| | Precipitation | Soil moisture | Data sources | Explanation of terms
.
Summary
Since the last report (March 24, 2008), moderate amounts of precipitation (10 to 25) mm were recorded over much of the reporting area, with the exception of the northern and central parts of the Peace Region and an isolated pocket in the Southern Region where less than 10 mm was recorded. Precipitation accumulations along the foothills and across most parts of the eastern border of the province were above 25 mm. Some of the stations in the southern foothills and southeastern parts of the province reported more than 60 mm.
Growing season precipitation accumulations relative to long term normal to date (April 1, 2008 to April 27, 2007), are at least near normal across most of the reporting area with the exception of large parts of the Southern Region, the central parts of the Peace Region, and other isolated pockets elsewhere that were below normal. Areas drier than this can be found in the extreme southwestern parts of the Southern Region where growing season accumulations graded down to very low.
To date most of the reporting area is in now snow free with the exception of the foothills and isolated pockets along the eastern border of the province.
Long term precipitation (180 day) accumulations to date are generally well below normal across most the reporting area with large parts of the Northern, Central and Southern Regions classified as extremely low, a condition occurring less than once in 25 years. Overall, it has been unusually dry over the past six-months, therefore, over the next several weeks, it is important that timely, normal to above normal precipitation amounts are received, so that recharge of the root zone soil moisture reserves occurs, thus providing some protection against short-term dry spells over the coming growing season.
The regular springtime soil moisture survey, covering most of the reporting area is underway, however, progress has been slowed down by recent snowfall events and lingering frost a depth, delaying the May 1 target date. Weather permitting; the survey completion is expected by mid May. The spring soil moisture survey is used to initialize the soil moisture model with a more accurate representation of soil moisture conditions across the reporting area and correct for inaccuracies in the model estimates. In the absence of the spring soil moisture survey, the soil moisture status discussions and maps presented here have been generated from model runs that were initialized with last spring's soil moisture conditions.
Since the November 7, 2007 drought report, soil moisture reserves have stayed relatively the same across most areas. Typically, the period extending through early fall and into late spring period is an important time for soil moisture recharge that has not occurred due to significant precipitation deficits that extend back as far as last July. This has lead to significant deficits across many parts of the province with parts of the Northern, Southern and Peace Region classified as having extremely low reserves. These areas are currently particularly drought sensitive and above normal precipitation will be need in these areas in order to see the crops through short-term hot and dry spells that frequently occur during the growing season.
A large selection of related maps can be found at http://www.agric.gov.ab.ca/acis, under the Quick Viewer tab.
Current Situation
Precipitation
Precipitation since the March 24, 2008 Drought Report (Figure 1) - Significant amounts of precipitation (less than 25 mm) were recorded along the western foothills and along the eastern border of province with lesser amounts (15 to 25 mm) being recorded across a wide area in between these areas. Across the Peace Region, precipitation graded from 25 mm in the south to less than 10 mm in some areas in the central and northern parts of the region.
Peace Region: Precipitation accumulations across the region were greatest at the High Prairie AGDM station (28.6 mm) followed by the Valleyview AGDM station (25 mm) in the south, with the lowest amounts being recorded at the Hawk Hills AGCM (5 mm) station near Manning, and at the La Crete AGDM (5.6 mm) station in the far northern parts of the region. Elsewhere, precipitation generally ranged from 10 to 16 mm.
Northern Region: Precipitation accumulations across the region graded from highs of 43.1 mm at the Little Paddle Headwaters station near Barrhead in the west and 37.6 mm at the Cold Lake A station and 37.8 mm at the Lloydminster A station in the east, down 10 to 14 mm across the central and southern parts of the region.
Central Region: Precipitation accumulations across the region varied from a highs of 40.6 mm at the Calgary International Airport station followed by 35.20 mm at the Water Valley Station in the south west to lows of 10 to 13 mm in several isolated pockets in the south-central and north-central parts of the region. Precipitation accumulations across the rest of the region graded from above 25 mm in the western and eastern parts of the region to 15 to 20 mm across a vast area in the centre and south.
