| | Drought indices | Precipitation | Soil moisture | Explanation of terms
Since the last Drought Report (July 27, 2005) relatively dry conditions have prevailed across most of the reporting areas. Across all regions precipitation was generally less than 20 mm, with most of the Southern Region receiving less than 10 mm. The greatest amounts of precipitation were measured in the western parts of the Northern and Central Regions where more than 50 mm was recorded at several locations.
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, grading 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. In contrast, across much of the western parts of the Northern Region, GSP has been Below Normal. Both the west half of the Peace Region and northern tip of the Peace Region received Near Normal GSP to date, while all other areas, in the region, received Below Normal GSP.
In the Northern Region the number and size of areas classified as Drought Alert are beginning to increase, however, these areas remain relatively small, but do suggest that if dry conditions persist, a rapid emergence of Drought Alert across much of the area is possible. A few small pockets of Drought Alert can also be found emerging in the Central Region, indicative of a drying trend that needs to be watched closely. Except for these pockets, all other areas are reporting at least Near Normal conditions.
The 90-day trend in drought conditions in Southern and Central Regions is generally towards Normal to Above Normal, mostly due to heavy rainfall in June. For the most part other regions are reporting a trend towards Normal, except for a few widely scattered, isolated pockets in the Northern Region, that are trending towards Drought Alert.
Since the last report, available soil moisture reserves have dropped sharply, with most of the east half of the Southern, Central and Northern Regions and much of the northern Peace Region reporting less than 25 mm of available moisture. For the Southern Region, low soil moisture reserves are Normal for this time of year, however, elsewhere less than 25 mm of soil moisture reserves generally represent Well Below Normal to Extreme Deficits. Actively growing crops in these areas with less than 25 mm of plant available water will need precipitation immediately, especially in the Central, Northern and Peace Regions.
Current Situation
Drought Indices
Long-term Drought (Figure 1) - Currently, 1.8 % of the reporting area is in Drought Alert, up slightly from the 1.3 % reported in the July 27, 2005 Drought Report. This increase is due to the appearance of a number of small pockets in the central parts of the Northern and Central Regions and the growth of existing pockets. There are 9 pockets of Drought Alert identified, the two largest affecting the City of Edmonton and surrounding lands, and most of the west half of M.D. of Wainwright. Many of the surrounding weather stations adjacent to these pockets have values that are on the low side of the Normal scale. Therefore, if dry conditions prevail through these regions, expect to see these pockets of Drought Alert to grow rapidly. Elsewhere, all other areas are showing Normal wiith areas in the south, along the foothills, showing Above Normal to Wet..
Recent (90-day) trend in long-term Drought conditions (Figure 2) - In much of the western parts of the Central Region and most of the Southern Region, record setting precipitation events received during the month of June have resulted in a 90-day trend towards Above Normal, with a few small pockets trending towards Wet. Since the last report, however, Below Normal precipitation across much of the south has resulted in a decrease in the size of the area classified as Above Normal. Across the rest of the reporting area, the trend is towards Normal, with the exception of the northeastern part of the Northern Region, which is trending towards Above Normal, and one small pocket on the border between the counties of Barrhead and Lac Ste Anne, which is trending towards Drought Alert. This suggests that, if the current 90-day pattern persists, then most of those areas currently reporting as Drought Alert should return to Normal over the next few months.
Precipitation
Precipitation since the July 27th, 2005 Drought Report (Figure 3) - Since the last Drought Report, dry weather has prevailed over most of the province. In the Southern Region, most areas received less than 10 mm of precipitation. Over most of the east half of the Central and Northern Regions, less than 20 mm of precipitation was recorded. However, several areas received between 30-40 mm. The most precipitation recorded in the province was in the County of Woodlands, where more than 60 mm fell. In the Peace Region, much of the south half received less than 20 mm, the north 20-30 mm, and the west 30-40 mm.
