Regulated deficit irrigation: smarter watering with less water

Use less water and still get the same harvest, or even better fruit: that is the promise of regulated deficit irrigation (RDI). It works, but only if you know exactly when and how much. Irrigating on gut feeling misses exactly that insight.

Agriculture accounts for more than two thirds of all fresh water used worldwide. In Belgium and the Netherlands too, dry summers and irrigation bans are no longer the exception, especially in fruit growing. So the question is no longer whether to save water, but how to do it without harming your crop.

In 2022, researchers from Northwest A&F University and Shanghai Jiao Tong University brought forty years of RDI research together in a single review (Yang et al., 2022). In this article, we summarize their key findings and look at what they mean for anyone who wants to irrigate smarter today.

What is regulated deficit irrigation?

With RDI, you deliberately give less water than the plant needs for maximum growth, either for the whole season or during a chosen growth stage. The amount is usually expressed as a percentage of crop evapotranspiration (ETc). As a rule of thumb, RDI supplies no more than 70% of ETc over the season. How much per growth stage is still an open question.

The authors distinguish three forms:

Stage-based deficit irrigation. The oldest and simplest form: you cut back on water during growth stages when the plant hardly suffers from it, and water fully during the sensitive stages.
Partial root-zone drying (PRD). You irrigate only half of the root zone, either always the same side or alternating left and right. The dry roots send a stress signal to the leaves, the stomata partly close and the plant transpires less. Meanwhile, the wet half supplies enough water to keep growing.
Subsurface irrigation. The drip line is buried in the soil, right at the roots. Almost no water is lost, but installation costs more.

How does deficit irrigation work inside the plant?

Roots and shoots compete for the same resources. Under a mild water deficit, the plant produces the hormone abscisic acid (ABA). It closes the stomata, slows leaf growth and sends sugars to the roots and fruit. The plant invests less in foliage and more in reproduction.

Fruit is also less sensitive to water deficit than leaves and shoots. When the plant gets normal water again after the stress period, a compensation effect often follows: the stored reserves go to the fruit, which catches up. Stress that is too severe or lasts too long has the opposite effect: cell walls lose their elasticity and yield drops.

Results of deficit irrigation by crop

The review identifies four benefits: more efficient use of water and nutrients, better quality, maintained or even higher yield, and plants that cope better with later drought. A selection of the studies the authors discuss:

Crop	Approach	Result
Citrus	RDI in two cultivars	50–55% water savings without yield loss; fewer split and oversized fruits
Apple	55–70% water from bloom to fruit set	Highest water use efficiency, positive effect on fruit quality
Pear	PRD compared with conventional RDI	Both methods equivalent: the amount of water determines the effect, not the technique
Peach	RDI	Less pruning wood, same yield, more bioactive compounds in the fruit
Wine grape	50–70% ETc from veraison onward	More anthocyanins and polyphenols, no yield loss
Cauliflower	Moderate to severe RDI in the juvenile stage	Up to 24% less water with the same head yield and the same gross income
Onion	75% of water requirement during bulb maturation	4% lower yield, 9% higher irrigation efficiency

For fruit, it is mostly about quality: sugar content, color, firmness and fruit size determine the price more than raw kilos. For vegetables, researchers also look at storage life. Severe RDI, for example, extended the shelf life of cauliflower.

Pitfalls: when deficit irrigation costs you the harvest

RDI does not simply mean "giving a bit less water". The same savings can improve quality in one stage and cost you the harvest in another. The review gives clear examples:

Timing. A grapevine that runs short during berry growth yields less. Bell pepper lost 13 to 20% of its yield with a deficit mid-season, while a deficit late in the season actually gave the highest productivity.
Dose. In bell pepper, a 50% deficit hit harder than a 25% deficit. In blueberry, a moderate deficit gave the same harvest and quality, but a severe deficit caused oxidative damage.
Duration. Short, mild stress trains the plant. Prolonged, severe stress breaks it down.
Context. The authors note that there are still no concrete guidelines per crop, soil type and climate.

That last point may be the most important. RDI requires a narrow margin: enough stress to steer the plant, but not so much that it suffers damage. A fixed calendar or an ETc percentage does not take into account your plot, your soil or next week's heat wave.

Measuring water stress to irrigate smarter

The review is clear: RDI relies on precise water control, with drip irrigation and measuring equipment. The authors name leaf water potential as one of the best measures of water deficit in the plant. At the same time, they point to two barriers to wider adoption: the cost of that equipment and the lack of simple, practical procedures.

That is exactly where sensor technology has taken a big step in recent years. Leaf water potential is measured with a pressure chamber, by hand and before sunrise. That works for a trial, not for day-to-day management of an orchard. If you want to apply RDI in practice, you are better off combining a few measurements that deliver data continuously and remotely:

Soil moisture at multiple depths. Shows how much water is available in the root zone, and whether an irrigation round actually reaches the roots. With PRD, it also shows whether the dry and wet zones really differ. See our multi-depth soil moisture sensor, and learn how to check that your data is right in Is your soil moisture sensor still measuring correctly?
Trunk diameter with a dendrometer. A tree shrinks during the day and swells again at night. If that recovery fails to happen, the tree is under stress, often before you see anything in the leaves. This way you measure the water stress of the plant itself, which is exactly what RDI needs. Read more in What is a dendrometer? or discover the precision dendrometer for fruit growers.
Evaporative demand of the air (VPD). Determines how hard the plant has to work to transpire water. When VPD is high, a planned stress period gets out of hand faster. See What is vapor pressure deficit (VPD)?

The value lies in the combination. In the PlantData.Live portal, soil moisture, trunk diameter and weather data sit next to your irrigation log. So you see not only how much water you gave, but also how the soil and the tree responded. Alerts warn you when stress crosses the agreed threshold, so you can adjust before damage occurs. That turns RDI from a calendar-based guess into a strategy you can track and refine per plot.

Deficit irrigation in Belgium and the Netherlands: worthwhile even without water scarcity

RDI was developed for dry regions. Still, the authors point out that growers with enough water can benefit too, because it can improve the quality of the harvest. What often holds it back is habit: a lush green tree feels safe, even if part of that growth comes at the expense of the fruit.

The key points:

Mild, well-timed stress, especially early in the season, saves water and can improve quality.
The right stage and dose differ per crop, plot and year.
By continuously measuring how soil and plant respond, you can apply RDI safely and adjust before damage occurs.

Want to know what this could look like for your crop? Contact us for tailored advice.

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Source

Yang, B., Fu, P., Lu, J., Ma, F., Sun, X. & Fang, Y. (2022). Regulated deficit irrigation: an effective way to solve the shortage of agricultural water for horticulture. Stress Biology. https://doi.org/10.1007/s44154-022-00050-5

The article is published open access under a Creative Commons Attribution 4.0 license. The figures in this article come from the studies the authors discuss in their review.