| | Precipitation | Snow pack | Data Sources | Explanation of Terms
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Summary
Since the last drought report (March 4, 2007), precipitation was highly variable across the reporting area. Across the Peace Region, precipitation ranged from 20 to 30 mm in the north to 50 to 60 mm in the centre and 40 to 50 mm in the southwest. Across the Northern Region, precipitation ranged from less than 10 mm in west-central and southeastern locations, to 20 to 30 mm in the north and west. In the Central Region precipitation ranged from less than 10 mm in the east, up to 50 to 60 mm in the west. In the Southern Region, precipitation ranged from less than 10 mm in the southeast, to well over 80 mm along the foothills.
Cold Season (October 1, 2006 to March 31, 2007) precipitation accumulations relative to long term normal, ranged widely throughout the reporting area. Across the Peace Region, Cold Season accumulations ranged from moderately low in the north to extremely high in the centre, with the previously dry areas in the west receiving high to very high accumulations. Across the Northern Region, precipitation accumulations for the cold season ranged from extremely high in the west to a small pocket of very low in the south. Across the Central Region, Cold Season precipitation accumulations ranged from near normal in the west to moderately low and low across central locations and low in the northeast. Across the Southern Region, Cold Season precipitation ranged from very high in a few pockets in the west along the foot hills, to moderately low and low in the centre, down to extremely low in the southeast, with the exception of the Cypress Hills where extremely high accumulations were recorded.
The 365-day precipitation accumulations relative to long term normal in the northwestern Peace Region improved slightly since the March 4th, 2007 report and now range from moderately low to near normal. However, a pocket of extremely low still persists, in the north eastern parts of the M.D. of Clear Hills (northwest Peace Region). Across the Northern Region accumulations over the past 356 days are generally near normal, with areas grading to moderately low in east central locations. Across the Central Region most areas are classified as having at least near normal precipitation accumulations over the past 365-days with a few pockets in south-central locations, grading down to moderately low and one pocket in north-central part of the region grading to high. Across the southern half of the Southern Region, the 365 day precipitation accumulations have generally decreased since the last report, with several areas grading down to very low. Elsewhere in the Southern Region conditions are generally near normal.
Currently most of the snow pack is gone across the Northern, Central Regions. Precipitation received over the past few days has brought some snow cover to most of the southern region (less than 10 cm) and most of this is expected to melt over the next day or so. In contrast, across the Peace Region and the northeastern parts of the Central Region, significant snow packs still exist. Most of the western half of the Peace Region in estimated to have more the 100 mm of water equivalent in the snow pack with the greatest amounts (greater than 150 mm) in the Swan Hills. Many of these areas have snow packs that are classified as extremely high and, as such, Alberta Environment has indicated that flood risk is high for Northern Alberta
http://environment.alberta.ca/forecasting/ForecastersComments/index.html
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 March 4, 2007 Drought Report (Figure 1) - Since the last report, precipitation accumulations have been highly variable across the reporting area, ranging from less than 10 mm across parts of the Northern and Central Regions and southeast corner of the Southern Region to more than (50 to 60 mm) across parts of the Peace Region and well over 60 mm along the foothills in the western parts of the Southern Region.
Peace Region: Across the region, precipitation accumulations were lowest in the southeast (20 to 30 mm), and highest in the centre near the town of Peace River (60 to 70 mm) and in the north near the town of High Level (50 to 60 mm).
Northern Region: Precipitation accumulations ranged from less than 10 mm in the southeast to 20 to 30 mm across the west and northern parts of the region, with several widely scattered stations recording between 10 to 20 mm.
Central Region: Precipitation accumulations ranged from less than 10 mm in the east, up to 10 to 20 mm across most of the central portions of the region, with the greatest amounts (50 to 60 mm) being recorded along the foothills in the southwest.
Southern Region: Precipitation was highly variable across the region with the least amounts recorded in the extreme southeast near the town of Onefour (10 to 20) mm and the greatest amounts (>80 mm) recorded at several stations in the foothills and mountains.
Cold Season Precipitation Accumulations relative to Long Term Normal (1961 - 2005) (Figure 2) - The cold season is defined as the period between October 1 and March 31. This is a critical period for soil moisture recharge, particularly when the soils are not frozen. During this time, evaporation rates are low and thus the precipitation that falls as rain is an important source of soil moisture recharge for the following growing season. In addition, over winter snow pack accumulations can contribute to soil moisture recharge when melt conditions are favourable. However, generally snowmelt water is more important for recharging surface water bodies and ground water stores.
