| | Summary | Drought indices | Precipitation | Soil Moisture | Explanation of terms
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Introduction
A new classification scheme has been introduced for the all maps that describe current conditions relative to long-term normals. This permits direct comparisons between different maps and provides a more intuitive description of how often certain conditions are likely to occur. The classification system is as follows:
| Class or Condition | Frequency of Occurrence |
| extremely low | drier than this less than 1 in 30-years |
| very low | drier than this less than 1 in 20-years |
| low | drier than this less than 1 in 10-years |
| moderately low | drier than this less than 1 in 4-years |
| near normal | similar to this 5 out of 10-years |
| moderately high | wetter than this less than 1 in 4-years |
| high | wetter than this less than 1 in 10-years |
| very high | wetter than this less than 1 in 20-years |
| extremely high | wetter than this less than 1 in 30-years |
Summary
Precipitation during May brought much needed relief to dry conditions in the Peace Region and the northwestern parts of the Northern Region.
Since the April 10th, 2006 Drought Report, precipitation totals were highest in the Peace Region, (greater than 120 mm), followed by the Northern Region where many areas received between 80 to 100 mm. The least amount of precipitation (20 to 30 mm) was recorded in central parts of the Central Region. Across the Southern Region precipitation was quite variable, ranging from 80 to 90 mm in the north, to 30 to 40 mm in the southeast corner.
Over the past 365-days, several areas in the Peace Region recorded extremely low precipitation accumulations. However, recent precipitation has greatly improved soil moisture reserves for cropping. Several areas in the central parts of the Northern Region have low precipitation accumulations. Elsewhere across the reporting area, precipitation accumulations have been at least near normal with some parts of the Southern, Central and the extreme eastern parts of the Northern Region having extremely high precipitation accumulations.
Since the start of the growing season, April 1, 2006, much of the province has received at least near normal precipitation accumulations, with much of the Southern, Peace and the northern parts of the Northern Regions receiving moderately high to extremely high precipitation accumulations.
Soil moisture reserves are at least near normal over much of the reporting area with several locations in the Southern, Central and Northern regions showing extremely high reserves. In central parts of the Peace Region, soil moisture reserves are near normal due to rains in May, with moderately low to extremely low reserves in the north and extreme south.
Current Situation
Drought indices
Long Term Drought: 365-day precipitation accumulations relative to long term normal (Figure 1) - The Long Term Drought Index map, based on the Standardized Precipitation Index (SPI), previously seen in reports prior to the 2006 growing season has been discontinued and replaced by a similar, but more intuitive, descriptor of long term precipitation patterns (Figure 1). This new method compares precipitation accumulations over the past 365-days against the ranked precipitation accumulations during the same period, for all years through 1961 to present. The percentile for the current accumulations is then computed and expressed as a frequency of occurrence, relative to the long term normal. This provides a more intuitive assessment of current conditions by answering the question, "how often does this occur?".
Peace Region: Three large pockets of extremely low precipitation accumulations exist, one in the south, one in the center, and one in the north. These areas quickly grade in a category of low accumulation with much of the Peace Region being classified as at least moderately low, with the exception of the west, which is classified as near normal.
Northern Region: Across much of the western half of the region, precipitation accumulations are ranked as moderately low with the exception of a few pockets of low. Across the east half of the region, precipitation accumulations are at least near normal with the extreme eastern locations, around the town of Lloydminster, reporting extremely high precipitation accumulations.
Central Region: The northern parts of the region recorded near normal precipitation accumulations, grading rapidly to extremely high across the south half of the region.
Southern Region: Precipitation accumulations are near normal in the southeast, grading to extremely high in the north and west with a large pocket of at least near normal found between the Cites of Lethbridge and Calgary.
Precipitation
90-day precipitation accumulations relative to long term normal (Figure 2) - Across most of the reporting area, precipitation accumulations over the last 90-days have been at least near normal.
Peace Region: Across most of the region, at least near normal accumulations were recorded with central parts recording moderately high accumulations, grading to extremely high in the west, centered on the Saddle Hills area.
