| | Drought indices | Precipitation | Soil moisture | Explanation of terms
Summary
Since the last Drought Report (June 27, 2005) storm events brought precipitation to some areas of the province with total accumulations ranging from 0-103 mm. Across the reporting area, precipitation totals were highly variable as a result of a number of localized thunderstorms that occurred during the reporting period. Southwestern Alberta continued to receive the highest amounts of rainfall, greater than 70 mm. Other areas that received the most rainfall (40 - 60 mm) include the northwestern part of the Northern Region, as well as the central part of the South Region. Precipitation totals less than 10 mm were recorded in the Peace, Northern, and Central Regions, as well as at a station in the eastern part of the Southern Region. Overall, the Peace Region, most of the Central Region and the central part of the Northern Region received less than 25 mm, which is considered to be Below Normal to Much Below Normal for this period.
Since the start of the growing season (April 1, 2005), precipitation across most of the Southern Region was Much Above Normal, changing to Near Normal to Above Normal as we move to the east corner of the region. For central parts of the Central Region, precipitation was Near Normal, grading to Above Normal in the east and west. In the Northern Region, the central areas received Below Normal precipitation with most of the other areas receiving at least near Normal precipitation. The Peace Region has received Below Normal precipitation with the exception of the northern part, with Above Normal conditions.
Currently, in the Northern Region, the western part of Two Hills and Minburn Counties, the area centered on the city of Edmonton expanding to the bordering Counties and a pocket in the western part of the County of Wainwright are classified as Drought Alert.
The 90-day trend in drought conditions across the entire Southern and Central Regions ranges from Normal to Wet. The Drought Alert areas are in the Northern Region, centered on Edmonton and the northern part of Leduc County and around Twin Lakes. Despite recent precipitation deficits, most of the Peace Region 90 day trend is towards Normal. However, if the rest of July remains dry, much of the south and central part of the Peace Region will trend towards Drought Alert.
In general, since the last report, the available soil moisture dropped by an average of 25 mm across most of the reporting area with the exception of the western and northern part of the non Peace Region agricultural areas of the province. Currently, most of the Peace Region available soil moisture categories are 50 to 75 mm and 25 to 50 mm, with a pocket area of below 25 mm. Similar moisture levels are exhibited in south, central and southeast part of the Northern Region, the central part of the Central Regions and the southeast part of the Southern Region.
The situation would be worse had it not been for previously adequate soil moisture reserves, which were established last fall and early this spring. Areas with soil moisture reserves ranging between 50-75mm can be found in all the regions. These areas will need precipitation soon to insure crop growth is not affected by lack of moisture.
Current Situation
Drought indices
Long-term Drought (Figure 1) Currently 3% of the reporting area is in Drought Alert, up from the 0.3% reported in the June 27, 2005 Drought Report. Currently, three areas of Drought Alert exist in the Central Region. These include an area centered on the City of Edmonton expanding to the Strathcona, Sturgeon and Leduc Counties; the second area is comprised of the western parts of the Two Hills and Minburn counties, expanding to Lamont and Beaver Counties; the third being a pocket in the western part of the County of Wainwright.
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, then expect to see Drought Alert areas grow rapidly.
Currently 84% of the reporting area is classified as Normal, up from the 74% reported during the last report, mainly, as a result the decrease in the Above Normal category from 21% to 11% and the Wet category from 4.2% to 1.8%.
Recent (90-day) trend in long-term Drought conditions (Figure 2) Over the past 90-days, precipitation in the Southern Region has resulted in a trend toward Above Normal or Wet. Precipitation since the last report has shown improvements in the western part of the Peace Region, with some areas trending towards at least Below Normal. But, in the central part of the Northern Region, below normal precipitation resulted in the expansion of the area under Below Normal condition. Similarly, in agricultural Regions other than the Peace, areas under the Above Normal category are shifting to Normal conditions.
Precipitation
The 90-day precipitation departures cover the period from April 13, 2005 to July 11, 2005. This is similar to that shown on the Growing Season Precipitation Departures map. As a result, only the Growing Season Precipitation Departures map is presented here (see section below entitled 'Growing season precipitation to date' ( Figure 4 and Figure 5).
Precipitation since the June 27th, 2005 Drought Report (Figure 3). Since the June 27, 2005 Drought Report, storm events brought precipitation over much of the province. The western part of the Southern Region continued to receive the highest amount of precipitation, for the most part greater than 70 mm and up to 103 mm, followed by the area north and west of the Northern Region (40 -60 mm) and the central part of the South Region. Elsewhere in the Peace, Northern, and Central Regions, as well as at a station in eastern part of the Southern Region precipitation totals less than 10 mm were recorded.
In the Southern Region, the highest precipitation total since the last report was recorded at Streeter Creek (103.6 mm) and the lowest at Seven Persons (7.9 mm). Most of the central part of the Southern Region, received an average of 30 mm.
In the Central Region, precipitation totals since the last report were less than 34 mm, with an average of 18 mm. The lowest amount was reported at Morrin AGDM (5.6 mm), where the drier area is centered.
In the Northern Region, precipitation totals generally exceeded 45 mm, with an average of 29 mm. A total of six stations in the vicinity of Edmonton, Breton Plots, and Twin lakes recorded less than 10 mm. Vegreville received a total of 15. 6 mm.
The Peace Region continues to be the driest region and the precipitation totals since the last report were less than 35.5 mm, with an average of 17.5 mm. The lowest recorded was at Spring Creek (3.8 mm).
