| | Drought indices | Precipitation | Soil moisture | Explanation of terms
Since the last Drought Report (August 8th, 2005) the south half of the province was generally cool with Above Normal precipitation. In contrast, the north half of the province graded from Above Normal precipitation in the east to Much Below Normal throughout most of the Peace Region. Precipitation was heaviest along the foothills and across most of the Central Region, ranging between 90 to 150 mm. Many areas in the Peace Region received less than 10 mm.
Since the start of the growing season (April 1, 2005), precipitation in the western part of the Southern Region was Much Above Normal, mostly due to extreme precipitation events occurring during the first half of June and more recent precipitation over the past few weeks. This grades to Above Normal in central locations and then to Near Normal in the eastern part of the Region. Across much of the Central Region and the east half of the Northern Region, growing season precipitation (GSP) has been at least Near Normal, with many areas reporting Above Normal. In contrast, across much of the central parts of the Northern Region and over much of the Peace Region, GSP has been Below Normal. Elsewhere in the Peace Region, portions of the north and west, received Near Normal GSP.
Currently only the Northern Region has areas showing Drought Alert, these include: most of Sturgeon County and the City of Edmonton, much of Westlock County, and localized areas in the Counties of St. Paul, Minburn and Camrose and one small pocket east of the town of Barrhead. All other Regions in the reporting area have at least Near Normal conditions.
The 90-day trend in Drought conditions in Southern and Central Regions is generally towards Normal to Above Normal. For the most part, all other regions are reporting a trend towards Normal except for a few widely scattered, isolated pockets in the Northern Region, which are tending towards Drought Alert. In addition, dry conditions in the Peace Region over the past 90-days, have resulted in a trend towards Drought Alert in two locations, the largest around the town of Manning and a smaller one around the town of Valleyview.
Since the last report, available soil moisture reserves have increased dramatically across much of the west half of the Southern Region and over many parts of the Central Region. Soil Moisture reserves are still dropping across much of the Northern Region with the most notable declines occurring in the Peace Region. Currently, most of Peace Region is reporting less than 25mm of soil moisture; a condition requiring immediate precipitation for late seeded crops and pasture. However, too much precipitation at this time would impact harvesting efforts and decrease yield quality.
Current Situation
Drought Indices
Long-term drought (Figure 1) - Recent rains throughout the southern and eastern parts of the Northern Region decreased current Drought Alert areas to about 0.9 % of the reporting area, down from the 1.8 % reported in the August 8th, 2005 Drought Report. Currently there are six pockets of Drought Alert in the province, the largest of which includes the City of Edmonton and much of Sturgeon County. A relatively large pocket can also be found centrally in Westlock County, as well as other small pockets affecting an area east of the town of Barrhead, the Counties of St. Paul, Minburn and Camrose. If dry conditions prevail through these regions, expect to see these pockets of Drought Alert grow. Elsewhere, all other areas are showing Normal with areas in the southern half of the reporting area, along the foothills, showing Above Normal to Wet.
Recent (90-day) trend in long-term drought conditions (Figure 2) - Across much of the Central Region and most of the Southern Region, precipitation events received during the month of June, coupled with precipitation received over the past few weeks, have resulted in a 90-day trend towards Above Normal conditions, with a number of locations trending towards Wet. Across the Northern and Peace Regions, the current trend is generally towards Normal. However, two areas tending towards Drought Alert can be found, one centered on the town of Manning and a smaller one centered on Valleyview.
The 90-day precipitation map depicts much of the Peace Region as having below to much below normal precipitation (Figure 4). These trends are not captured on the 90-day trend map (Figure 2) as magnitude of these precipitation deficits were not enough to bring the 90-day trend down to a Drought Alert condition. However, it is important to note that precipitation deficits are occurring over much of the Peace Region, and if this pattern continues, expect to see those areas currently showing a 90-day trend towards Drought Alert to increase in size.
In the Northern Region, the pockets of Drought Alert around the City of Edmonton including much of the County of Sturgeon, east of Barrhead, and in the County of St. Paul are showing a 90-day trend towards Drought Alert.. If conditions do not improve in these areas, expect to see these areas increase in size. Elsewhere, other areas in the Northern Region currently classified as Drought Alert are tending towards Normal. If these conditions persist, expect to see these Drought Alert areas disappear over the next few months.
Precipitation
Precipitation since the August 8th, 2005 Drought Report (Figure 3) - Since the last Drought Report, wet cool weather has generally persisted over much of the south half of the reporting area, up to and including the city of Edmonton. In the Southern Region, precipitation was greatest in the west, with more than 150 mm falling in and around Cardston, grading to 36.9 mm in the east near the town of Schuler. Most of the Central Region, received more than 60 mm with amount in excess of 120mm common through out. In the Northern Region, precipitation amounts dropped of with as little as 10-20 mm being reported in the north central parts and more than 170 mm being recorded in the east, around Lloydminster. Across the Peace Region, precipitation was light with less than 10 mm recorded in many areas, with most of the region receiving between 10 – 20 mm. The greatest amounts of precipitation recorded in the Peace Region were at House Mountain (48.9 mm) and Fairview (41.2 mm).
