| | Summary | Drought indices | Precipitation | Soil moisture | Explanation of terms
Summary
Since the October 3, 2004 Drought Report, a significant snowfall event occurred over most of the province halting harvesting efforts across much of the north half of the province. Since that time, cool cloudy weather has slowed melting and much of the snow pack is estimated to still remain. Precipitation totals across the province ranged from 0-10 mm in much of southeast Alberta and less than 20 mm over most of the east half of the province and the north half of the Peace region. Precipitation increased towards the foothills with west central Alberta receiving up to 90 mm near Rocky Mountain House and up to 60 mm over the Swan Hills.
Areas classified as Drought Alert since the last report have remained largely unchanged with slight decreases in affected areas occurring in east central Alberta. Drought Alert areas currently persist in several locations around the province including the MD of Mackenzie in the northern Peace, two areas in central Alberta, one in the central part of eastern Alberta and one in south western Alberta.
Precipitation during the past 90 days has resulted in a trend toward at least Normal, or low risk of Drought, for most of the reporting area, including all areas currently in Drought Alert status. If the 90-day trend towards Normal persists, existing long-term Drought Alert conditions should normalize in most areas within in a few months.
Since much of the precipitation fell as snow, and snow cover is estimated to remain in many areas north of Red Deer, soil moisture levels currently mapped in these areas do not reflect recent precipitation. However, soil moisture reserves have improved relative to normal in the south with some areas previously reported as Well Below Normal now estimated Below Normal to Normal. Currently soil moisture reserves are Well Below Normal in a broad band stretching through the central portions of the province including the Special Areas, isolated parts of western Alberta and the northern tip of the Peace region. Within these areas Extreme Deficits are estimated to occur in the northern Peace and a few pockets south of Edmonton. In contrast, much of the southern and central Peace region, parts of western Alberta and southwest Alberta are Above to Well Above Normal with some areas in Extreme Surplus.
Drought Indices
Long-term Drought (Figure 1):
Currently 6.8% of the reporting area is in Drought Alert, slightly less than that reported in the October 3, 2004 Drought Report (7.6%). Most of this decrease occurred in east central Alberta. Currently areas of Drought Alert can be found in the northern Peace country affecting the MD of Mackenzie, in east central Alberta, affecting parts of the counties of Two Hills, Minburn, Beaver, Camrose, Wainwright and Starland. Similarly, in the east, Special Areas 3 and 4 and the MD of Acadia are in Drought Alert. In southern Alberta, three small pockets of Drought Alert exist, one in the county of Forty Mile, one in the county of Cardston, and a new one, emerging in the MD of Pincher Creek. The appearance of the Pincher Creek pocket, in close proximity to the Cardston pocket, suggests that much of southwestern Alberta, currently classified as Normal is on the low end of this range and further precipitation deficits could result in future expansion of the areas classified as Drought Alert.
The areas previously classified as Normal have remained largely the same and currently stand at 87% of the reporting area. In addition, a few small pockets of land classified as Above Normal can be found throughout the province accounting for just over 6% of the reporting area.
Recent trends (Figure 2):
Over the last 90 days, all areas in the province, including those in Drought Alert, are trending to Normal or better conditions. In the southern Peace region and western Alberta, conditions are trending to Above Normal as indicated by the Improving and Greatly Improving classifications.
Precipitation
Precipitation over last 90 days (Figure 3)
Over the last 90 days, the tip of the northern Peace region has received Below Normal precipitation, however, the extent of this area has decreased since the last report. In all other areas of the Peace, Above Normal to Much Above Normal precipitation has been recorded. Several small circular areas of Below Normal precipitation can be found scattered throughout Alberta. These areas are centered on weather stations and suggest that the adjacent areas in the Near Normal category are sitting near the low end of the category range. Future precipitation deficits in these areas could lead to relatively rapid increases in the areas classified as Below Normal.
The largest area of Below Normal precipitation can be found in east central Alberta, extending from the county of Wainwright down to the MD of Acadia. As a result of a series of precipitation events in September and October, western Alberta reports Above to Much Above Normal. Most of the east half of the province has recorded Near Normal precipitation over the last 90 days.
Precipitation totals since the last report (Figure 4)
Since the October 3, 2004 Drought Report, significant snowfall accumulations have occurred in the north half of the province as a result of a large disturbance moving through the province in mid October. Since that time, below average temperatures have slowed the melting process and much of the north half of the province is still predicted to be under snow.
Since the last report, between 5-10 mm of precipitation was recorded in southeast Alberta, and 10-20 mm in the east half of the province and the northern and central Peace regions. Moving westward toward the foothills, precipitation amounts increased steadily with maximum accumulations being recorded at Rocky Mountain House (>90 mm), and the Swan Hills (>60 mm).
