| | Drought indices | Precipitation | Explanation of terms
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Recent snowfall over most of the reporting area has brought some relief to the abnormally dry conditions that have been experienced over most of Alberta since the beginning of November. Snow is forecast to continue over then next several days and should further improve this winter's moisture deficit.
Since the January 31, 2006 Drought Report precipitation totals across the province ranged from more than 167 mm in southern foothills to less than 10 mm in the eastern part of the Central and Southern Regions. From February 1st to March 5th most of the Northern, Central and Southern Regions received at least Near Normal precipitation with the exceptions in the eastern parts of the Central Region, the southeastern parts of the Northern Region, and the southeastern corner of the Southern Region that received Below Normal precipitation. Across much of the Peace Region, Below Normal precipitation fell, grading to Well Above Normal in the southwest.
Areas classified as Drought Alert since the last report have been significantly reduced due to Near Normal and Above Normal precipitation accumulations. Currently, areas classified as Drought Alert persist in areas north of Edmonton, extending to Sturgeon County and part of Westlock County, as well as the central and southeastern portions of the Peace Region.
Snow pack accumulations are still Much Below Normal over most of the reporting area except across the foothills in the Southern Region and southeastern parts of the Peace Region. Further snow pack accumulations across most of the reporting area are required for adequate spring runoff, which is needed to recharge surface water bodies and groundwater supplies.
Drought Indices
Long-term drought (Figure 1)
Long term, or hydrologic, drought is a result of the cumulative effect of several dry months and primarily impacts livestock feed and water supplies and may affect annual crops. Long term drought classification is determined from precipitation totals over a 365-day period using the Standardized Precipitation Index (SPI).
Since the January 31st, 2006 report, areas classified as Drought Alert decreased from 14.9 % to 5.3 %. Currently in the Northern Region, areas classified as Drought Alert include parts of the Sturgeon and Westlock Counties, and the northern part of Edmonton. In the Peace Region, areas classified as Drought Alert exist in parts of the MDs of Greenview and Big Lakes in the south and part of MD of Northern Sunrise and the Northern Lights County in the north central. Other areas are reporting Normal to Above Normal conditions, with some areas in foothills reporting Wet conditions. Most of this was due to Above Normal precipitations received during the growing season and early fall.
Note that at this time of year a classification of Drought Alert does not indicate a significant threat to agricultural production, as most of the precipitation required to grow crops is received during the growing season. Dry winter conditions can be quickly reversed with significant early spring precipitation events.
Recent (90-day) trend in long-term drought conditions (Figure 2)
Because of the Near Normal and Above Normal precipitation received since the last report, the 90-day trend across most parts of the Northern, Central and Southern Regions have shifted away from trending towards Drought, to trending towards Drought Alert and Normal, with the exceptions of a few remaining isolated pockets that are still trending towards Drought. The 90-day trend in most parts of the Peace Region is towards Normal, with the exception of the central and the southeastern portions trending towards Drought Alert conditions. With more precipitation expected over the next week, it is quite possible that the 90-day trend will continue to shift towards Normal and Above Normal conditions.
Precipitation
Precipitation since the January 31st, 2006 drought report (Figure 3)
In the Southern Region, precipitation accumulations were greatest in the foothills areas with 167.3 mm being recorded at Spionkop Creek station, rapidly grading to an average of 14 mm for most of the stations in the rest of the region except at Milk River (24.2 mm), Medicine Hat (30.0 mm), Medicine Lodge (5.6 mm) and Schuler AGDM (9.3 mm) stations. Across the Central Region, precipitation was greatest in the west (average of 24.2 mm), with the exception of 7.6 mm at Dickson Dam Auto station, grading to an average of 16.3 mm across the eastern half of the region. In the Northern Region, the northeastern part recorded an average of 28.5 mm while in southwest an average of 22.9 mm was recorded. Across the rest of the region an average of 16 mm was recorded with exceptions at Dapp AGDM (6.2 mm) in Westlock County, Wainwright A (8.1 mm) in M.D. Wainwright, Vermilion AGDM (9.1 mm) in the County of Vermilion River and Bodo AGDM (8.6 mm) in the M.D. of Provost. In the Peace Region, the west received the highest precipitation with 47.2 mm recorded at Beaverlodge CDA, in the County of Grande Prairie and 22.7 mm at Cleardale AGDM station in M.D. Clear Hills. Precipitation across the rest of the region averaged 14.5 mm with the lowest at the Peoria AGDM (9.9 mm) station.
