| | Drought indices | Precipitation | Soil moisture | Explanation of terms
Summary
Since the last Drought Report (July 11, 2005) several storm events resulted in much needed precipitation across the southern half of the Peace Region and across much of the Northern Region. Since most of this precipitation was associated with thunderstorm activity, total accumulations were variable and for some locations, may be different than what is reported here. Precipitation measured across the south half of the Peace Region ranged from 31- 91 mm with the exception of the northern Peace Region, where less than 20 mm was recorded at Manning. Across the Central Region precipitation totals were between 10-40 mm, while the southern Region was dryer still recording between 0-30 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, grading to Above Normal in central locations and then to Near Normal in the eastern part of the region. In most parts of the Central and Northern Regions, growing season precipitation has been Near Normal with a few areas reporting Above Normal, and some areas, recording Below Normal precipitation, the biggest of which can be found in the area which includes the city of Edmonton and Red Deer extending north as far as the county of Barrhead, and south as far as Lacombe county. The west half and northern tip of the Peace Region received Near Normal while all other areas received Below Normal growing season precipitation.
Currently Drought Alert areas consist of several pockets in the Northern Region including Sturgeon County and areas surrounding Edmonton, and other isolated pockets east of Edmonton. In the Central Region, a pocket of Drought Alert is emerging in the northeast corner of Special Area 4.
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, all other regions are reporting a trend towards Normal.
Since the last report, available soil moisture reserves dropped by an average of 25 mm across most of the north half of the province and by about 50 mm across much of the south. This is typical for this time of year as plants reach maximum vegetative growth and evaporative demand is high. Areas with less than 25 mm of soil moisture reserves exist across much of the east half of the Southern and Central regions and parts of the north-central Peace Region. Actively growing crops in these areas will need precipitation immediately to prevent or alleviate moisture stress. The situation in the Peace Region would be worse had it not been for previously adequate soil moisture reserves, which were established last fall and early this spring.
Current Situation
Drought indices
Long-term Drought (Figure 1). Currently 1.3 % of the reporting area is in Drought Alert, down from the 3% reported in the July 11, 2005 Drought Report. In the Northern Region, the largest Drought Alert area includes Sturgeon County, the City of Edmonton and some of the neighboring counties. Elsewhere in the region, some pockets exist in the counties of Minburn, St. Paul, Wainwright, and between the border of Barrhead and Lac Ste Anne Counties. In the Central Region, a pocket of Drought Alert is emerging in the northeast corner of Special Area 3.
Historically in Alberta, monthly precipitation totals begin a slow and steady decline towards winter lows beginning in August. As such the Drought Index (SPI), which is a measure of yearly precipitation deviations from Normal, should gradually become less responsive to precipitation deviations. Currently 86% of the reporting area is classified as Normal, 12% as Above Normal, and 1.7% as 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 current 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 current conditions persist, most of those areas currently reporting as Drought Alert should return to normal over the next few months.
Precipitation
Precipitation since the July 11th, 2005 Drought Report (Figure 3). Since the last Drought Report (July 11, 2005) a series of thunderstorm events brought significant precipitation to most parts of the Northern and Peace Regions with recorded amounts ranging from about 46-92 mm with 91.50 mm recorded at Fairview, 83.2 mm at Cleardale, 81.9 mm at High Prairie, and 88.5 mm at Spring Creek. However, much of the northern tip of the Peace Region was apparently missed by these events, with as little as 18.9 mm being recorded at Manning and 28.2 mm at High Level.
In the Northern Region, the highest amount of precipitation (86.9 mm) was reported at Cold Lake and the lowest amount (24.0 mm) was reported in the County of Westlock near Dapp. Across the Central Region, the greatest precipitation, 43.8 mm was record near Rocky Mountain House and the lowest, 3 mm, in the southeast near Esther. The Southern Region received the least amount of precipitation with totals generally below 20 mm with several stations in the southeast reporting less than 5 mm. Some exceptions did occur in the foothills near Pincher Creek with 72.5 mm and near Streeter Creek with 58.5 mm precipitation.
