| | Current situation | Drought indices | Precipitation | Soil moisture | Explanation of terms
Summary
November was much warmer and drier than normal. As a result, much of the early snow that fell in October melted, contributing to increases in soil moisture levels for some areas in the province. This was despite below normal precipitation falling since the October 31, 2004 Report.
Significant snowfall over much of the north half of the province in the first week of December has resulted in a return to winter like conditions with complete snow cover estimated to occur over most of the province. Since the October 31, 2004 Report, precipitation totals ranged 0-10 mm through most of south central, central and eastern Alberta. Between 10 and 20 mm fell in western Alberta and northeastern Alberta with the most precipitation falling in the Peace River Region (generally between 30-40 mm). Of significant note is the fact that the northern tip of the Peace Region, an area in dire need of moisture, received between 30-40 mm of precipitation.
Areas classified as Drought Alert since the last report have remained largely unchanged with one exception being the northern tip of the Peace River Region, where a slight decrease was estimated to have occurred. Drought Alert areas currently persist in several small pockets around the province including the M.D of Mackenzie in the northern Peace, east central Alberta, the central part of eastern Alberta and a few locations in southwestern 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 most areas currently in Drought Alert status. The greatest change has occurred in a large area in southeastern Alberta where Below Normal precipitation has lead to a trend towards Drought Alert. If the 90-day trend towards Normal persists, existing long-term Drought Alert conditions should normalize in most areas within in a few months.
Much of the precipitation that fell as snow in October has melted as a result of above normal temperatures extending through most of November. To capture his melt event, the fall soil moisture map has been updated as significant increases in soil moisture levels are predicted to have occurred.
Soil moisture reserves heading into winter currently are at least Normal in most parts of the province. Exceptions include south central and extreme southern Alberta where soil moisture reserves range from Well Below Normal to Below Normal. In addition, soil moisture reserves in the Extreme Deficit category can be found in a few small pockets in the northern tip of the Peace Region along with Well Below Normal to Below Normal soil moisture reserves.
Current Situation
Drought Indices
Long term drought (Figure 1): Currently 5.4% of the reporting area is in Drought Alert, slightly less than that reported in the October 31, 2004 Drought Report (6.8%). Most of this decrease occurred in the northern tip of the Peace Region. Currently areas of Drought Alert can be found in the northern Peace Region affecting the M.D of Mackenzie, in east central Alberta, affecting parts of the counties of Two Hills, Minburn, Beaver, Camrose, Flagstaff, Wainwright, and Starland. Similarly, in the east, Special Areas 3 and 4 and the M.D. of Acadia are in Drought Alert. In southern Alberta, three small pockets of Drought Alert exist, two in county of Cardston and one in the M.D. of Pincher Creek. In southwestern Alberta, the close proximity of these areas to one another suggests that much of southwestern Alberta, which is 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 88% of the reporting area. In addition, a few pockets of land classified as Above Normal can be found throughout the province, accounting for slightly less than 7% of the reporting area.
Recent trends (Figure 2): Over the last 90 days, almost all areas in Alberta, including those in Drought Alert are trending to conditions that are Normal or better. The only exception occurs in the county of Starland where Drought Alert conditions are persisting. This area is part of a larger area in south central Alberta that is currently trending toward Drought Alert.
Precipitation
Precipitation over last 90 days (Figure 3): For the northern tip of the Peace Region the 90-day precipitation totals have finally resulted in a return to Normal. However, the period represented by this 90-day time frame (September 7th to December 5th) typically only accounts for a small portion of the total annual precipitation in this region. Non-the less this condition is welcomed. For the remainder of the Peace Region, the 90 day precipitation totals range from Normal in central regions to Much Above Normal in the west and southwest.