90-day precipitation departures (Figure 4) - The 90-day precipitation departures span the period from May 31, 2005 to August 08, 2005. Across most of the Southern and Central Region, precipitation was Much Above Normal, despite Well Below Normal precipitation in the month of July. Across much of the Northern Region, precipitation over the past 90 days has been Near Normal with Below Normal amounts falling in central and east central locations. The Peace Region has been very dry over the past 90 days with much of the region reporting Below Normal and one large pocket around Manning reporting Much Below Normal. In the Peace Region 90-day precipitation deficits are not reflected in the recent 90-day Trend map (Figure 2), but serve as a warning: if dry conditions persist, expect to see Drought Alert areas emerging in this region over the next few months.
Growing season precipitation (GSP) to date (Figure 5 and Figure 6) - In the Peace Region, GSP since the last Drought Report has only changed marginally ranging from 153 mm in central parts to over 250 mm in the extreme western and southeastern locations. In the Northern Region, GSP ranged from 175-200 mm in a number of central and east central locations to upwards of 400 mm in the northeast and west. In the Central Region, the driest location is around the Drumheller area (212.8 mm) ranging to over 425 mm in the east around the Town of Consort (Special Area 4) and along the foothills in the west. Across the Southern Region, the driest locations were in the southeast around Onefour 178.8 mm, ranging to well above 500 mm along the foothills.
In the Peace Region, GSP expressed as a percent of Normal (Figure 6), was generally Below Normal across the entire region, with the exception of parts of the west, and small areas in the North and east, where Near Normal GSP was recorded. Across most of the east half of the Northern and Central Regions, GSP to date was at least Near Normal with two notable areas reporting Much Above Normal GSP (County of Vermillion river and Special area 4) and one area in the County of St. Paul, reporting Below Normal. In the west half of the Northern Region, GSP expressed as a percent of Normal was Below Normal, with a few locations grading to Normal. Across the Central Region, precipitation ranged from Normal in central locations, to Above Normal in the west, and Much Above Normal in the east (Special Area 4). Across the Southern Region, August rains brought most the west half of the Region to Well Above Normal GSP. Even the southeast corner of the Southern Region is now reporting Near Normal precipitation. Currently warm, dry weather is needed in order to bring crops in.
Precipitation departures for the month of August, to date (Figure 7) - So far, across the southern half of Alberta, August precipitation was generally Well Above Normal with the exception of the extreme southeast where precipitation was Above Normal. Across the north half of Alberta, the situation is quite different. Throughout most of the Peace Region, August precipitation to date has been Well Below Normal, with some pockets of Normal in the extreme northwest and northern portions of this region. In the Northern Region, precipitation was Below Normal in the west, grading sharply to Much Above Normal in extreme south and eastern locations.
Normal precipitation for the month September (Figure 8) - September marks the transition between the wet season (May – August) and the drier winter months (October – March). During this month precipitation totals typically decline significantly with precipitation totals ranging from just over 20 mm in the Special Areas to less than 60 mm in the Swan Hills and Foothills. Provincially, on average about 9% of the annual precipitation falls in September.
Soil Moisture
Soil moisture in the agricultural regions of Alberta (Figure 9) - Soil moisture reserves across most of the south half of the province improved significantly since the last report, a trend that is not normal for this time of year. This was due to Above Normal precipitation during the month of August Across the Peace Region, soil moisture reserves continued to decline and now much of the region is predicted to have less than 25 mm of reserves. Soil moisture reserves are also low throughout much of the central parts and western parts of the Northern Region, but do improve eastward towards the Saskatchewan border.
Areas with soil moisture reserves below 25 mm need precipitation in order to prevent moisture stress in some of the late seeded crops and perennial stands. However, this need for precipitation is direct conflict with the need to have warm dry weather for harvesting operations.
Significant depletion of soil moisture reserves is typical through August, as a result of vigorous vegetative growth and a decline in monthly precipitation accumulations. This drying trend typically continues into early September and then soil moisture reserves typically increase due to a significant reduction in evaporative demands, brought on by killing frosts, colder temperatures and shorter day lengths.
Current soil moisture departures (Figure 10 and Figure 11) - Soil moisture deficits are computed relative to Normal, which is the 30-year statistical average taken from the years 1971-2000(Figure 10 ) .
In the Peace Region, most areas are reporting Well Below Normal soil moisture reserves, except for the extreme northwest and eastern portions where Near Normal reserves are found (Figure 11). Of note are the increasing soil moisture deficits in northern and extreme southern areas in the Peace Region. Other areas in the province that are reporting soil moisture deficits include most of the western half and central parts of the Northern Region, where soil moisture departures are Well Below Normal to Below Normal. Across most of the south half of the region, soil moisture departures are Well Above Normal, and should provide some protection against dry spells in the near future.
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, Food and Rural Development
Edmonton, AB T6R 5T6
Contact: Ralph Wright @ (780) 427-3556
This report was created on August 28, 2005.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of August 8, 2005. |
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