| | Preciptation | Snow pack | Data sources | Explanation of terms
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Summary
Since the last drought report (February 11, 2008) precipitation totals across the province ranged from more than 30 mm in the northern and western parts of the Peace Region, and also across the Swan Hills, Cypress Hills, southern foothills and the south-eastern corner of the province to less than 10 mm across a vast area stretching from the southern part of the Northern Region to southern border of the Southern Region. The rest of the reporting area recorded between 10-30 mm.
To date, cold season precipitation (Oct 1 to March 31) accumulations relative to long term normal have been below normal across most of the reporting area. Across much of the Peace Region and the northern half of the Northern Region accumulations were low to moderately low, with some areas grading to very low. Across the southern parts of the Northern Region and over large parts of the Central and Southern Regions accumulations ranged from very low to extremely low, with the latter seen on average not more than once in 25 years. Although it has been an unusually dry winter across much of the reporting area, it is helpful to put the current deficits into perspective. Across the Plains, cold season precipitation accumulations typically ranges between 90 to 130 mm, accounting for only about 25 to 30 percent of the annual average precipitation. Given the fact that the current cold season deficits are about 25 to 50 percent of the normal, the net precipitation deficits ranges between 50 to 75 mm. Thus normal to above average precipitation over the next few months will be needed to recharge soil moisture reserves and provide some protection against short term dry spells over the growing season.
Most of the reporting area is virtually snow free south of Highway 16. Snow packs continue to linger in the north-western parts of the Northern Region where they range from 50 to 60 mm of snow water equivalent (SWE), and also across much of the Peace Region, where the SWE ranges up to as much as 60 to 70 mm in the extreme north. The absence of snow pack accumulations across the Southern Region are normal for this time of year and grade to moderately low in the northern parts of the Central Region and over most for the Northern Region. Across the Peace snow packs are variable and range from low in the southeast to near normal in the central and northern parts of the region.
A large selection of related maps can be found at http://www.agric.gov.ab.ca/acis under the Quick Viewer tab.
Current Situation
Precipitation
Precipitation since the February 11, 2008 Drought Report (Figure 1) - Since the last report, moderate amounts of precipitation (15 to 25 mm) were recorded over much of the Peace Region, the central and northern parts of the Northern Region, the western parts of the Central Region and the along the foothills and also in the eastern parts of the Southern Region. Precipitation accumulations across most of the Central Region, extending to the Northern and Southern Regions continued to be the lowest in the reporting area with total recorded amounts of less than 10 mm being very common.
Peace Region: Precipitation accumulations across the region where highest at the High Level A station (35.3 mm) in the north, with 40.5 mm at Doig Lookout station in the M.D. of Clear Hills and lowest at the Grande Prairie AGDM station (6.8 mm). In general, precipitation accumulations in the northern-half of the region ranged from 20 to 40 mm while in the south-half, from 10 to 20 mm.
Northern Region: Precipitation accumulations across the region ranged from 15 to 20 mm in the north, and central areas down to 5 to 10 mm in the south-central and southwestern portions of the region. The greatest amounts of precipitation were measured at the Oliver AGDM station (29.2 mm), north of Edmonton City, and at the Smoky Lake AGDM station (22.7 mm) in Smoky Lake County. The lowest accumulations were measured at the Camrose station (4.2 mm) in the County of Camrose, with several stations in the surrounding counties reporting less than 10 mm.
Central Region: Precipitation accumulations across most of central and eastern parts of the region continued to be the lowest in the reporting area. Precipitation accumulations were highest in the west, ranging from 20 to 25 mm and lowest in the central and east central areas where a number of stations reported less than 5 mm.
Southern Region: Precipitation accumulations across the region graded from more than 35 mm at most of the stations along the foothills and in the eastern parts of the region, down to 5 to 10 mm across most of the northern and central plains and pockets in the southern parts of the region. In the eastern part of the region, Medicine Lodge station (50.3 mm) recorded the most precipitation. In the mountain areas adjacent to the Southern Region, a few stations reported precipitation accumulations in excess of 90 mm.
Cold Season Precipitation Accumulations relative to Long Term Normal (1961 - 2005) (Figure 2) - Across the agricultural areas of the province, on average, the 6 month cold season (Oct 1 to March 31) accounts for about 25 to 30 percent of the average annual precipitation. While over winter snow pack and the resulting snow melt runoff are extremely important for replenishing surface water stores, they are typically less important for recharging growing season soil moisture reserves. Fall and spring precipitation events occurring on thawed soils are responsible for the recharge of much of the soil moisture for the coming growing season.
Cold season accumulations to date are generally well below normal across most the reporting area with large parts of the Northern, Central and Southern Regions receiving extremely low accumulations. Average to above average spring precipitation will be needed in these areas in order to recharge soil moisture reserves for the coming growing season.
Peace Region: Cold season precipitation accumulations in the region varied from near normal conditions in the north and central parts of the region to low with pockets of very low in the southern parts of the region.
Northern Region: Cold season precipitation accumulations in the region graded from near normal and moderately low in the northern and central parts to very low and extremely low in the southern parts of the region.
Central Region: Cold season precipitation accumulations across most of the region varied from, extremely low to very low, the lowest in the entire reporting area, with the exception of the east where small pocket that grades to near normal can be found.
Southern Region: Cold season precipitation accumulations of extremely low to very low stretched across a north-south corridor in the central plains of the region, that grade to near normal and moderately high in the both the extreme southwest and also along eastern parts of the region.
Average Precipitation Accumulations for April (Figure 3) - April generally marks the end of the dry season in most parts of Alberta with precipitation normally increasing to 30-40 mm over much of the province with the exception of the southwest where 50-60 mm is average. However in the Peace Region dry weather typically persists throughout April, with average accumulations here ranging from 10 to 20 mm and improving in May, when precipitation amounts average about 40 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: SWE's in the region ranged from 50 to 70 mm, in the western and northern parts to 10 to 30 mm in the southwest and less than 10 mm in the south-eastern parts of the region (Figure 5). Relative to normal, over most of the north, snow packs were near normal and in the south were classified as moderately low to low.
Northern Region: SWE's in this region ranged from 40 to 60 mm in the north and northeast to less than 1 mm, across most of the southern and southwestern parts of the region. These accumulations, relative to normal, ranged from moderately high to high, in the north and northeast corner to moderately low across most of the rest of the region with a few isolated pockets grading to low.
Central Region. Most of the snows have disappeared across most of the region. Relative to normal, snow pack conditions range from moderately low in the north half of the region to near normal across the south half of the region.
Southern Region: The snows across most of the plains of the region are gone with snow packs still lingering in the foothills and also in the Cypress Hills. Relative to normal these conditions are near normal for most of the region, with the exception of the extreme southwest were snow pack accumulations are classified as extremely high.
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 (AENV) and Environment Canada (EC). The data undergoes a preliminary computer assisted QA/QC check performed by Alberta Agriculture and Rural Development (ARD) staff. Suspicious values are checked and verified and daily missing values are filled using archived data from AENV 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 under further review by ARD, AENV and EC.
Historical Weather data
Historical weather data was provided by Environment Canada. This data was then 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
Alberta Agriculture, and Rural Development
Edmonton, AB T6R 5T6
Contact: Ralph Wright; ph (780) 427-3556
This report was created on March 24, 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 February 11, 2008. |
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