| | Summary | Current Situation | Explanation of Terms
Summary
Precipitation totals across the province ranged from 10-20 mm, with the exception of the Peace Region where precipitation accumulations were variable and ranged between 10 mm in north central areas to upwards of 90 mm west of Grande Prairie and in extreme east central locals. Relative to the 1971-2000 normals for the month of December, these accumulations rank as at least Normal, with much of the Peace region receiving Much Above Normal. Areas receiving below to much Below Normal precipitation include south central and south western Alberta. Of significant note is the fact that the northern tip of the Peace region, an area in dire need of moisture received Much Above Normal precipitation (30-40 mm) for this time of year.
Areas classified as Drought Alert since the December 5, 2004 report have remained largely unchanged with the exception of the northern tip of the Peace Region, where a return to Normal conditions is estimated to have occurred. Drought Alert areas currently persist in a few isolated pockets including east central Alberta, parts of Special Areas 3 and 4, and 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 much of the reporting area, including most 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.
Although snow pack in stubble fields is considered to be much below average to below average through most of Alberta, at least Above Normal snow packs are estimated to occur in the northern tip of the Peace Region and other large pockets in the central, western and south eastern portions of the Peace Region.
Current Situation
Drought Indices
Long term Drought (Figure 1): Currently 3.4% of the reporting area is in Drought Alert, a notable decrease since the December 5, 2004 Drought Report (5.4%). Most of this decrease occurred in the northern tip of the Peace Region. Areas of Drought Alert persist in east central Alberta, affecting parts of the counties of Two Hills, Minburn, Beaver, Camrose, Flagstaff, Wainwright, and Starland. In the east, parts of Special Areas 3 and 4 and the M.D. of Acadia are in Drought Alert. In southern Alberta, two small pockets of Drought Alert exist, one in the county of Cardston and one in the M.D. of Pincher Creek. Over the last few drought reports, these areas have been consistently shrinking in size suggesting a gradual return to a Normal precipitation regime.
The areas previously classified as Normal have decreased by 3% since the last report, now standing at 85% of the reporting area. This decrease has been due to a corresponding increase in the Above Normal category, which is now 10% of the reporting area, up from 7% reported in the December 5, 2004 Drought Report.
Recent trends (Figure 2): Over the last 90 days in most areas in the Province, including those in Drought Alert, the trend is toward Normal or better conditions. Notable exceptions occur in the counties of Two Hills, Minburn and Wainwright, a small pocket straddling the border between Special Areas 3 and 4 and in the M.D. of Pincher Creek. These areas are currently in Drought Alert and are expected to remain in this category if current conditions persist.
Precipitation
Precipitation over last 90 days (Figure 3): For the northern tip of the Peace Region the 90-day precipitation totals have maintained their status of return to Normal. October 3 to December 31 typically accounts for only a small portion of the total annual precipitation in this region. None the less, this condition is welcomed. For the remainder of the Peace Region, the 90-day precipitation totals range from Below Normal in central regions to Much Above Normal in the west and southwest.
For the rest of Alberta, most of the west half of this region has reported Near Normal or better precipitation accumulations. In contrast, in the east half accumulations of precipitation are Below Normal with a number isolated pockets reporting much Below Normal precipitation. 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 otherwise 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, which translates to a total precipitation deficit over the past 90 days of approximately 30-40 mm.
Precipitation totals since the last report (Figure 4): Since the December 5, 2004 report, accumulations of precipitation ranged from less than 10 mm in parts of southern and east central Alberta to upwards of 90 mm in the Peace region just west of Grand Prairie and parts of the eastern Peace Region. However, the majority of the Peace Region received somewhat less than this with amounts ranging between 20-40 mm. Less precipitation (between 10-20 mm) was recorded in the rest of Alberta.
Precipitation percent of Normal for December 2004 (Figure 5): December precipitation was at least Normal for about 60% of the reporting area with large parts of the Peace River Region receiving Much Above Normal precipitation. Most of the west half of southern Alberta received Below Normal precipitation with several areas receiving much Below Normal precipitation. These trends were seen in much of eastern Alberta, and parts of northern and western Alberta. However, from a yearly perspective, these deviations from Normal do not account for a significant proportion of the yearly precipitation totals, as December 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 (October through to March) progresses the categories on the Precipitation Departures for December 2004 map become very sensitive to small deviations in total precipitation amounts. Over the year 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 December (Figure 6): December is a typically a dry month across 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.
Snow pack conditions: Modeled snow pack conditions are shown in Figure 7. This map represents the current snow pack in stubble fields and reflects a 30% precipitation loss due to blowing; in addition to losses due to sublimation and snow melt process. 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 hence, goes to directly runoff.
Snow water equivalents range from less than 1 mm in the south around the City of Lethbridge and then between 1-10 mm for most of southern Alberta. As one moves northward stored water in the snow pack increases to 10-25mm in the north half of the province with a few pockets of 25 -50mm of snow water equivalent. In the Peace Region, the snow water equivalent ranges from 25-75 mm with between 50-75 mm found in the northern tip of the Peace, the Swan Hills and west of Grande Prairie.
Snow pack conditions percent of Normal (Figure 8): During the melt in November much of the early winter snow pack disappeared. As a result of this and Below Normal precipitation in November, snow pack conditions for this time of year are much Below Normal for most of Alberta with isolated pockets of near to Above Normal in central Alberta. For the Peace Region the snow pack is variable ranging from much Below Normal in south central areas to Much Above Normal in southeast, east central and southwestern locals.
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.
Snow pack (reported during the winter season only)
Snow pack snow water equivalents are modeled for stubble fields. Snow water equivalent is defined as the equivalent depth of water, expressed as mm, that the snow pack contains if it were to be melted. Snow water equivalent is computed from precipitation and subsequent losses due to sublimation and snow melt processes. In the model, if precipitation falls when the mean daily temperature is less than 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.
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 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 T6H 5T6
Contact: Ralph Wright; ph 780-427-3556;
This report was created on December 31, 2004.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of December 5, 2004 |
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