Southern Region: Precipitation accumulations across the region mainly graded from more than 35 mm at most of the stations along the foothills and in the eastern parts of the region, down to 10 to 15 mm in two pockets located in the north-central and southern parts of the region, with the exception of 7.7 mm recorded at the Vauxhall CDA station. Most of the central plains areas recorded 15 to 20 mm of precipitation. In the eastern parts of the region, the Medicine Lodge station recorded 61.7 mm followed by 42. 5 mm at the Medicine Hat A station. In the foothill areas, a few stations reported precipitation accumulations in excess of 70 mm.
Growing Season Precipitation Accumulations relative to Long Term Normal (1961 - 2005) (Figure 2) - Growing season precipitation accumulations relative to long term normal to date (April 1, 2008 to April 27, 2008), were at least near normal across most of the reporting area with the exception of below normal accumulations in large pockets located in the Southern and Peace Regions, as well as several small pockets scattered across the Northern and Central Regions.
Peace Region: Near normal growing season precipitation was received throughout most of the region, with the exception of an extended area in the centre and the north where moderately low accumulations have occurred.
Northern Region: With the exception of isolated pockets classified as having moderately low accumulations, most of the region received near normal growing season precipitation to date.
Central Region: Most of the region has received at least near normal growing season precipitation to date with the exception of some scattered pockets in the centre and southern part of the region where moderately low accumulations were recorded.
Southern Region: Most of the central parts of the region received below normal growing season precipitation accumulations to date, with parts of the southwest grading to low or very low. Across much of the east half of the region and the extreme northwest, accumulations to date are at least near normal.
180-Day Precipitation Accumulations relative to Long Term Normal (1961 - 2005) (Figure 3) - Long term precipitation (180 day) accumulations to date are generally well below normal across most the reporting area with large parts of the Northern, Central and Southern Regions classified as extremely low, a condition occurring less than once in 25 years. Average to above average spring precipitation will be needed in these areas in order to recharge soil moisture reserves for the coming growing season.
Peace Region: 180-day precipitation accumulations in the region varied from near normal conditions in the northern and central parts of the region to low with pockets of very low in the south-western parts of the region.
Northern Region: 180-day precipitation accumulations in the region graded from near normal and moderately low in the northern and central parts to very low and extremely low in the southern parts of the region.
Central Region: 180-day precipitation accumulations across most of the region varied from, extremely low to very low, the lowest in the entire reporting area, with the exception of the extreme southwest where a small pocket that grades to near normal can be found.
Southern Region: 180-day precipitation accumulations of extremely low to very low stretched across a north-south corridor in the central plains of the region, that grades to near normal and moderately high in the both the extreme southwest and also along eastern parts of the region.
Average Precipitation Accumulations for April (Figure 4) - The month of May generally marks the beginning of a significantly wetter period over most of the province, accounting for about 10 percent of average annual precipitation. In May, the southwestern part of the Southern Region typically receives more precipitation than any other area in the province, with average accumulations in the 50 to 80 mm range. In contrast, across most of the Peace Region and eastern portions of the Northern, Central and Southern Regions, average precipitation ranges between 40 to 50 mm, increasing westward to upwards of 70 mm along the foothills.
Soil Moisture
Soil moisture in the agricultural regions of Alberta ( Figure 5 and Figure 6) - Weather conditions in the past few weeks have delayed the regular spring soil moisture survey that is used to initialize the soil moisture model. Thus, the soil moisture maps and status discussions included in this report are based on results from continuous model runs that were initialized using information gathered in the spring of 2007.
Peace Region: Soil moisture levels generally varied from a range of 50 to 75 mm in the south-eastern parts of the region, down to 25 to 50 mm in the central and northern parts of the region. Modeled soil moisture reserves relative to long term normal varied from near normal in the extreme southeast to a mix of low and very low across most of the rest of the region.