Growing season precipitation to date (Figure 4 and Figure 5). To date, growing season precipitation has been highly variable across the province, grading from 97 mm in the central and 68 mm in the north part of the Peace Region to above 500 mm in the southwestern area of the Southern Region. Elsewhere in the Peace Region, a range of 103-168 mm was recorded in the north, and 110-129 in the west and 96-138 mm in the south. Across the Northern Region, Twin Lakes (89 mm) and Vegreville (88.5 mm) received the least amount of precipitation, followed by two stations in the central part of the region, around Edmonton, with total precipitation of 105 and 107mm increasing to a greater than 190-200 mm in the northern, eastern and western localities. In the Central Region, the driest locations can be found in the central areas, through the Counties of Starland and part of Lacombe where 119-133.mm was recorded. These amounts quickly increase in the southwest to over 400 mm and in the east to above 250 mm. The wettest part in the reporting area can be found in the western portions of the Southern Region where over 500 mm has fallen in the foothills and mountains, grading to a low of 134-171 mm in the southeastern portion and only 7.9 mm at Seven Persons (Figure 4).
Growing season precipitation, expressed as a percent of Normal, is variable across the reporting area with the wettest locations found in the southwest part of the South Region and the driest in most parts of the Peace Region and the central part of the Northern Region. Most of the Southern Region has reported Near Normal to Much Above Normal precipitation mainly as a result of major precipitation events that fell during June. Most of the central region received Near Normal precipitation grading to Above Normal to the west and east. The north part of Lacombe and most of Ponoka Counties received less than 80% of Normal, making this the driest part of the Central Region.
In the Northern Region, several counties centered around the City of Edmonton received Below Normal precipitation, including Sturgeon, Strathcona, Leduc, Wetaskiwin, Parkland and part of Camrose, Lac Ste Annie and Barrhead. Other pockets of Below Normal precipitation are located in the counties of St. Paul, Minburn and Wainwright.
Most of the Peace Region, with exception of the northern part, is currently reporting Below Normal precipitation. (Figure 5).
Normal precipitation for the month July (Figure 6). 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 (Figure 6).
Soil Moisture
Soil moisture in the agricultural regions of Alberta as of July 11, 2005 (Figure 7). Soil moisture reserves along the foothills and the northeastern parts of the Northern Region are typically well above 100 mm. The lowest soil moisture reserves are situated in the central Peace Region, near Fairview (less than 25 mm and in the Central Region, in the southern corner of Starland County. Elsewhere, low soil moisture reserves (25 - 50 mm) can be found in the central part of the Northern Region, around Edmonton and in the eastern part, in St Paul and Two Hill Counties. In the Central Region, low soil moisture content (25 -50 mm) covers the entire Starland County and parts of the counties west, north and east of Starland. The low soil moisture also covers most of the Paintearth County, expanding into Provost and Flagstaff Counties. In the eastern part, low soil moisture content is centered on the town of Esther. In the Southern Region low soil moisture areas are found in the southeastern and northeastern part of the region.
For those areas with less than 25 mm of moisture reserves, precipitation will be needed immediately and for those with less than 50 mm in the next few days to prevent stress from lack of moisture. Areas with 50-75 mm will need precipitation within the next week or so, provided that crops have well established root systems, capable of extracting soil water at depth.
All other areas of the province have at least 75-100 mm of soil moisture reserves, adequate protection against short-term dry spells. Regardless, in those areas with less than 100 mm of reserves, Near Normal well-timed precipitation is needed to provide adequate moisture for crop growth. Those areas with greater than 125 mm of reserves will need warm dry weather over the next few weeks to prevent localized yield reductions due to ponding in low lying field areas. This is particularly true along the Foothills, between Rocky Mountain House and the US border. In these ares, ponding has already localized yield reduction.
Current soil moisture deficit (Figure 8). Soil moisture deficits are computed relative to Normal (which is the 30 year statistical average taken from the years 1971-2000).
In the Southern Region, no areas are reporting Below Normal soil moisture reserves. 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 month of June and first week of July. Localized crop production has been reduced due to ponding and excessive moisture. In the Central Region, soil moisture reserves are Near Normal in several areas, below normal and much below normal in Starland and part of Stettler Counties. The eastern part is reporting Well Above Normal to Extreme Surplus. In the Northern Region most of the eastern half of the region is at Near Normal, grading to Well Above Normal in the northern and eastern locations, and at some areas to Extreme Surplus category. But in most part of the west half of the region Well Below Normal soil moisture reserves with pockets of extremely below normal found around Edmonton, south of Leduc and Barrhead County. In the Peace Region, most of the western and northern part were Near Normal, but much of the central and the southeast locations have Well Below Normal soil moisture reserves. Two areas in the mid west border of the Big Lakes and the south central part of the Northern Sunrise counties are reporting Extreme Deficit conditions.
It is important to point out that given extreme rainfall along the foothills, one would assume that Extreme Surpluses should show up in these areas as shown in Figure 8, and indeed Extreme Surpluses likely exist as most of the soils in these regions are at or near saturation. The modeling process currently being used assumes that field capacity is attained in one day and any excess soil moisture above field capacity is rapidly lost to deep drainage. In reality when extreme quantities of precipitation occurs, which happened over much of the foothills, it is difficult to account for subsurface lateral flow across saturated landscapes as surface seeps, relatively shallow impermeable layers and a host of other factors come into play as soil water is redistributed. In addition, the foothills areas in this region are typically near field capacity at this time of year (Figure 9), thus when soil moisture deficits are computed by subtracting current soil moisture levels that are estimated to be near field capacity, from a 30-year normal that is near field capacity, only marginal surpluses will appear.
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 11, 2005.
Drought analysis is currently scheduled at monthly intervals between October 30 and May 1. This report updates the previous report of June 27, 2005. |
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