Irrigation - How Much?

So, you made the big move to drip irrigation. Smart move! Now it's time to schedule your irrigation. It might look like a big deal, but don't worry, it's easy. Actually, irrigation is all about balancing the water availability in the soil. Soil has a capacity to hold water, and plants consume that water. Our job is to give the soil back the water that the plants have consumed. This way your plant never dries out and won't suffer from stresses.

OK, so now that that's cleared up we need to apply the amount of water the crop is consuming in order to stay balanced. You may wonder, but how do we calculate a fields water consumption? The water consumption of a field is called evapotranspiration, or ET for short.

ET is the sum of water that the plants consume by transpiration along with evaporation from the soil. ET is calculated based on meteorological data considering temperature, humidity, wind speed and radiation. However, wouldn’t every plant transpire a different amount of water? Yes indeed!  The water consumption of a specific prop in a specific field is called ETc. To find the, ETc we follow a simple equation. Don't worry, it's really easy...

Let's take it step by step.

The measured value of ET is called ETo. This represents the evapotranspiration from a standardized vegetated surface of grass. Where do you get the ETo values from? Either from a weather station located on your farm or from the NETAFIM ETo application. So how do you move from a reference grass to your crop in its specific growth stage? Each crop in every growth stage has a factor called Kc. The crop factor represents the properties of the crop. So, we simply multiply the ETo by the crop factor. How do you find out a crops Kc? Simple. Just do a quick Internet search, consult the crop literature or ask an irrigation specialist. In order to work out the actual water consumption of your crop in a specific field, at a specific growth stage - that is, to get the, ETc, we simply multiply the ETo by the crop factor.

Here's an example. Let's say you have a tomato field. According to the local weather station, yesterday's ETo was 6.5mm. The tomato plants in the field is in the vegetative growth stage. And according to the Kc values, your Kc is 0.64. Now if we do the math… an ETo of 6.5 millimeters multiplied by a factor of 0.64 = 4.16 millimeters, and that's your tomato fields actual water consumption for yesterday. This means your field needs 4.16 millimeters or 416 cubic meters per hectare for its optimal water balance.

ETo: Reference Evapotranspiration  x  Kc: Crop Factor  =  ETc: Crop evapotranspiration

ETo        x             Kc           =             ETc

6.5mm x             0.64       =             4.16mm

 ETc is a dynamic value because the ETo changes daily according to the weather and the Kc changes periodically according to the crop growth stage.

Now, what happens if you're not irrigating daily? In this case, you need to sum the ETc of the days since your last irrigation event. The sum is the amount you'll need to irrigate.

Now that you know more about precision irrigation...It's time to irrigate!

About the Author

Christoff Boltman is the Seeds for Africa / H2GRO Hydroponic specialist. Christoff has been growing hydroponically for the last 15 years and welcomes the opportunity to share his experience and knowledge with our customers. Feel free to pop in at our showroom and Christoff will be delighted to show you around and advise you on what would best suit your needs.