Northern Region: moderately high precipitation accumulations dominate the north half of the region, grading to high and extremely high, in the county of Athabasca and Westlock County as well as near the Town of Lloydminster in the East.
Central Region: Most of the central and eastern part of the region recorded at least near normal precipitation with the exception of few pockets of moderately low accumulations in the western half of the Region.
Southern Region: Most of the region recorded at least normal precipitation with the exception of a few pockets of moderately low precipitation accumulations with one pocket grading to very low in the west and one pocket of moderately low in the far southeastern portion of the region.
Precipitation since the April 11th, 2006 Drought Report (Figure 3)
Peace Region: Precipitation accumulations ranged from 130 mm at the White Mtn. LO station in the Saddle hills areas to less 31.9 mm at the Fort Vermillion RS station in the north. The central parts of the Peace Region received 80 to 100 mm grading to 40 to 50 mm across much of the northern parts of the Region.
Northern Region: Precipitation accumulations ranged from 102.6 mm in the west at the Violet Grove CS station, to 52.0 mm at the Camrose station. In general, precipitation was greatest in the west, exceeding 80 mm, in many areas, and the lowest in the center (50 to 60 mm) then increasing again in the east, with a number of stations recording accumulations of more than 70 mm.
Central Region: The greatest amount of precipitation was received at the Consort AGDM station (79.0 mm) in the northeast and the least amount (23.7 mm) at the Parlby Creek Near Mirror station in the north. Since the last report, this region received the least amount of precipitation over the reporting area, with several stations in the central and western portions recording less than 50 mm.
Southern Region: Precipitation was variable with the greatest amounts recorded at the Beauvais station (125.4 mm) in the west, followed by the Strathmore IMCIN station in the North (91.9 mm) and the least amount (35.2 mm) recorded at the One Four CDA station in the southeast. Overall, the west half of the region received over 50 mm of precipitation while the east was drier receiving less than 50 mm of precipitation.
Growing season precipitation (GSP) to date (Figure 4) - Precipitation accumulations for the growing season to date were very similar to those reported with the April 11, 2006 Drought Report map, as there is only a 10 day difference between the two maps. The main differences exist in the northwestern parts of the Northern Region where an additional 10 to 30 mm fell and in southwestern parts of the Southern Region where up to 50 mm of additional precipitation fell.
Growing season precipitation accumulations relative to long term normal (Figure 5) - Growing season precipitation to date has generally been at least near normal over most of the reporting area. However, several pockets of moderately low and low growing season precipitation accumulations were recorded in the Central Region.
Peace Region: The south half received moderately high growing season precipitation accumulations to date, grading to extremely high in the west. Across the north, growing season precipitation was near normal.
Northern Region: Across the northwest, growing season precipitation was the greatest with some areas classified as extremely high. This grades to moderately high in central locations, down to near normal in the extreme east and over much of the south half of the region
Central Region: Across the northwest, growing season precipitation to date was generally Normal with several pockets grading into the low and moderately low categories. Across the east half of the region, precipitation was near normal with parts of the east and much of the southeast recording moderately high amounts of precipitation.
Southern Region: Across most of the region, growing season precipitation was at least near normal with a few pockets along the Canada/US border recording moderately low amounts of precipitation.
Normal precipitation accumulations for June (Figure 6) - June is typically one of the wettest months in the year with precipitation accumulations ranging from 60 to 70 mm in the southeast to greater than 100 mm across the west parts of the Northern Region and southeastern parts of the Peace Region. If near normal precipitation falls then soil moisture reserves should be average to extremely high across the reporting region.
Soil moisture
Soil moisture in the agricultural regions of Alberta (Figure 7) - Soil moisture conditions are generally good throughout the reporting area, due to significant precipitation received since March and also in the south, due to significant precipitation received late last summer and during the fall. Spring and fall are critical times for soil moisture recharge as evapotranspiration is relatively low due to reduced, live vegetative cover. Thus, precipitation received during these periods is critical for replenishing soil moisture levels.
