| | Drought indices | Precipitation | Soil moisture | Explanation of terms
Summary
Since the last Drought Report (May 18, 2005), a major precipitation event during the first week of June brought in excess of 300 mm to the western parts of the Southern Region. The bulk of this precipitation fell mainly west of Highway 2 between Calgary and Lethbridge and west of Highway 4 between Lethbridge and Coutts. Precipitation totals in the Central Region dropped off dramatically, with less than 10mm being recorded in many areas north of the City of Red Deer and over much of the west half of the Northern Region. In the Peace Region, precipitation ranged from less than 20mm in the east and parts of the west with amounts greater than 60 mm recorded around Grande Prairie.
Since the start of the growing season (April 1, 2005), precipitation across most of the Southern Region was Much Above Normal with exception of the extreme southeast corner of the region, which reported Below Normal precipitation. For central parts of the Central Region, precipitation was Much Below Normal, grading to Normal in the east, and most of the west. A similar situation exists in the Northern Region. Central parts of the Peace Region, reported at least Near Normal precipitation, falling to Below Normal in the southeast and northwest.
Areas classified as Drought Alert since the last report have disappeared in the Southern Region, and decreased slightly in size in the Central Region, and increased slightly in the Northern region. Currently the Peace Region is classified as mostly Normal with some pockets of Above Normal in central and south central localities.
The 90-day trend in drought conditions across the entire southern region ranges from Normal to Wet. In the Central Region a small pocket trending towards Drought has emerged, centered on the town of Lacombe. Drought Alert extends north from this point up to, but not including the City of Edmonton. Elsewhere in the northern region small pockets trending towards Drought Alert can be found in Westlock County and the County of Athabasca. Most of the Peace Region is trending towards at least Normal, with one area trending towards Drought Alert, centered on the town of High Prairie.
The lowest soil moisture levels in the province (25-50mm) are currently found in the Counties of Stettler and Paintearth and the southern parts of Flagstaff County, as well as a small area straddling the border between Special Areas 3 and 4, centered on the town of Esther. Other areas reporting relatively low soil moisture reserves can be found in Smoky Lake County, the Counties of Two Hills, St Paul and the M.D. of Bonnyville. Overall, soil moisture levels are much better than they were at this time last year.
Current Situation
Drought Indices
Long-term drought (Figure 1). Currently 1.8 % of the reporting area is in Drought Alert, up slightly from the 1.1% reported in the May 18, 2005 Drought Report. Two small pockets of Drought Alert currently exist in the Central Region, one centered on the town of Lacombe, and one larger one, centered on the town of Ester, straddling the border between Special Areas 3 and 4. In the Northern Region, a relatively large pocket of Drought Alert can be found east of the City of Edmonton, mostly confined within Beaver County, and the Counties of Minburn and Two Hills. A second pocket can be found nearby in the County of Thorhild. Currently no Drought or Drought Alert areas can be found in the Southern or Peace Regions.
Historically in Alberta, about 48% of average annual precipitation is received through the July-August period. Since the long-term Drought index (SPI) is a measure of yearly precipitation deviations from Normal, precipitation totals through July-August will carry significant weight in the SPI calculations. Thus, over the next few months, the SPI will be quite responsive to precipitation departures from Normal. As a result, if precipitation trends remain Below Average across the current Drought Alert areas, then expect to see Drought Alert areas growing rapidly in size.
The areas previously classified as Normal have increased by 4% since the last report and now stand at 89.7% of the reporting area. The area classified as Above Normal has increased from 5.3% to 7.7 % of the reporting area.
Recent precipitation in the Southern Region has moved some areas into the Wet category, which currently occupies about 0.8% of the reporting area.
Recent (90-day) trend in long-term drought conditions (Figure 2). Over the past 90-days, precipitation surpluses in the Southern Region have seen much of the area trending towards Above Normal or Wet, a major turnaround from that reported in the May 18, 2005 Drought Report. However, in the Central and Northern Regions, absence of precipitation, particularly during the last two months (April and May) have seen some areas moving towards Drought Alert, one large one, south of the City of Edmonton and north of the City of Red Deer, and within this region, one area, centered on the town of Lacombe is trending towards Drought. Elsewhere in the Northern Region, a few pockets are trending towards Drought Alert (County of Athabasca and Minburn and Westlock County). Most the Peace Region is trending to at least Normal with one area centered on the town of High Prairie, tending towards Drought Alert.
Precipitation
90-day precipitation departures (Figure 3). Over the past 90-days, precipitation has been highly variable across the province. Most of the Southern Region has reported Near to Much Above Normal precipitation, most of which fell in the first week of June. In contrast, much of the central portions of the Central and Northern Regions and much of the southwest portions of the Peace Region are reporting Below Normal precipitation. Within these areas, a few small pockets of Much Below Normal are being reported; one in Westlock County, one in Leduc County, and one in Special Area 3. However, adequate deep soil moisture reserves still exist in Westlock and Leduc County, but precipitation is much needed in these areas, as surface moisture supplies are low.
