| | Precipitation | Snow pack | Data sources | Explanation of terms
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Summary
Since the last drought report (December 14, 2007) significant precipitation was recorded over much of the Peace Region (greater than 30 mm), parts of the Northern Regions (greater than30 mm), across the foothills (greater than 50 mm) and in isolated pockets in the Central and Southern Regions (greater than 25 mm). In contrast, precipitation accumulations across a wide area covering most of the Central Region and extending well into the Southern Region continued to be the lowest in the reporting area, ranging from 5 to 15 mm.
Cold season precipitation (Oct 1 to March 31) accumulations, to date, across most of the Peace Region, the north part of the Northern Region, the Foothills, and isolated pockets along the eastern border of the Central and Southern Regions, ranged from moderately low to moderately high. In contrast, precipitation accumulations across an extended area in the south half of the reporting area, varied from moderately low to extremely low.
Modeled snow pack accumulations across the northern half of the Peace Region, the north eastern parts of the Northern Region and south half of the Southern Regions were generally near normal. Across other parts of the reporting area snow cover was generally moderately low with most of the Central Region and the south half of the Northern Region grading down to at least low with some pockets grading down to extremely low.
A large selection of related maps can be found at www.agric.gov.ab.ca\acis, under the Quick Viewer tab.
Current Situation
Precipitation
Precipitation since the December 14, 2007 Drought Report (Figure 1)
Since the last report, significant precipitation was recorded over much of the Peace Region, the northern parts of the Northern Region, the southern foothill areas including some pocket areas in the Southern and Central Region east borders. However, precipitation accumulations across most of the Central Region and much of the northern and central parts of the Southern Region continued to be the lowest in the reporting area.
Peace Region: Precipitation accumulations across the region ranged from lows of 20.5 mm at the Valleyview AGDM station in the south and 21.8 mm at the La Crete AGDM station in the north to highs of 43.5 mm and 49 mm at Cleardale AGCM and Spirit River Auto stations respectively, in the west.
Northern Region: Precipitation accumulations graded from highs of 40.9 mm at Smoky Lake AGDM station in the north and 37.4 mm at the Cold Lake A station, in the north-eastern part of the region to lows of 7.9 mm and 9.8 mm at Camrose and Bellshell AGCM stations respectively, in the south.
Central Region: Precipitation accumulations across the region were the lowest compared to the rest of the reporting Regions. Precipitation accumulations across the region graded from 20 to 25 mm in the northeast, 10 to 15 mm in the centre, up to a high of 28.1 mm at the Oyen AGDM station recorded in the east.
Southern Region: Precipitation accumulations across the region varied from more than 60 mm at most of the stations in foothills to less than 10 mm in the northern part of the region. Precipitation totals across most of the plains were in the range of 10 to 25 mm with the exception of few stations, in the north-central parts of the region that recording less than 10 mm. In the eastern part of the region, Medicine Lodge station recorded 42.1 mm. In the mountain areas, a few stations reported precipitation accumulations in excess of 300 mm.
Cold Season Precipitation Accumulations relative to Long Term Normal (1961 - 2005) (Figure 2)
Cold Season (Oct 1 to March 31) precipitation accumulations relative to long term normal, to date, are generally much below normal across the entire region, with some exceptions where accumulations are at least near normal. Areas classified as at least near normal include the northern parts of the Peace Region, the northeast corner of the Northern Region and few other widely scattered pockets located in other regions. Elsewhere in the reporting area, accumulations were at least moderately low with large areas recording very low accumulations, with large parts of the Central Region classified as having extremely low accumulations.
Peace Region: Cold season precipitation accumulations in the region were generally moderately low in the south half of the region, grading to near normal across the north.
Northern Region: Cold season precipitation accumulations in the region graded from very low to extremely low in the south half of the region, up to near normal in the extreme northeast with a few pockets here grading to at least moderately high.
Central Region: Cold season precipitation accumulations across the region, with the exception of a pocket in the east, remain the lowest across the entire reporting area. Accumulations ranged from very low in the west and east to extremely low over an extended area across the central and southern parts of the region.
Southern Region: Cold season precipitation accumulations across the plains, along the foothills and in the east varied from moderately low to very low in the north and north-eastern part of the region large areas grading to very low and several scattered pockets grading to extremely low.
Average Precipitation Accumulations for February (Figure 3)
Across the province, February is typically the driest month of the year, receiving only about 3.5 percent of the annual precipitation. During this month precipitation totals ranges 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 that receive 20-30 mm and parts of the foothills in the Southern Region that receive up to 40-50 mm.
