Tracing Irrigation Water with Blue Dye and a Shovel

The Vegetable Open House at the Simcoe Research Station provided us with an opportunity to use blue dye to see where the irrigation water goes. A drip irrigation system under plastic was set up in a pepper crop. The drip tape had 0.23 U.S. gallons per hour emitters spaced at 12". We used a Dositron™ fertigation injector to add Blazon™ dye to the irrigation water. We then ran the system for 1, 2, 3, 4, and 6 hours to show the effect of various irrigation times on water distribution in the soil. The results are pictured on the right.

What you can't tell from the pictures is that the top 6" of soil was fairly loose, corresponding to tillage depth. This also happened to be where the majority of the pepper roots were. Below this layer, the soil was more dense. You can see this in the 1 and 2 hour photographs on the right, where the looser layer of soil is dry. The dryness and the pattern of blue dye indicate that 2 hours of irrigation does not wet the entire soil volume of the rooting zone.

At 3 hours, the soil volume of the upper layer was entirely wetted and the blue dye from the emitters had spread laterally and almost met. It appears that the actual wetting front was a bit ahead of the blue dye, indicating a small amount of binding of the dye to the soil. The dye is mostly held in the upper layer, with just a few fingers breaking into the lower layer. This is a nice demonstration of water following the path of least resistance. It doesn't move into the more dense layer until the top layer is full, and the water has nowhere else to go.

At 4 hours and beyond, a significant amount of water was leaching into the lower layer, below the root zone and out of reach of the crop. Three hours appears about optimum in this case.

Before we start making too many generalizations, let's think about some of the factors affecting optimum irrigation system run-time. There is irrigation frequency, emitter volume and spacing, crop stage, crop type, crop rooting depth, and soil factors. Of course there is the weather which is less important if the crop is grown under plastic as was the case at this site.

How then, do we sort through all these factors and figure out how long to run the system? One of the sensor-based technologies would work well here. A sensor placed in the root zone close to an emitter would tell you when to turn the system on because it's getting dry. Another sensor placed just below the tilled layer would tell you when to turn the water off because it's leaving the rooting zone. If you prefer a low-tech, low-cost, show-me approach, a regular soil probe can be used. After the irrigation system starts, every hour you can pull a soil core from the area beneath a few emitters. Tap the soil out of the probe onto a piece of paper or plastic and find the depth of the wetting front. You should be able to find it either by the colour change or by feel. If you do this through a few irrigation cycles, you'll soon get a good idea of how long you should run the system to get the water down to the right depth for your particular situation.

In this pepper field, normal irrigation time was 4 hours. A 25% savings in water, energy, and fertigation nitrogen costs could be seen by using one of these monitoring techniques. On the other hand, under-irrigating could be costing you in reduced yield and quality, for instance from calcium deficiency disorders. It might be worth the time to pencil out a few of these scenarios and get an idea of weather you can afford not to have some sort of monitoring method in place.

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