| | Drought indices | Precipitation | Explanation of terms
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March snows brought winter-like conditions and much needed snow pack accumulations to much of the north half of the reporting area. This was a welcome relief to the unusually dry winter conditions experienced in most of the province. Across most of the South and Central regions precipitation has continued to accumulate, resulting in little concern regarding spring soil moisture shortages.
Since the March 05th, 2006 Drought Report, precipitation totals across the province ranged from more than 90 mm in parts of the Southern Region, up to 45 mm in the east half of the Central and Northern Regions and up to 30 mm in parts of the Peace Region. Most of this precipitation in the Central, Northern and Peace Regions fell as snow. During this period (March 6th to Apr 10th), most of the Northern, Central and Southern Regions received at least normal precipitation, with many locations receiving well above normal precipitation. The west half of the Peace Region received the lowest amounts of precipitation, less than 20 mm, which was Below to Much Below Normal. In contrast, the northern and eastern parts of the Peace Region received Near Normal precipitation.
Since the last report, the areas previously classified, as Drought Alert have remained largely the same, with the exception of the extreme southeastern parts of the Peace Region where the size of area classified as Drought Alert increased slightly. Currently, areas of Drought Alert persist north of Edmonton city extending to Sturgeon County and parts of Lac Ste. Anne County, as well as in the central and southeastern portions of the Peace Region.
Warm weather in April was sufficient to melt most of the snow pack across the agricultural areas of the province. In the Peace Region and parts of the Northern Region, spring precipitation is needed to recharge soil moisture levels insuring sufficient stores against short-term dry spells. Precipitation across much of the Central and Southern Region in the fall and since February has been sufficient to recharge soil moisture levels thus insuring some resistance to short term dry spells in the near future.
Currently the Peace Region is the driest area of the province, suffering from moisture deficits since August. However, in the Peace Region February to April tend to be the driest months of the year, and as a result, appreciable precipitation should not be expected until May and June. Average monthly precipitation totals for the Peace Region are 19 mm (February), 18 mm (March), 19 mm (April), 43 mm (May) and 83 mm (June ).
Drought Indices
Long-term Drought (Figure 1)
Since the March 5th, 2006 report, areas classified as Drought Alert increased by less than 0.1% and are 5.4% of the reporting area. All areas previously classified as Drought Alert have decreased noticeably in size, except for the extreme southeastern parts of the Peace Region where the Drought Alert areas have increased by about 40%. In the southeast parts of the Peace Region areas classified as Drought Alert currently include parts of the MD's of Greenview and Big Lakes and have expanded to also include the M.D. of Smokey River. In the north central Peace Region, parts of the M.D of Northern Lights and the County of Northern Sunrise are also classified as Drought Alert. In the Northern Region, areas classified as Drought Alert include parts of the Counties of Sturgeon and Lac Ste Anne, and the northern part of Edmonton city. All other areas in the Province are reporting at least Near Normal conditions, with some areas classified as Above Normal found through out most of the Southern Region and the extreme eastern parts of the Central and Northern Regions.
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 Alert, to trending towards Normal. Exceptions include a few remaining isolated pockets in the south eastern Peace Region and western parts of the Central Region that are trending towards Drought Alert.
Precipitation
Precipitation since the March 5th, 2006 Drought Report (Figure 3)
In the Southern Region, precipitation accumulations were greatest in the County of Forty Mile at the Bow Island station, with 111.3 mm and the Spionkop Creek station (89.6 mm) in the foothills. Average precipitation accumulations in the Lethbridge area were 52.8 mm. Most of the south half of the Southern Region received 40 to 50 mm grading to 20 to 30 mm in the rest of the region with the exception of less than 10 mm of precipitation recorded at Vicary Creek (9.20 mm) and Livingstone A (5.3 mm) stations in the foothill areas and at the Onefour CDA station with 12.50 mm in southeast corner of the region. Across the Central Region, precipitation was greatest at the Consort AGDM station (45.2 mm). Across Special Area 4, an average of 35.8 mm was recorded across three stations. The rest of the region received 20 to 30 mm with the exception of the station at Esther (17.6 mm) in the east and a large pocket in the northwestern corner that averaged 12.8 mm, the lowest for the region. In the Northern Region, the highest precipitation accumulations were at the Vermilion AGDM station (45.8 mm) in the County of Vermilion River and at the Dapp AGDM station (42.1 mm) in Westlock County. The central, northern parts and a pocket in southeastern part of the region recorded an average of 37.4 mm while the rest of the region recorded 20 to 30 mm with the exception of four isolated pockets with 10 to 20 mm of total precipitation. In the Peace Region, the south and the south-central portions received below 10 mm, the lowest in the reporting area. Much of the western and central parts of the region received 10 to 20 mm, while the rest of the region received 20 to 30 mm the exception of the Fort Vermillion station, in the northeastern corn of the region, which recorded 16.9 mm.