Northern Region: Soil moisture levels across a broad corridor stretching north to south across the central portions the region ranged from 25 to 50 mm where conditions were the driest in the area and then graded upwards to 50 to 100 mm in the east and west. Modeled soil moisture reserves relative to long term normal graded from above near normal conditions along the eastern border of the region to a mix of extremely low to low in the centre and south east, with the lowest reserves (extremely low) found in a large pocket south of Edmonton.
Central Region: Soil moisture levels along the western border of the region are in the 75 to 100 mm range, while the rest of the region is in 25 to 50 mm range, with the exception of a relatively large pocket area with below 25 mm found in the east central portions of the region. Modeled soil moisture reserves relative to long term normal across most parts of the region varied from moderately low to low with some areas grading down to very low.
Southern Region: Soil moisture levels across most of the region are in the 25 to 50 mm range, with the exception of the southeast corner and the foothill areas of the region were moisture levels grade up to more than 75 mm. Relative to long term normal, soil moisture reserves are lowest in the central and west central portions of the region grading from low to extremely low. In contrast, along the eastern and extreme southern parts of the region, soil moisture reserves are near normal.
Data Sources
Near Real Time Weather data
Daily and hourly near-real-time raw weather data is brought in via daily data feeds from Alberta Environment (AENV) and Environment Canada (EC). The data undergoes a preliminary computer assisted QA/QC check performed by Alberta Agriculture and Rural Development (ARD) staff. Suspicious values are checked and verified and daily missing values are filled using archived data from AENV databases or from the EC web site. If daily data is still missing, it is estimated using data from nearby stations. Maps describing current conditions are based on preliminary data that is subject to change under further review by ARD, AENV and EC.
Historical Weather data
Historical weather data was provided by Environment Canada. This data was then converted to a 10 km daily gridded weather data set that used all available daily data to generate historical climate and soil moisture normals.
Explanation of Terms
Precipitation Accumulations-Frequency of Occurrence
Precipitation accumulations, expressed as a frequency of occurrence are computed for various periods and can be found on our web site at www.agric.gov.ab.ca\acis, under the Quick Viewer tab. Maps are routinely produced for the following periods:
- Past 365-days
- Past 180-days,
- Past 90-days
- Past 30-days,
- Growing season to date -Starting April 1st
- Cold Season to date- Starting October 1st
Selected maps from this series are included in this report.
Precipitation accumulations for each period are then determined by ranking the precipitation accumulations during similar periods 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:
| Description | Frequency of Occurrence |
| extremely low | drier than this, on average, less than once in 25-years |
| very low | drier than this, on average, less than once in 12-years |
| low | drier than this, on average, less than once in 6-years |
| moderately low | drier than this, on average, less than once in 3-years |
| near normal | on average, this occurs at least once in 3-years |
| moderately high | wetter than this, on average, less than once in 3-years |
| high | wetter than this, on average, less than once in 6-years |
| very high | wetter than this, on average, less than once in 12-years |
| extremely high | wetter than this, on average, less than once in 25-years |
This same scheme is then used for similar maps of soil moisture and snow pack accumulations so that comparisons can readily be made across the various map types.
Snow pack (reported during the winter season only)
Snow pack snow water equivalents (SWE) are modeled for stubble fields. SWE is defined as the equivalent depth of water (mm) that the snow pack contains if it were to be melted. SWE 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 percent 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 SWE.
Soil moisture
Soil moisture is measured as millimetres (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 reported on from May through to October.
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. Poor 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 modifier of low, indicate a need for more precipitation to restore reserves.
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 average soil moisture (spring or fall), computed using model runs dating back from 1961 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 since 1961 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 average. However, currently this process is unable to account for snow currently existing on the ground and as such is not as accurate where snow packs exist.
Report prepared by the Drought Reporting Team
Ralph Wright, Daniel Itenfisu and Isabel Simons-Everett
Alberta Agriculture, and Rural Development
Edmonton, AB T6R 5T6
Contact: Ralph Wright; ph 780-427-3556
This report was created on May 2, 2008.
Drought analysis is currently scheduled at monthly intervals between October 31 and April 31, and twice monthly from May 1 to September 3h. This report updates the previous report of March 24, 2008. |
|