What drives irrigation decisions in a wet year?

By Brian Henriott
Hortau Grower Support

During the rainy season in California, irrigation season may be the furthest thing from a grower’s mind.

After attending seminars and agricultural tradeshows for the past few months, one concept that I have heard repeatedly now is “banking” rain water.

I think that this can be a very misunderstood and misleading concept. Rain events are no more than an irrigation that nature initiates rather than the farmer.

It is true, that during the winter respiration, transpiration, and evaporation all slow down, but they do not stop. Your crop’s need for water is not zero just because it is dormant.

In seasonal and evergreen crops, such as citrus, the demand may be less due to lower temperatures, reduced solar radiation, and increased humidity, but it is still there.

In orchards and vineyards that are dormant through the winter, water still moves through the profile and evaporates into the atmosphere.  Whether the plant is actively pulling water from the soil, it still must maintain moisture within itself in order to survive dormancy.

I think that “banking” should be thought more of as a checking account rather than a savings. Yes, nature is adding in to the account, but it is also making withdrawals at the same time.

The “Bank” is more like a piggy bank. There is a maximum quantity of water the soil profile can hold. After that maximum is met, additional water become surface runoff or it percolates down through the soil profile and adds to the aquifer.

So, we really need to determine the depth of your soil profile that can be affectively managed, and what the water holding capacity of that soil is.

I have heard many growers talk about roots going 4, 5, 6 feet deep, and it is true that roots can be found at that depth, but we are talking about a very small percentage of the active root zone.

As a grower, your resources should be applied to the area where they have the greatest impact. Typically, the soil has the most water holding capacity in the top 2- 3 feet of the profile. That is also the portion of the profile that contains a vast majority of the active roots. So, it is that top 2-3 feet that growers should manage and focus resources on.

Depending on soil type, the top 3 feet of soil can hold at most 7-8 inches of water.

In other words, the maximum amount of water it could ever hold is only 7-8 inches.

That said, that number is not the precise measurement. Soils that hold that greatest volume of water (heavy, clay-based soils), also hold water extremely tightly, making much of it unavailable to the crop.

Plants in heavy clay soil begin to experience stress before even 50% of the water holding capacity is used up. That leaves less than 3-4 inches of available water in the top 3 feet of the profile. What does this mean? It means that the soil “bank” for water isn’t much of a bank at all.

Now that we have established that growers should not rely on a “big rain year” to provide crops with water well into the spring, we are back to the question of when to start irrigation?

Or in dry winters, should you even stop irrigating? I have heard of growers in some deciduous crops say they don’t start irrigation until after a specified date, regardless of the last rainfall.

If the wide array of variables such as rain, wind, temperature, and humidity were consistent year after year, it might be acceptable to start after a certain date, but we all know that that is not the case. So, calendar-based irrigations aren’t accurate.

What about Et? Calculating evapotranspiration is a good way to get an estimate for what your crop is using. But even when accurately measuring on-site variables, in the end, it is still an estimate, not a direct measurement.

Furthermore, Et does not take into account the state of your soil’s bank of water.  Et, may assume the crop is pulling water when there is no longer water available for the plant.  Alternatively, Et could recommend applying 1.5” of water when the soil only has the capacity to hold 0.75”.

Soil tension is a direct measurement that’s not affected by soil type, soil EC, or the wide array of environmental variables our crops face. Tensiometers measure soil tension – the same force that plant roots contend with to draw water from the soil.

Hortau’s digital tensiometers measure tension in real time at three different depths.

This gives growers insight into their soil profile like never before, and because tension is being tracked in real time, tension is shown as a trend line at each of the three depths.

Using a web-based application, growers can quickly see if the tension is rising or falling allowing growers to better gauge the timing and duration of irrigations.

In the end, it shouldn’t matter if it’s a dry or wet year.

When it comes to irrigating our crops, if there’s no available soil moisture, the plant is stressed.

Crop water stress equates to increased pest and disease pressure, and decreased yields.

If a grower has tensiometers in the field, knowing when to start irrigating is a much easier decision.

About the Author

Brian Henriott is Hortau’s grower support specialist for Mid-Northern Central Valley. He is a native Californian originally from the Bay Area, and attended Cal Poly San Luis Obispo, majoring in horticultural production as an undergrad and then in plant protection as a grad student. At Cal Poly, Brian developed a passion  for entomology, eventually transitioning that interest into a two-year lecturer position at Cal Poly. After college, Brian worked for four years on the Central Coast as a harvest manager in the vegetable industry before joining Hortau full-time to help growers integrate soil tension management into their operation.