90-day precipitation departures (Figure 4) - The 90-day precipitation departures span the period from May 11, 2005 to August 08, 2005. This map is similar to the GSP Departures map (Figure 6), thus see the next section.
Growing season precipitation (GSP) to date (Figure 5 and Figure 6) - In the Peace Region, GSP ranged from 145 mm in central parts to over 250 mm in the extreme western and southeastern locations. In the Northern Region, GSP ranged from 150-175mm in a number of central and east central locations to upwards of 300 mm in the northeast and west. In the Central Region, the driest location is around the Drumheller area (138.0 mm) ranging up to 320 mm in the east near Consort (Special Area 4) and upwards of 400 mm in the extreme west. Across the Southern Region, the direst locations were in the southeast around Onefour, less than 140 mm ranging to well above 500 mm along the foothills (Figure 5).
GSP expressed as a percent of Normal (Figure 6) - GPS was generally Near Normal in the north and through many parts of the west. Below Normal precipitation was recorded through most of the east half of the region. Across most of the east half of the Northern and Central Regions, GSP to date was Near Normal, with the exception of a large pocket of Below Normal in and around the M.D. of Wainwright. In addition, Below Normal precipitation was recorded throughout most of the western edge of the Northern Region. Across the Central Region, precipitation grades from Below Normal in the north, to Well Above Normal in the west and extreme southern areas. An isolated pocket of Below Normal can also be found in and around Starland County. Despite recent dry weather across most of the Southern Region, most of the area is showing at least Near Normal GSP, resulting from extremely wet weather in June. However, in the south east corner of the region, in and around the town of Onefour, Below Normal precipitation was recorded.
Precipitation departures for the Month of July (Figure 7) - Across most of Alberta, July precipitation was Below Normal, with the exception of the southern half of the Peace Region and the northeastern parts of the Northern and the Central Region. Much of the Southern Region recorded Much Below Normal precipitation, which permitted croplands in the west to dry following excessive rains in June. Precipitation is now needed, especially in the southeast.
Normal precipitation for the month August (Figure 8) - Historically precipitation totals typically drop off in August across all of the reporting area, accounting for an average about 14% of the annual precipitation (Figure 8).
Soil Moisture
Soil moisture in the agricultural regions of Alberta as of August 8, 2005 (Figure 9) - Soil moisture reserves across the province dropped significantly since the last report. However, this is a normal trend for this time of year as evaporative demand peaks and rainfall totals decline. However, due to below average precipitation in June, the decline in soil moisture reserves was greater than normal.
Soil Moisture reserves are below 25 mm for most of the east half of the Southern, Central and Northern Regions as well as across much of the Northern Peace Region. These low reserves are typical in the Southern Region and eastern parts of the Central Region, however, for forages and hay, precipitation would be welcome. In the dry areas of the Central, Northern and Peace Regions, immediate precipitation would aid in finishing late seeded crops and help to alleviate potential moisture stress in perennial crops.
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 reverses as soil moisture reserves typically increase due to significantly reduced evaporative demands brought on by killing frosts, cooler temperatures and shorter day lengths.
Current soil moisture deficit (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, the central, northern and southeastern areas is reporting Well Below Normal soil moisture reserves while most of the area in the rest of the region is Near Normal with the exception of the western part which is reporting Above and Well Above Normal conditions (Figure 11). Of note are the increasing soil moisture deficits in northern and extreme southern areas in the Peace Region. Elsewhere, in the west half of the Northern Region and central parts of the Central Region, large soil moisture deficits are rapidly emerging due to Below Normal precipitation patterns. Currently these areas are showing Well Below Normal to Extreme Deficits in soil moisture.
What is needed now is Normal August rainfall amounts, in the form of a few drenching rains, followed by warm dry weather to aid in harvesting. Across most of the south, although soil moisture reserves are low, they are rated as Normal for this time of year. The south would also benefit from a few days of wet weather followed by warmer drier weather through the harvesting period.
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; ph 780-427-3556
This report was created on August 8, 2005.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of July 27, 2005 |
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