Pulse Beat Individual Articles

Growing Impacts of Multi-Year Precipitation Deficits

Timi Ojo, Manitoba Agriculture and Resource Development – Fall/Winter (December) Pulse Beat 2021

The impact of water availability for optimal crop yield quality and quantity is well known. However, after almost a decade of being at the wet end of the pendulum, the growing impact of successive dry years appears to be a ‘new normal’ in Manitoba and much of the prairies. In a typical year, the average growing season precipitation does not meet crop water demand. The total average precipitation between May and August ranges from about 270– 310 mm. Apart from the total growing season precipitation, the timing of the precipitation is very important. For most cereals and dry beans, effective precipitation during May, June and July is important. Precipitation received in August is generally too late to make any difference in yield. For soybeans and other crops that require a longer growing season, August precipitation may provide a late boost to yield. However, crops will not recover all the yield potential lost from previous dry months.

Apart from growing season precipi­tation, the water holding capacity of a soil determines how much water it can store. Comparing crop-water demand vs. supply shows how the scale tips between deficit and excess moisture:

  • Deficit Moisture — spring soil moisture + growing season precipitation < crop water requirement
  • Excess Moisture — spring soil moisture + growing season precipitation > crop water requirement

On average, the growing season precip­itation (May–August) in Manitoba can supply about two-thirds of the 400–450 mm of moisture required for growing soybeans. This does not account for the precipitation that may be lost to either runoff during periods of high rainfall intensity or deep percolation below the root zone. The amount of soil water in the spring can be the difference between a good yield and a poor one.

The following five indicators were used to analyze the 2021 growing season:

  1. Previous Fall Soil Moisture Status — The amount of water in the soil at freeze-up provides a good indication of soil moisture status at the start of the next growing season. Limited change in soil moisture status is expected over winter due to frozen soil that limits water infiltration and minimal evapotranspiration, mostly due to sub-zero air temperatures, no vegetation and snow cover. The 2020 fall soil moisture map showed that most locations had about 150–225 mm of available moisture heading into the 2021 spring (Figure 2a). Many of these locations were closer to 150 mm. In contrast, the 2019 fall soil moisture map showed many locations had higher available soil moisture heading into the 2020 growing season because of the rain and snowstorm received during the 2019 Thanksgiving weekend (Figure 2b). At the time of writing this article (early October), air temperatures have been consistently more than 12°C above seasonal average and there is no widespread frost across agro-Manitoba yet, making 2021 one of the latest, first fall frosts recorded. These factors indicate that the 2021 fall moisture map would be drier than 2020, except there are several days of soaking rainfall before freeze-up.
  2. Growing Season Precipitation — The 2021 growing season started with dry topsoils due to a lack of overwinter snow accumulation (about 30–40% of historical average) and early snowmelt. Many areas received much-needed rain during the third week of May as concerns grew about dry seedbeds. Crops tried to hang on during the early part of the growing season with localized rainfall in some areas. However, as crops approached the active vegetative stage in late June into July, the precipitation deficit intensified. St. Adolphe received a total of 191 mm between May 15 and September 30. However, only 21.6 mm of rain fell between June 10 and August 8. Figure 3 shows the worsening crop water deficit over the last three growing seasons. In early August, the difference between the amount of precipitation received and the expected crop water use for dry beans was -250 mm, with an additional -100 mm for most other crops. With this much deficit, the soils would have been depleted of any available soil moisture.
  3. Record Heat — Limited precipitation and intense heat are two sides of the same drought coin. All locations ended the 2021 growing season with above-normal heat unit accumulation. Recent warm years typically resulted in locations receiving about 10% more growing degree-day accumulation than historical averages. Some areas such as Arborg, Moosehorn and Minnedosa were at least 20% above the normal growing degree heat accumulation at the end of the 2021 season. In Winnipeg, 2021 tied the previous record set in 1988 for the most days in a year (35 days) with above 30°C air temperature.
  4. Wind Speed — Higher wind speed increases the rate of water loss by promoting soil surface dryness and higher transpiration. Analysis of wind speed at 25 locations in the spring showed that both the maximum and average wind speeds in 2021 were greater than the 15-year average. On June 11, Clearwater, Somerset, Waskada, Snowflake, Killarney, Manitou and Windygates had wind gusts of at least 100 km/hr.
  5. Groundwater Level — Manitoba and much of the Canadian prairies were in a ‘wet cycle’ for much of the last decade until 2016 (Figure 4). The hydrograph at Oak Lake, west of Brandon, showed that the groundwater level is currently at the lowest it has been in 12 years, at about 2.1 meters below the ground level. The 2017 growing season was a dry year. However, crop yield was generally above average, largely because of the impact of groundwater level close to the root zone.

The 2021 growing season can be summar­ized as being very dry, hot and windy. Although it is uncertain when the dry cycle will end and early indication from current fall soil moisture status is not very promising, one can hope for a better 2022 growing season with days of soaking rains to get things back on the right track.