Snow pack conditions (Figure 4 and Figure 5)
Modeled snow pack conditions expressed as snow water equivalent (SWE) are shown in Figure 4. Snow pack accumulations relative to long term normal are shown in Figure 5. These maps represent the current snow pack estimates in stubble fields and reflect a 30 percent precipitation loss due to blowing; in addition to losses due to sublimation and snow melt process.
Peace Region: SWEs in the region graded from 70 mm in the west and northwest to 30 mm in the south and south-eastern part of the region, and the accumulation relative to long term normal ranged from moderately high in the north to low in the south and south east (Figure 5).
Northern Region: In this region SWEs graded evenly from a range of 40 to 60 mm across the north to 10 to 30 mm across the south. These accumulations relative to normal ranged from high to moderately high in the north, down to low and very low, with one pocket of extremely low found in the County of Camrose.
Central Region: Across most of the region SWEs are in the 10 to 20 mm range, with the exception of the northwest and southeast corners of the region where they are estimated to be in the 20 to 30 mm range. For much of the area, SWEs relative to normal are generally low with east-central and northeastern parts of the region grading down to very low.
Southern Region: SWEs across most of the plains of the region are in the 10 to 20 mm, including pockets with less than 10 mm accumulation, grading to above 90 mm along the foothills and to 30 to 40 mm in a pocket in the east. The snow accumulations relative to long term normal across most parts of the region are at least near normal, except in the north and the southeastern corner of the region were accumulations are classified as at least moderately low.
Data Sources
Near Real Time Weather data
Daily and hourly near-real-time raw weather data is brought in via daily data feeds from Alberta Environment (ENV) and Environment Canada (EC). The data undergoes a preliminary computer assisted QA/QC check performed by Alberta Agriculture (AF) staff. Suspicious values are checked and verified and daily missing values are filled using archived data from ENV databases or from the EC web site. If daily data is still missing, it is estimated using data from nearby stations. Maps describing current conditions are based on preliminary data that is subject to change following further review by AF, ENV and EC.
Historical Weather data
The historical weather data provided by EC was converted to a 10 km daily gridded weather data set that used all available daily data to generate historical climate and soil moisture normals.
Explanation of Terms
Precipitation Accumulations - Frequency of Occurrence
Precipitation accumulations, expressed as a frequency of occurrence are computed for various periods and can be found on our web site at www.agric.gov.ab.ca\acis, under the Quick Viewer tab. Maps are routinely produced for the following periods:
· Past 365-days
· Past 180-days
· Past 90-days
· Past 30-days
· Growing season to date -Starting April 1
· Cold Season to date- Starting October 1
Selected maps from this series are included in this report.
Precipitation accumulations for each period are then determined by ranking the precipitation accumulations during similar periods period dating back from 1961 to present. The current accumulation is compared to the ranked values, yielding the frequency of occurrence, based on percentiles. The percentile points were then put into arbitrary but intuitive classification fields that describe the current state as drier, near or wetter than the long term normal. The resulting map thus answers the question "how often does this occur?" The classifications are as follows:
| Description | Frequency of Occurrence |
| extremely low | drier than this, on average, less than once in 25-years |
| very low | drier than this, on average, less than once in 12-years |
| low | drier than this, on average, less than once in 6-years |
| moderately low | drier than this, on average, less than once in 3-years |
| near normal | on average, this occurs at least once in 3-years |
| moderately high | wetter than this, on average, less than once in 3-years |
| high | wetter than this, on average, less than once in 6-years |
| very high | wetter than this, on average, less than once in 12-years |
| extremely high | wetter than this, on average, less than once in 25-years |
This same scheme is then used for similar maps of soil moisture and snow pack accumulations so that comparisons can readily be made across the various map types.
Snow pack (reported during the winter season only)
Snow pack snow water equivalents (SWE) are modeled for stubble fields. SWE is defined as the equivalent depth of water (mm) that the snow pack contains if it were to be melted. SWE 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 percent of the total precipitation is allowed to accumulate resulting in a 30 percent 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 SWE.
Soil moisture (reported during the growing season months only)
Soil moisture is measured as millimetres (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 reported on from May through to October.
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. Poor 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. Current soil moisture levels are compared against soil moisture levels for the same day in each year from 1961 to present. The frequency of occurrence is computed based on the percentile points, using the same method that was used for similar maps that were generated for precipitation. The frequency of occurrence is then plotted using the same class scheme as is used in the long-term (hydrologic) drought map (see table above). Soil moisture reserves with a modifier of low indicate a need for more precipitation to restore reserves.
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 average soil moisture (spring or fall), computed using model runs dating back from 1961 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 since 1961 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 average. However, currently this process is unable to account for snow currently existing on the ground and as such is not as 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 February 11, 2008.
Drought analysis is currently scheduled at monthly intervals between October 31 and April 31, and twice monthly from May 1 to September 30. This report updates the previous report of December 14, 2007. |
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