In general, dry winter conditions in the South have been alleviated due to good soil moisture carry over from the fall. Across the north, extremely dry conditions over the past year or so have been alleviated from significant precipitation accumulations this spring. However, continued precipitation is needed in the Peace Region, particularly in the northern part to build sufficient soil moisture reserves to withstand short-term dry spells that may occur during the growing season. Areas with less than 50 mm of soil moisture require at least Above Normal precipitation over the next few weeks to build soil moisture reserves for the growing season ahead.
Peace Region: May precipitation has brought much needed relief to most of the areas that were facing low soil moisture reserves, due to low precipitation accumulations over the past year or so. (Figure 1). Across central portions of the region, soil moisture ranges between 75 to 100 mm, decreasing in the south to 50 to 75 mm and in the north soil moisture levels are down to 25 to 50 mm..
Northern Region: Most of the region has soil moisture levels greater than 75 mm, with the wettest areas to the east around Lloydminster greater than 125 mm).
Central Region: Most of the region has soil moisture levels of greater than 75 mm with parts of the southwest estimated to have over 125 mm of moisture. Central locations are the driest with 50 -75 mm.
Southern Region: Most of the west half of the region has soil moisture levels of greater than 125 mm grading to a low of 50 to 75 mm in the extreme south east.
Long term average soil moisture conditions (Figure 8) - The long-term average soil moisture conditions are shown in Figure 8. Note that at this time of year soil moisture reserves are typically lowest in the eastern parts of the central region (20 to 25 mm) and grade up to 100 to 125 all along the foothills. Across the Peace Region, normal soil moisture reserves for this time of year are typically 75 to 100 mm with the driest areas found in the central Peace Region (50 to 75 mm)
Current soil moisture reserves relative to long term normal (Figure 9) - This map replaces the previous soil moisture departure map seen in reports prior to 2006 growing season and uses the same classification scheme as the precipitation maps that describe current accumulations relative to long term normals. Soil moisture reserves computed for the report date (Figure 7), were compared to modeled soil moisture values, using the historical weather data from the same date during the 1961 to current period, and are expressed as a "frequency of occurrence", which shows how often current soil moisture reserves of a similar magnitude have occurred in the past (1961 to 2005).
Soil moisture reserves are at least near normal over much of the reporting areas with parts of the Southern, Central and Northern Regions showing extremely high reserves. In the Peace Region, parts of the south have moderately low reserves and parts of the north extremely low reserves.
Peace Region: Soil moisture reserves through the central parts of the region are near norma. Across the north, soil moisture reserves are modularly low grading to extremely low east of Fort Vermillion. Across the extreme south, reserves dip to moderately low south of Grande Prairie.
Northern Region: The extreme eastern portions of the region are classified as high to extremely high reserves, grading to near normal across the rest of the region, with a few pockets of moderately high reserves found north of City of Edmonton..
Central Region: Much of the south half of the region has soil moisture reserves grading into the extremely high category, grading to near normal in the northwest.
Southern Region: Much of the northern half of the region has soil moisture reserves in the extremely high category, grading to moderately high in central locations, to near normal in the
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 by ranking the precipitation accumulations during the same 365-day 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:
| Class or Condition | Frequency of Occurrence | Probability of Occurrence |
| extremely low | drier than this less than 1 in 30-years | 0.033 |
| very low | drier than this less than 1 in 20-years | 0.050 |
| low | drier than this less than 1 in 10-years | 0.100 |
| moderately low | drier than this less than 1 in 4-years | 0.250 |
| near normal | similar to this 5 out of 10-years | 0.500 |
| moderately high | wetter than this less than 1 in 4-years | 0.250 |
| high | wetter than this less than 1 in 10-years | 0.10 |
| very high | wetter than this less than 1 in 20-years | 0.050 |
| extremely high | wetter than this less than 1 in 30-years | 0.033 |
Precipitation accumulations-frequency of occurrence.
Precipitation accumulations, expressed as a frequency of occurrence are computed for various periods. They are computed using the same technique and same time frame (1961 to current) that was used for the long term (Hydrologic Drought Index) and share the same class scheme (see table above). The only difference being the period of time over which precipitation accumulations were computed.
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 is 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. 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 low classification, 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 were 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 near 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 May 28th, 2006.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of April 11, 2006. |
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