90-day precipitation departures (Figure 4)
Over the past 90-days, most of the Southern and Central Regions received Much Below Normal to Below Normal precipitation accumulations with the exception of few pockets and in the foothills with Near Normal and Above Normal precipitation. Similarly, in the Northern Region most locations received Much Below Normal to Below Normal precipitation, with the exception of two pockets in the northeast with Near Normal precipitation. Likewise, most of the Peace Region received Much Below Normal and Below Normal precipitation with the exception of the southwestern corner of the region where Near Normal to Above Normal precipitation was recorded.
On average, precipitation accumulations during the period from December to February, only amount to about 60 mm, which is about 13% of the average annual precipitation. Thus, deficits at this time of year are not as serious as they would be during the growing season. A few significant snowfalls or early spring rains will be sufficient to mask the deficits experienced over the past few months.
Precipitation departures for the month of February (Figure 5)
For the month of February, precipitation accumulations were generally Below Normal with several large pockets of Much Below Normal stretching across a broad band extending from the central Peace Region, through the central and southeastern parts of the Northern Region, and across much of the eastern parts of the Central Region to the central parts of the Southern Region. However, in early March, precipitation fell across most of these areas, easing concerns of winter precipitation shortages.
For most of the west half of the Central Region, the northeastern part of the Northern Region, and large parts of the Southern Region as well as the northern and the western part of the Peace Region precipitation departures for February varied from Near Normal to Much Above Normal. However, February tends to be the driest month of the year (approximately 10-20 mm total) and as such, departures from normal at this time of year have minimal impact on annual precipitation totals.
Normal precipitation for the month of March (Figure 6)
March is a relatively dry month, falling on the heels of February which is historically the driest month of the year. Precipitation received in March typically accounts for about 4.1 % of the total annual precipitation, with the southwest regions of the province receiving the most precipitation (typically between 20-40mm).
Snow pack conditions (Figure 7)
Modeled snow pack conditions expressed as snow water equivalent (SWE) are shown in Figure 7. This represents the current snow pack estimates in stubble fields and reflects a 30% precipitation loss due to blowing; in addition to losses due to sublimation and snow melt process.
SWE's across much of the western and central parts of the Northern and Central Regions as well as the southeastern corner of the Peace Region range from 10 to 20 mm. Across the Plains of the Southern Region, the SWE's are less than 10 mm while the SWE's in some of the foothills areas are above 200 mm. In the Peace Region, SWE's in the west varied from 30 to 70 mm and from 10 to 30 across the rest of the region.
SWE's expressed as a percent of Normal as of March 5th, as shown in Figure 8, are Much Below Normal across most parts of the reporting area. Exceptions to this include parts of the foothills in the south and southwestern corner of the Peace Region with Near Normal to Much Above Normal conditions.
Winter season (October1 to March 31) precipitation accumulations (Figure 9)
In the agricultural areas of the province, average precipitation totals during relatively dry, winter season (October 1 to March 31) account for about 27% of the total annual precipitation. As such, for many areas, large monthly deficits relative to normal during this time do not necessarily represent significant moisture deficits, nor do large monthly surpluses relative to normal represent an immediate amelioration of dry conditions. Figure 9 shows the percent of average annual precipitation received during the dry season, from October 1 to March 31.
The importance of winter precipitation varies from region to region, and even within a region. In the Central and Northern Regions, over winter precipitation normally accounts for about 24 % of the total average annual precipitation while across much of the Peace Region, precipitation during this period accounts for 30-35% of the average annual precipitation. Thus, the precipitation over this period is relatively more important to the Peace Region's annual water balance. Across the plains of the Southern Region, over-winter precipitation accounts for about 25-30% of the average annual precipitation. In the Southern Region, west of the City of Lethbridge, winter precipitation becomes increasingly more important, particularly in the mountains and foothills, a source of much irrigation water, where the October to March precipitation accounts for up to 50 % of the average annual precipitation.
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 March 5th, 2006.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of January 31st, 2005. |
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