90-day precipitation departures (Figure 4). The 90-day precipitation departures span the period from April 29, 2005 to July 27, 2005 resulting in a map that is similar to the Growing Season Precipitation Departures map (Figure 6). Thus, see the section below entitled Growing Season Precipitation to Date
Growing season precipitation to date (Figure 5 and Figure 6). To date, growing season precipitation has been highly variable across the province, grading from 117 mm in the central part of the Peace Region to above 530 mm in the foothill areas of the Southern Region.
In the Peace Region, precipitation ranged from 117 mm in central parts to over 190 mm in the extreme western and southeastern locations. In the Northern Region, Twin Lakes (127. 3 mm) and Wainwright (147.5 mm) received the least amount of precipitation and Cold Lake (316.1 mm) the highest. The City of Edmonton and its surrounding area received an average of 170 mm. In the Central Region, the driest Station is at Drumheller (135.6 mm) and the wettest at Cop Upper (426.2). In Southern Region the growing season precipitation decreased from 531.3 mm at Pincher Creek in the western part to 136 mm at Onefour, which located in the southeastern corner of the region (Figure 5).
Growing season precipitation expressed as a percent of normal (Figure 6) was generally near normal for much of the western and northern parts of the Peace Region grading to below normal for much of the east half of the Peace Region. Across most of the east half of the Northern and Central Regions, growing season precipitation to date was Near Normal with a large area of Below Normal extending across most the western part of the Northern Region, down through to the north western parts of the Central Region. Across the southern region, precipitation was Much Above Normal in the west, grading to Near Normal in the southeast. For the Southern Region, these trends are in large part due to record setting precipitation amounts recorded through the first half of June.
Normal precipitation for the month July and August (Figure 7 and Figure 8). July is typically one of the wettest months in Alberta, with the exception of Southern Alberta where June is the wettest month, accounting for about 18% of the average annual precipitation Average July precipitation ranges from 30-40 mm in the east central portions of the Southern Region, up to 120-140mm in the Swan Hills and areas surrounding Rocky Mountain House Historically precipitation totals typically drop off in August across all of the reporting area accounting for on average about 14% of the annual precipitation (Figure 8).
Soil moisture
Soil moisture in the agricultural regions of Alberta as of July 27, 2005 (Figure 9). Soil moisture reserves along the foothills and across the northern border of the Northern Region are generally well above 75 mm. Across the south half of the Peace Region, soil moisture reserves are generally well above 50 mm with the direst areas being found in central locations where less than 25mm of soil moisture reserve exist, a condition requiring immediate precipitation. Across the Northern Region, soil moisture reserves are typically greater than 50 mm with much of the east central areas showing less than 50 mm. The west half of the Central Region is beginning to dry out with much of the central and eastern areas showing less than 25 mm of soil moisture reserves. Much of the east half of the Southern Region has less than 25 mm of soil moisture reserves. In these areas further precipitation is needed immediately to prevent actively growing crops from entering or continuing to be moisture stressed. Areas with 25-75 mm also need precipitation within the next week or so to sustain good crop growth. Areas of the province with at least 75-100 mm of soil moisture reserves have adequate protection against short-term dry spells. In areas with less than 100 mm of reserves, Near Normal and well-timed precipitation is needed to provide adequate moisture for crop growth
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 is an emerging pocket of Extreme deficit in the northern part of the Peace Region. In the Northern Region, the counties reporting Well below Normal include Barrhead, Lac Ste Anne, Parkland, Brazeau, Leduc, Wetaskiwin and Camrose, extending into the central region affecting the counties of Ponoka, Stettler, Starland, Red Deer and Paintearth. Most of the areas in the Southern Region are Near Normal with a pocket of Above Normal and Well Above Normal, which is due to the major precipitation received during the month of June. However, an area with Below Normal soil moisture reserves has immerged in the eastern part of the region due to the Much Below Normal precipitation received since the last report.
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 July 27, 2005.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of July 11, 2005 |
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