Peace Region: Most of the southern half of the region has received at least high Cold Season precipitation accumulations to date, with central locations grading to extremely high and west central locations grading to very high. Across the northern half of the region, accumulations were at lest near normal, grading to moderately low in the extreme northeast.
Northern Region: Across most of the region, cold season precipitation accumulations were least near normal with several locations in the west and extreme northeast, grading to extremely high. Moderately low accumulations were recorded in a small pocket in the east-central part of the region, while low accumulations were recorded in south-central and southeastern locales. One pocket of very low was recorded in the centre of the County of Camrose.
Central Region: Cold Season precipitation across most of the west half of the region and parts of the southeast were near normal. Much of the east half of the region recorded at least moderately low accumulations, with a few areas in the northeast and south central parts of the region recording low accumulations.
Southern Region: Cold season precipitation was highly variable across the region, ranging from extremely low in the southeast (Cypress County), moderately low to low in south-central locals, and up to very high in parts of the west. A notable exception to the extremely low Cold Season precipitation accumulations in the southeast occurs in the Cypress Hills where extremely high accumulations have been recorded.
Long Term Conditions: 365-Day Precipitation Accumulations relative to Long Term Normal (1961 - 2005) (Figure 3) - Precipitation accumulations over the past year were variable across all regions.
Peace Region: The northwest and southeastern parts of the Peace Region are experiencing minor net annual precipitation deficits and are currently classified as moderately low, with a pocket of low recorded in the southeast, near the town of Valleyveiw. Elsewhere accumulations are at least near normal with the exception of the north-central parts of region where areas of moderately low exists which grade into very low in the northeast corner of the M.D. of Clear Hills.
Northern Region: Generally over the Northern Region, 365-day accumulations are near normal with the exception of a large band of moderately low, running north to south through the east parts of the region stretching from St. Paul to Camrose.
Central Region: Much of the region has recorded at least near normal precipitation over the past 365-days with a few widely scattered pockets across the south half of the region, classified as moderately low.
Southern Region: Across the Southern Region, 365-day precipitation accumulations are extremely variable, with three pockets of very low scattered widely throughout the south half of the region. In contrast, most of the north half of the region has experienced near normal accumulations.
Average Precipitation Accumulations for April (Figure 4) - April generally marks the end of the dry season in most 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 ranging from 10 to 20 mm and improve in May when precipitation amounts average about 40 mm.
Snow pack conditions (Figure 5 and Figure 6)
Modeled snow pack conditions expressed as snow water equivalent (SWE) are shown in Figure 5. Snow pack accumulations relative to long term normal are shown in Figure 6. These maps represent the current snow pack estimates in stubble fields, which reflect a 30% precipitation loss due to blowing, in addition to losses due to sublimation and snow melt process.
Peace Region: Significant snow packs still exist across most of the Peace Region with SWEs being highest in the southwest and southern parts of the region (125 to 150 mm), grading down to 20 to 30 mm in the southeast and north central locations. For much of the west half and central parts of the region, snow packs are classified as at least high, with several large areas grading to very high and a few smaller areas grading to extremely high. Elsewhere snow packs are generally near normal to moderately high.
Northern Region: Most of the snow pack in the Northern Region is gone with the exception of some snow cover remaining (SWE's, of 50 to 60 mm) in the northeast and extreme north. The snow packs here are classified as moderately high with a few pockets, grading to high.
Central Region. The snow pack is largely gone from this region.
Southern Region: Precipitation received during the last two days has brought some snow cover to most of the region (SWEs 1 to 10 mm). However, warm conditions forecast for the next few days will melt most of this new snow. Historically snow cover on April 11th is unusual and as such the snow pack accumulation relative to long term normal map shows that much of the area has very high to extremely high snow cover for this time of year. Strictly speaking this map is accurate since snow cover at this exact time of year is a relatively rare occurrence, however, snow cover is light and it should disappear over the next day or two.
Currently, the Cypress Hills has a well-established snow pack where SWEs are estimated to be as high as 40 to 50 mm. Snow packs in the southwest, in the mountains and foothills, are significant (greater than 50 mm SWE) and still persist. The snow pack in the Cypress Hills is classified as high and the snow pack in the mountains and foothills is classified as moderately high with a few small pockets grading to 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 (AAF) 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. All maps describing current conditions are based on preliminary data that is subject to change under further review by AF, 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 Occurence |
| 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, Food and Rural Development
Edmonton, AB T6R 5T6
Contact: Ralph Wright; ph 780-427-3556
This report was created on April 12, 2007.
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 March 4, 2007 |
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