The importance of sticking to your crop’s water budget

Keep this crucial concept in mind when choosing crops to grow

“Growing season rainfall varies a lot depending on where you are in the province and the crops that you’re growing,” says Provincial meteorology specialist, Timi Ojo.

A ‘perfect’ growing season would have just enough heat and moisture at the right times to make a bumper crop. The problem of course is that it rarely happens.

With moisture being so critical to both short- and long-season crops, if producers had a better idea of their crop water budget going into a new season, it might help make seeding decisions easier.

Provincial meteorology specialist, Timi Ojo, and his colleagues at Manitoba Agriculture, are working on producing current, real-time data for producers to help them make those decisions based on crop water supply, demand and precipitation contribution.

So let’s go back to that ‘perfect’ growing season. In his presentation at the recent Manitoba Agronomists Conference, Ojo confirmed that there’s no such thing, at least in Manitoba.

“Growing season rainfall varies a lot depending on where you are in the province and the crops that you’re growing,” he said.

Using data from Manitoba Agricultural Services Corporation, the No. 1 cause of crop loss in 2016 was excess moisture and in 2017 it was drought and heat. MASC’s aggregated data over the last 50 years or so shows a pretty even split between the two states.

“Oftentimes in any given year we are either at one end of the pendulum or the other,” said Ojo.

x photo: Manitoba Agriculture

So how do you figure out the crop water budget for the season ahead?

Just like any other budget, the crop water budget has income (inputs) and expenses (outputs). Affecting the input side is the available water in the root zone which changes depending on contributions from rainfall or irrigation. There is also capillary rise, which depends on the depth of the groundwater level.

On the output side the main source of water loss is evapotranspiration from soil or crops, deep percolation particularly in sandy soils, run-off and subsurface flow.

In calculating the water balance, (which is whether there is enough soil moisture in the spring, plus growing season precipitation, to meet crop water demand), it’s necessary to begin with the available water-holding capacity of the soil and how much water can realistically be expected.

“That’s our income,” says Ojo. “On the expense side is the crop water demand or how much do the crops actually use, so the difference between the two is the water balance.”

How much water is in the soil?

Every fall, Manitoba Agriculture publishes a fall moisture map on its website which shows available water content in the soil just before freeze-up for over 100 different locations across the province. To explain water-holding capacity, Ojo used the example of a sponge. “When you have saturated conditions, it means that the soil is holding too much water than it can handle,” he said. “If you soak a sponge in water, and you lift the sponge up, excess water is going to drain from the sponge, so that’s saturation.”

There’s a concept called field capacity, which is the maximum amount of water the soil can hold by itself without any draining occurring. “So when the excess moisture has drained away from the sponge, the sponge still holds some water, that’s field capacity,” said Ojo.

Then there is the permanent wilting point: the lowest amount of moisture in the soil that is not available to crops, which doesn’t mean there is no moisture in the soil.

“The soil still has some water,” said Ojo. “As an example, clay loam soil’s permanent wilting point is about 6.1 inches, so there’s still quite a bit of moisture in the soil but it’s not available to crops because the force at which the soil is holding on to the water is greater than the force at which the plant roots can actually extract the water from the soil.”

The available water-holding capacity is the moisture level between the field capacity and the permanent wilting point, expressed as a percentage.

How much precipitation will we get?

The climate normal precipitation across agro-Manitoba during May to September is about 12 to 14 inches, but it can vary widely at different locations, which makes it a bit of a moving target. For shorter-season crops the average precipitation would be closer to nine inches, because only rainfall received between May to July is likely to be useful to many crops.

To calculate the water balance, Ojo used the climate normal precipitation from May to July for shorter-season crops and 14 inches for longer-season crops like corn and soybeans and assumes a 25 per cent loss due to run-off or drainage.

“For short-season crops, you would have, on average, a little over 15 inches of moisture availability (from soil available and precipitation) during the growing season and for long-season crops you would have about 17 inches,” he said.

How much water will we lose?

Moisture is lost throughout the growing season either through evapotranspiration or crop use.

Evapotranspiration rates are entirely weather driven, based on energy radiation, wind speed and humidity. If producers don’t have a weather station on their land that can measure radiative energy, they can look at the difference between daily minimum and maximum temperatures. When the difference in degrees is higher there will be more intense radiative energy which causes more evapotranspiration. When the relative humidity is lower, more moisture will also be lost from the soil and crop surface, and higher wind speeds will also contribute to higher moisture loss.

Evapotranspiration rates are also affected by the crop coefficient factor, which determines how much water a growing crop is actively transpiring and varies for different crops and at different stages of plant growth water, and can also be affected by environmental stress like drought.

Taking all these factors into account, based on actual figures from Brunkild in 2018, the crop water demand for a wheat crop, for example, was 402 mm and the growing season available water was 373 mm, so there was a water budget deficit of 29 mm (about one inch).

Timing of rainfall critical

A huge factor in water budget is timing of precipitation, possibly more so than the actual total amount.

“I always prefer to look at the timing as well because it makes as much impact in the yield,” said Ojo. Precipitation might be normal for the year, but if only an inch of rain falls between the critical flowering period for many crops from mid-July to late August it can badly affect yield. That’s when the groundwater contribution is important, said Ojo.

“The last eight to nine years almost everywhere across the Prairies, we seem to have seen a spike in groundwater,” he said.

That might be making the case for producers to plant crops like early-maturing soybeans to compensate for lower-than-normal precipitation, but how long can they expect groundwater to compensate for below-normal precipitation?

“Groundwater is a two-edged sword,” said Ojo. “It’s helpful sometimes for it to be close to the surface but it also comes with a salinity issue when it’s really close to the surface.”

So, can Ojo or anyone else really predict what the next growing season’s precipitation will be with any level of certainty? “What I can tell you today is that next year is going to be above normal,” he said.

Online tools for producers to track crop water availability

As part of Manitoba Agriculture’s Weather Program, producers have a few online tools that can help them check current weather conditions and also soil temperature and moisture levels.

New in 2018 is the Current Weather Viewer (link). Producers can access this interactive, online tool from their phones, iPads or computers and get hourly updates on current weather conditions, including air temperature, relative humidity, precipitation, wind speed and direction and soil temperature at hundreds of locations across Manitoba.

Weather stations across the province also monitor soil moisture at various locations which is analyzed to produce crop water availability estimates for crops that are published as weekly maps on the website, together with other forecast maps such as corn heat units and growing degree days. Producers can also consult Manitoba Crop Weather Reports (link) for the actual numbers used to compile the maps.

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