Precipitation since the May 18th, 2005 Drought Report (Figure 4). Since the May 18, 2005 Drought Report, a major precipitation event during the fist week of June brought in excess of 300 mm to the western parts of the Southern Region. The bulk of this precipitation fell mainly west of Highway 2 between Calgary and Lethbridge and west of Highway 4 between Lethbridge and the Coutts. Elsewhere in the Southern Region, precipitation totals were commonly in excess of 100 mm, with the exception of the southeast, which received between 40 and 50 mm. Since this event was mostly confined to the Southern Region, precipitation totals in the other three regions were significantly lower. In the Central Region less than 20 mm was recorded in many areas north of the City of Red Deer. Elsewhere in the Central Region, precipitation was greatest along the eastern and western borders with amounts typically exceeding 60 mm. In the Northern Region, precipitation was less than 20 mm in central and west central areas, grading to 30-40 mm in the west and 30-40 mm in the east around the town of Lloydminster. In the Peace Region, precipitation was ranged from less than 20 mm in the east and parts of the west, with the highest amounts recorded around Grande Prairie (>60 mm).
Growing season precipitation to date (Figure 5 and Figure 6) . To date, growing season precipitation has been highly variable across the province, ranging from less than 50 mm in a broad band north Red Deer, stretching up to the southern edge of the City of Edmonton. Elsewhere less than 50mm of precipitation has been recorded near the town of Ester, in the extreme eastern portions of the Central region, east of Edmonton around the town of Vergreville, and north of Edmonton in the counties of Westlock and Athabasca. In the Peace Region less that 50 mm was recorded around the Town of High Prairie and in the Saddle Hills. For the most part across the Central, Northern and Peace Regions, less than 75 mm has fallen with some exceptions in eastern and western locations of the Central and Northern Regions where upwards of 100 mm has fallen (Figure 5).
Growing season precipitation, expressed, as a percent of normal, has generally been Below Normal throughout most of the central portions of the Northern and Central Regions, with Much Below Normal falling in a large area between the City of Leduc and Red Deer. Other pockets of Much Below Normal can be found centered on the town of Ester in Special Area 3, and another just north of the town of Westlock and also around the town of Vergreville. In the Peace Region, Much Below Normal precipitation has been recorded in eastern and northwestern areas. Most the central Peace Region and the western and eastern portions of the Northern and Central Regions have reported Near Normal growing season precipitation to date. In the Southern Region, Much Above Normal growing season precipitation dominates with the exception of the southeastern corner of the region where Below Normal precipitation was recorded. (Figure 6).
Normal precipitation for the month June (Figure 7 ). June is typically the wettest month in Alberta, accounting for on average about 17% of the average annual precipitation, with total amounts ranging from 50-60 mm in the southeast up to 120-140 mm in and around the Swan Hills.
Soil Moisture
Soil moisture in the agricultural regions of Alberta as of June 07, 2005 (Figure 8). Soil moisture reserves over the west half of the reporting area are typically well above 100 mm with many areas showing greater than 125 mm. Much of the east half of the reporting areas has adequate reserves (>75 mm), with some exceptions (<75 mm) in the central portions of the Central Region and east central areas in the Northern Region. The lowest soil moisture reserves can be found in the central parts of the Central Region (25-50 mm) affecting the Counties of Stettler, Paintearth and Flagstaff, along with a small pocket in Special Areas 3 and 4, along the border with Saskatchewan. Other areas with moderately low reserves for this time of year (50-75 mm) can be found in east central parts of the Northern Region affecting the Counties of Minburn, Two Hills, St. Paul, Smoky Lake and the M.D. of Bonnyville.
Current soil moisture deficit (Figure 9). Soil moisture deficits are computed relative to Normal (which is the 30 year statistical average taken from the years 1971-2000). For the Southern Region, no areas are reporting Below Normal at this time. In fact, soil moisture reserves across most of the Southern Region range from Well Above Normal to Extreme Surplus, mostly due the heavy rains that occurred during the beginning of June.
Much of the rest of the province is reporting at least Normal soil moisture reserves with some exceptions where Below to Well Below Normal reserves exist, in the central parts of the Central Region and the east central parts of the Northern Region. Currently areas with Well Below Normal soil moisture reserves are found in the Counties of St. Paul, Smoky Lake, and the Counties of Wetaskiwin, Camrose, Stettler, Paintearth, and Ponoka County, along with a small pocket in Special Areas 3 and 4, along the border with Saskatchewan.
In the Peace Region, soil moisture reserves are Near Normal in eastern and central areas and Above to Well Above Normal in northern and western locations.
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, Isabel Simons-Everett and Kris Sabourin
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 June 07, 2005.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of May 18, 2005. |
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