90-day precipitation departures (Figure 4)
Due to late February and March precipitation events that occurred across most of the Southern, Central and Northern Regions, precipitation accumulations over the past 90-days were at least Near Normal, with several scattered isolated pockets ranging from Much Above Normal to Much Below Normal. Across the Peace Region, precipitation was also variable over the past 90 days, ranging from Much Below Normal in the extreme southern areas to Near Normal across the western, northern and eastern extremes.
Currently the Peace Region is the driest area of the province. However, March and April tend to be dry in this region with an average of 18 mm falling in March and 19 mm falling in April. Typically in the Peace Region monthly precipitation totals don’t bounce back from winter lows until May and June when on average 43 and 83 mm of precipitation is recorded, respectively. A few significant spring precipitation events will be sufficient to mask the deficits experienced over the past few months in the Peace Region, as the winter dry season has not lost its grip yet.
Precipitation departures for the month of March (Figure 5)
For the month of March, precipitation accumulations in much of the central and eastern part of the reporting areas ranged from Near Normal to Much Above Normal with the exception of isolated three pockets with Below Normal precipitation. In much of the western part of the Southern, Central Regions and the southwestern portion of the Peace Region precipitation ranged form Much Below to Below Normal. Most of the Northern Region received Much Above Normal precipitation.
Normal precipitation for the month of April (Figure 6)
April generally marks the end of the dry season in most of Alberta, with precipitation normally increasing to 30-40mm over much of the province with the exception of the southwest where 50-60 mm is Normal. However, in the Peace Region dry weather typically persists until May when precipitation amounts typically reach about 40 mm.
Snow pack conditions
Warm weather early in April was sufficient to melt most of the snow pack that had accumulated across the agricultural areas of the province.
Winter season precipitation departures (Figure 7)
Winter precipitation received from November 1 to March 31 was generally Below Normal across most of the reporting area with some exceptions. Scattered pockets of Near Normal precipitation occurred throughout the Southern Region, with small pockets of Much Below Normal in the southeast around Medicine Hat and northern areas in the County of Wheatland and Newell. In the Central Region, Near Normal precipitation was recorded in the extreme east and a small pocket in the north, which includes most of the County of Stettler. Much of the west half of the Northern Region received Much Below Normal precipitation with a few isolated pockets of the same in east central locations affecting the County of Two Hills and the M.D. of Wainwright. Across the southeastern and east central Peace Region, Much Below Normal precipitation was recorded, grading sharply to Near Normal up in the Saddle Hills area.
On average across the province, over winter precipitation for the five-month period spanning November to March only accounts for 20-25% of the average annual precipitation. In contrast, the four-month period, April to July, accounts for about 50% of the average annual precipitation. Thus, precipitation deficits over winter do not have as severe an impact on growing conditions as they do through the bulk of the growing season, when agricultural production needs the precipitation most.
Winter season (October 1 to March 31) precipitation accumulations (Figure 8)
In the agricultural areas of the province, average precipitation totals during the relatively dry winter months (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 8 shows the percent of average annual precipitation received 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
Technology and Innovation Branch
Agriculture and Rural Development
Edmonton, AB T6R 5T6
Contact: Ralph Wright; ph 780-427-3556
This report was created on April 10th, 2006.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of March 5th, 2006 |
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