For the rest of the Province most the north half and foothills regions have reported Near Normal or better precipitation accumulations. In contrast, there is a growing precipitation deficit in the southwest and south-central portions of the province with a dramatic increase in the size of the areas reporting Below to much Below Normal. Fortunately, this spans a relatively dry period in the year and total precipitation deficits are not as severe for a given percent of Normal class as they may be during historically wetter periods of the year. For example the areas currently classified as Well Below Normal typically receive between 60 to 75 mm of precipitation over this period witch translates to a total precipitation deficit over the past 90 days of approximately 30-40 mm
Precipitation totals since the last report (Figure 4): November precipitation levels were Well Below Normal for most of the province. Much of the precipitation received since the October 31, 2004 Drought Report occurred in the first part of December. Since the last report, less than 10 mm of precipitation fell through most of central and southern Alberta. Precipitation totals in the foothills and northwestern Alberta ranged between 10 and 20 mm with the most amounts being recorded in the Peace River Region where totals ranged between 20-30 mm in the north and central parts and upwards to 40 mm in the southwest and western areas. In the northern tip of the Peace Region, Near Normal precipitation was recorded, finally bringing some reprieve to the unusually dry weather experienced in this region since the spring.
Precipitation percent of Normal for November 2004 (Figure 5): November precipitation was much Below Normal for most of the reporting area except for the Peace River region. Here Below Normal precipitation was recorded in the north and central portions of the region and Near to Much Above Normal in the west central and eastern portions of the region. However, from a yearly perspective, these deviations from Normal do not account for a significant proportion of the yearly precipitation totals, as November 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 categories on the Percent of Normal Precipitation maps become very sensitive to small deviations in total precipitation amounts. Over a yearly timeframe, these seemingly large monthly deviations tend to have a relatively small impact on total yearly precipitation deviations. However, 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 December (Figure 6): November is a typically one of the driest months in the province with total amounts ranging between 10-20 mm for the east half of the province and between 20-30 mm for the Peace Region and most of western Alberta.
Soil Moisture
Current soil moisture conditions (Figure 7): Normally the last soil moisture map for the year is produced on November 1. However, warm weather during November caused significant snow pack melting, which further increased fall soil moisture reserves.
In the northern tip of the Peace Region, soil moisture levels were predicted to be less than 75 mm of plant available water over much of the M.D of Mackenzie and the northeast corner of the M.D. of Northern Lights. This represents a slight improvement over the last report. In the rest of the Peace Region, soil moisture increased along a southwest transect, ranging from greater than 75 mm in the northeast to nearly 150 mm in the Swan Hills (a condition that represents approximately 90% of field capacity).
By itself, Figure 7 does not provide a complete picture of soil moisture status in Alberta. Based on a 30-year model (1971-2000), we note that during November, it is not uncommon for the southeastern and much of eastern Alberta to have soil moisture levels at 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 (based on the 30-year statistical average). Since the October 31, 2004 Drought Report, soil moisture deficits have eased slightly, mainly due to snow pack melting, particularly throughout the central, north central and southern parts of the province. Currently, most of the province has at least Near Normal soil moisture reserves with exceptions being in central and east central Alberta was well as in parts of central and south central Alberta were reserves range from Below to Well Below Normal. In the north tip of the Peace Region, soil moisture reserves range from Below Normal in the east to a few pockets of Extreme Deficit in the west. The rest of the Peace Region and most of western Alberta has soil moisture reserves that are Well Above Normal, with a few areas estimated to have an Extreme Surplus.
Probability of Returning to Normal Spring Soil Moisture Conditions (Figure 10): A draft map showing the probability of returning to Normal spring soil moisture conditions is shown in Figure 10. Parts of central, north central and 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 attaining normal spring soil moisture reserves 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 insure adequate crop and pasture growth over the 2006 growing season.
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 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.
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 rainfall. Soil moisture is a valuable indicator of Drought potential because it shows 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 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, Food and Rural Development,
Edmonton, AB T6R 5T6
Contact: Ralph Wright; ph 780-427-3556;
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report, created on December 5, 2004, updates the previous report of October 31, 2004. |
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