected to contribute to the spring soil moisture reserves as much of the initial melt occurs over frozen or partially frozen soils and goes directly to runoff.
Warm weather in February resulted in significant melting of snow across much of Alberta. The SPWE is less than 1 mm across most of southern Alberta, including some locations well north of Calgary. In northern Alberta, the SPWE is variable ranging from 1-10 mm to as high as 30-40 mm. In the Peace Region, SPWE are highly variable containing 10-20mm in the extreme south to greater than 100 mm of SPWE in the north. In central Alberta, the snow pack is expected to contain between 20-50 mm of SPWE.
Snow pack water equivalent as a percent of normal (Figure 8) - During the melt in November much of the early winter snow pack disappeared. Continued melting since that time, particularly at the end of January and during the month of February, further reduced snow packs in open areas. The melt, coupled with Below Normal precipitation in many areas since November (Figure 3) has resulting in Much Below to Below Normal conditions. In the Peace Region, the snow pack is variable with respect to Normal ranging from Much Below Normal in southern regions to Much Above Normal across 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.
Report prepared by the Drought Reporting Team:
Ralph Wright, Katherine Altman, 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 February 28th, 2005.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of January 31, 2005. |
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