By Rebecca Shortt,  Engineering Specialist, Water Quantity

It has been hot and dry in SW Ontario in June.

What is the impact on plants? And how do we measure the water demand?

One thing I think we forget is that those days of high humidity are not as hard on the plants as days with low humidity which are very desiccating (however the humidity is hard on us humans!). uses Environment Canada weather data at a selection of Ontario weather stations to calculate the daily Evapotranspiration (ET).  This is a measure of how much water a mature grass cover crop would use if it had ample water available.

ET is calculated from temperature, humidity, windspeed and sometimes a measurement of solar radiation.

ET data can be used to calculate how much water we should be delivering to the crop with each irrigation.  Even a quick look at the graphs on can help us understand if the water demand has been typical or above (or below) average.

For the Delhi station, ET has been an average of 4.6mm/day in June and early July.  This is a little higher than typical.  See the graph below.  

Evapotranspiration (mm) graph taken from for the month of June 2022 in Delhi ON

To get your station data to look like this graph (above), click “Not cumulative in graphical display”.  See below:

Evapotranspiration calculator selection for not cumulative in graphical display

Rainfall must be measured at each field but you can use ET from a nearby weather station.  In SW Ontario, a weather station with ET data within 50km of your field site would be ideal.

Delhi has received very little rainfall and we currently have a moisture deficit of 118mm for the month of June.

Precipitation (mm) graph for the month of June 2022 for Delhi ON

With no rainfall relief in sight, we can use ET data to ensure the irrigation we are applying is matched to what the crop needs.

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