| | Drought indices | Precipitation | Explanation of terms
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January 2006 continued to be warmer and drier than normal in Alberta, with snow pack accumulations well below average. Since the last report, most of the province received much below to below normal precipitation resulting in an increase in areas classified as Drought and Drought Alert. Precipitation totals ranged from less than 5 mm in most parts of the reporting area, to more than 180 mm in some localities in the foothills of the Southern Region. Currently, areas in Drought Alert extend across much of the central parts of the Northern Region, as well as the central and southeastern portions of the Peace Region.
Across much of the reporting area, Much Below Normal precipitation over the past 90 days has resulted in the expansion of areas exhibiting a 90-day trend towards Drought, particularly in the central and/or western parts of the Northern, Central and Southern Regions. Moreover, four pockets trending towards Exceptional Drought have emerged in western parts of the Northern and Central Regions, with affected areas in the Municipal District (MD) of Big Lakes (Swan Hills), the central parts of Westlock County, parts of Red Deer, Mountain View and Clearwater Counties, and east of the City of Edmonton straddling the border between Parkland and Lac Ste Anne county. The emergence of small pockets that are trending towards Exceptional Drought serve as a warning that much of the areas currently trending towards Drought could move into an Exceptional Drought trend, if extreme precipitation deficits continue. The 90-day trend in the rest of the reporting area shows a trend towards Drought Alert and isolated scattered pockets of Normal conditions.
Clearly, this winter has been exceptionally warm and dry. If current trends continue into early spring, then serious moisture deficits will occur. However it is important to remember that on average in Alberta, only 25% of annual precipitation occurs in the six months, from October to March. Therefore, precipitation deficits at this time of year are not as severe as they might otherwise be during the growing season. That being said, however, snow pack accumulation is Much Below Normal over most of the agricultural region. Further snow pack accumulation is required, particularly in the Northern and Peace Regions, for adequate spring runoff, which is needed to recharge surface water bodies and groundwater supplies.
Drought Indices
Long-term Drought (Figure 1)
Since the December 31st, 2005 report, those areas classified as Drought Alert increased from 10.6 % to 14.9 % of the reporting area. Currently areas in Drought Alert in the Northern Region include Sturgeon and Westlock Counties, and parts of the counties of Athabasca, Thorhild, St. Paul, Two Hills, Camrose, Smoky Lake, Lamont, Strathcona, Leduc, Parkland and Wetaskiwin. Moreover, a pocket of Drought is emerging within the City of Edmonton (not visible on Figure 1) . Similarly, in the Peace Region Drought Alert exists in parts of the MD of Greenview, Big Lakes, and part of MD of Smoky River in the south and MD's of Northern Sunrise and Northern Lights. Other areas are reporting Normal to Above Normal conditions, with some areas in foothills reporting Wet conditions. Most of this was due to Above Normal precipitations received during the growing season and early fall.
Recent (90-day) trend in long-term Drought conditions (Figure 2)
Over most of the reporting area, Much Below Normal precipitation accumulations during the last 90 days has resulted in an expansions of those areas trending towards Drought Alert, Drought and Exceptional Drought. Across much of the Northern Region, the western parts of the Central Region, and the central parts of the Southern Region, the 90-day trend is towards Drought. Within these areas a few pockets of Exceptional Drought are emerging, specifically in western parts of the Northern and Central Regions, with affected areas that include the M.D of Big Lakes (Swan Hills), central parts of Westlock County, parts of Red Deer, Mountain View and Clearwater Counties, and east of the City of Edmonton straddling the border between Parkland and Lac Ste Anne county. The appearance of these areas on the map serves as a warning that much of the surrounding areas currently trending towards Drought are on the verge of trending towards Exceptional Drought.
If extreme precipitation deficits continue, expect to see areas trending towards Exceptional Drought to grow rapidly in size. Elsewhere in the reporting area, several pockets trending towards Normal can be found, with some in the northern and western parts of the Peace Region, across the eastern portions of the Central and Southern Regions and in the foothills of the Southern Region.
Precipitation
Precipitation since the December 31st, 2005 Drought Report (Figure 3)
Since the last Drought Report, most of the reporting area continued to receive Much Below Normal precipitation. Precipitation accumulations above 180 mm were recorded in parts of the foothills of the Southern Region, grading to below 5 mm at most stations in the northern and central parts of the region, and then increasing to 5 to 10 mm range along the Canada and US border and to 12 to 30 mm in the eastern part of the region with a high of 36.69 mm at Medicine Hat RCS station. Stations in the Central Region recorded an average of 5.5 mm, with the exception of 18.8 mm at Stettler AGDM station in the county of Stettler and 12.2 mm at Consort AGDM Station in Special Areas 4. Similarly, most of the Northern Region recordered below 10 mm (with an average of 6 mm), except the north central and the northeastern corner of the region that recorded an average of 15.3 mm of precipitation. In the Peace Region, the southwestern part received the highest precipitation with 45 mm recorded at Beaverlodge CDA, in the County of Grande Prairie and 28.42 mm at White Mtn Lo station in Saddle Hills County, followed by 26.6 mm at Ballater, station in the MD of Smoky River and 21.5 mm at High Level A station in the MD Mackenze. The rest of the region recorded an average of 15.9 mm, except at Valleyview AGDM station in the southeast at 7.99 mm.