Precipitation percent of Normal for October 2004 (Figure 5)
October precipitation ranged from Below to much Below Normal for much of southeast Alberta and Below Normal for most of the north, central and south central Peace region. Some isolated pockets of Below to Much Below Normal were also recorded in central Alberta and Special Areas 3 and 4. Much Above to Above Normal precipitation was recorded in the west half of Alberta with the greatest amounts falling along the foothills extending up into the Swan Hills. However, from a yearly perspective, these deviations from Normal do not account for a significant portion of the yearly precipitation totals, as October is historically a relatively dry month. 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, the precipitation categories on the Normal Precipitation maps become very sensitive to small deviations in total precipitation amounts. Over a yearly time frame these seemingly large monthly deviations tend to have a relatively small impact on total yearly precipitation deviations. However, in terms of trends it is desirable to see Normal or better precipitation patterns across the province given the serious deficits consistently experienced in many areas of the province over the past few years.
Normal precipitation for the month of October (Figure 6)
October marks the start of the dry season for most of Alberta, which typically spans the October to March over-winter period. Generally, monthly precipitation totals for November are less than those typically received in October with precipitation totals in the east half of the province for the month of November typically less than 20 mm and between 20-30mm for Peace County and most of western Alberta.
Soil Moisture
Current soil moisture conditions (Figure 7)
Due to the snowfall event in mid-October, much of the northern half of the province is estimated to still have some snow cover. As a result, soil moisture reserves have not fully benefited from recent precipitation. The long range forecast into the weekend predicts above average temperatures that should lead to further melting of the snow pack, resulting in soil moisture levels improving slightly.
For the northern tip of the Peace region, soil moisture levels showed no improvement over the October 3, 2004 Drought Report, with less than 50 mm of plant available water over much of the MD of Mackenzie and the northeast corner of the MD of Northern Lights. In the central Peace region, soil moisture levels are generally between 75 and 125 mm, with maximum levels of just over 140 mm being found in and around the Swan Hills.
For the east half of Alberta, soil moisture levels were generally below 50 mm with most of east central and extreme south eastern Alberta showing soil moisture levels below 25 mm. Areas on the map showing less that 25 mm of soil moisture will need good over-winter snow pack development coupled with a slow spring melt and/or spring rains prior to planting in order to ensure adequate moisture for planting or pasture re-growth. In the west half of Alberta, there is a strong gradient of increasing soil moisture levels that increases westward, towards the foothills, with maximum amounts of 150 mm estimated to occur in the foothills west of Red Deer.
By itself, Figure 7 does not provide a complete picture of soil moisture status in Alberta. Based on a 30-year model run spanning the 1971-2000 period, at this time of year it is not uncommon for the southeast to have soil moisture levels less than 50 mm (Figure 8).
Soil moisture deficits relative to 30-year normals (Figure 9)
Figure 9 shows soil moisture deficits relative to Normal (30-year statistical average). Since the October 3, 2004 Drought Report, soil moisture deficits have eased slightly, particularly throughout the central, north central and southern parts of the province. Well Below Normal soil moisture reserves are present in most of central, and north central Alberta. In addition, Well Below Normal soil moisture can be found in western Alberta in the MD of Yellowhead and the northern tip of the Peace region with the western portion of this area in the Extreme Deficit category. Well Above Normal soil moisture levels are estimated to occur in much of the Southern half of the Peace region, extending into extreme northwestern Alberta. In addition, other parts of the province with at least Well Above Normal soil moisture reserves can be found west of Red Deer, Calgary and Lethbridge.
Probability of returning to Normal spring soil moisture conditions
A draft map showing the probability of returning to Normal spring soil moisture conditions is shown in Figure 10. Parts of central, north central, west central Alberta, and the northern tip of the Peace region are estimated to have less than a 10% chance of achieving average spring soil moisture levels by May 1, 2005. In addition, large areas surrounding these locations have less than a 20% chance of receiving sufficient moisture over winter and during early spring in order to bring soil m moisture levels to average conditions by May 1st 2005. Areas with less than a 20% chance of returning to Normal spring soil moisture levels have low soil moisture reserves and have a greatly reduced capacity to resist dry conditions over the 2006 growing season. Unless soil moisture reserves are replenished prior to planting and spring re-growth in these areas, sustained Normal to Above Normal precipitation events will be needed to ensure adequate crop and pasture growth.
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 supply 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 -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.
Soil moisture (reported during the growing season months only)
Crops get the moisture they require from the reserve of soil moisture, which in turn is replenished by rainfall. Soil moisture is a valuable indicator of Drought potential because it shows the reserve of water available to a 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 these times 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 and to all times of the season. Below average soil moisture levels, at any time, indicate a need for more rain or snow 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 and Food,
Edmonton, AB T6R 5T6
Contact: Ralph Wright; ph 780-427-3556;
This report was created on October 31, 2004.
Drought analysis is currently scheduled at bi-weekly intervals between May 1 and October 30. This report updates the previous report of October 3, 2004 |
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