90-day precipitation departures (Figure 4)
Over the past 90-days, precipitation accumulations in the foothills areas of the Southern Region have been generally at least Near Normal. Precipitation accumulations recorded across most of the Southern Region were Much Below Normal with a few isolated pockets in eastern parts of the region that recorded Near Normal conditions. Similarly, across the Central Region, precipitation accumulation was Much Below Normal with the extreme eastern parts of the region recording Below Normal precipitation. Across most of the Northern Region, Much Below Normal precipitation was recorded with the exception of the northern tip of the region where Below Normal precipitation was recorded. Most of the Peace Region reported Much Below Normal precipitation, except for the northern tip, which recorded Below Normal precipitation, and the southwestern corner, which graded from Below Normal to Much Above Normal.
Precipitation departures for the month of January (Figure 5)
For the month of January, precipitation accumulations were generally Much Below Normal across most of the Northern, Central and Southern Regions, with exceptions in the eastern and western extremes of the Southern Region where some locations received Much Above Normal precipitation, and the south eastern parts of the Central Region which received Below Normal precipitation and the County of Stettler where Near Normal precipitation was recorded. Across most of the Peace Region, precipitation accumulations were Below Normal with a few pockets of Near Normal in the south east and north, and Above Normal in the extreme south west.
Normal precipitation for the month of February (Figure 6)
February is typically the driest month in Alberta, receiving only about 3.5 % of the annual precipitation. During this month precipitation totals range from 10 to 20 mm across most of the province, with the exception of a large area in the southeastern part of the Central Region that typically receives less than 10 mm. Areas that typically get more precipitation include parts of the Peace Region with 20-30 mm, and parts of the foot hills in the Southern Region that receive up to 40-50 mm..
Snow pack conditions (Figure 7)
Modeled snow pack conditions expressed as snow pack water equivalent (SPWE) are shown in Figure 7. This represents the current snow pack estimates in stubble fields and reflects a 30% precipitation loss due to blowing, in addition to losses due to sublimation and snow melt process. SPWE's across much of the western parts of the Northern, Central and Southern Regions are less than 1 mm with SPWE's increasing slightly to 1 to 10 mm towards the Saskatchewan border. In the Peace Region, snow pack development is slightly better, with 10-20 mm of SPWE through central and northern areas, ranging to 60 to 70mm in the southwest, down to 1 to 0 mm in the southeast.
Snow pack conditions percent of Normal (Figure 8)
Snow pack accumulations as of the end of January were Much Below Normal across most parts of the reporting area. Exceptions include parts of the foothills and a pocket in the eastern part of the Southern Region, the southwestern corner of the Peace Region (Near Normal) and the northern tip of the Peace Region (Below Normal).
Winter season (October1 to March 31) precipitation accumulations (Figure 9)
In the agricultural areas of the province, average precipitation totals during relatively dry, winter season (October 1 to March 31) account for about 27% of the total annual precipitation. As such, for many areas, large monthly deficits relative to normal during this time do not necessarily represent significant moisture deficits, nor do large monthly surpluses relative to normal represent an immediate amelioration of dry conditions. Figure 9 shows the percent of average annual precipitation received from October 1 to March 31.
The importance of winter precipitation varies from region to region, and even within a region. In the Central and Northern Regions, over winter precipitation normally accounts for about 24 % of the total average annual precipitation while across much of the Peace Region, precipitation during this period accounts for 30-35% of the average annual precipitation. Thus, the precipitation over this period is relatively more important to the Peace Region's annual water balance. Across the plains of the Southern Region, over-winter precipitation accounts for about 25-30% of the average annual precipitation. In the Southern Region, west of the City of Lethbridge, winter precipitation becomes increasingly more important, particularly in the mountains and foothills, a source of much irrigation water, where the October to March precipitation accounts for up to 50 % of the average annual precipitation.
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 average spring or fall conditions
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, Daniel Itenfisu and Isabel Simons-Everett
Conservation & Development Branch & Program
Alberta Agriculture and Food
Edmonton, AB T6R 5T6
Contact: Ralph Wright; ph 780-427-3556
This report was created on January 31, 2006.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of December 31, 2005. |
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