Luke Milliron, UCCE Orchards Advisor Butte, Glenn, and Tehama
Curt Pierce, UCCE Irrigation Advisor Glenn, Colusa, Tehama, and Shasta
Bruce Lampinen, UCCE Walnut Specialist, UC Davis
Allan Fulton, UCCE Irrigation and Water Resources Advisor Emeritus

All walnut growers want a healthy orchard. Can this still be achieved when crop prices are at rock bottom, and expenses are at an all-time high? A careful approach to irrigation in 2023 is one way to both save money and help ensure you have a healthy and productive orchard.

One of our favorite anecdotes comes from an Israeli irrigation researcher who once noted at a conference: “Growers don’t irrigate when the tree is stressed; they irrigate when they are stressed.” We saw the effects of this during the 2022 drought season when multiple walnut orchards we visited showed poor growth and yellowing leaves, a symptom of prolonged over-watering (Figure 1).

Currently, the pressure chamber is the only way to consistently irrigate a walnut orchard at an appropriate time for the trees, instead of when we (or the soil, the weather, and our neighbor) are stressed. Butte County growers who have recently adopted the pressure chamber have noted that they saved on PG&E and have healthier looking orchards by not over-irrigating during heat spells, all without reducing yield.

The UC’s best practice to save money and help ensure a healthy orchard and good nut quality is to use the pressure chamber and irrigate when trees are between 2 and 3 bars drier than the fully watered baseline season-long. If you start the 2023 season by waiting for the trees to reach this threshold (i.e., the trees are letting you know they are mildly water stressed) and apply subsequent irrigations at this threshold, you will save on water, PG&E bills, and make your trees LESS water-stressed come fall (likely because you didn’t suppress root growth in the spring by overwatering).

Barriers to adopting this practice:

Historical thinking. This new research has challenged the conventional wisdom that we must irrigate to keep up with ET to have healthy and high-yielding walnut orchards (Figure 2), as well as the now-outdated advice that it is best to maintain a “connection” between soil water layers at different depths in the soil profile.

Peer-pressure. You may feel more compelled to water if you observe more “stressed” neighbors start irrigating much earlier than the trees need. We have found that in past wet years, it is unpredictable as to when you will need to start irrigating. Although there will likely be adequate soil moisture, roots can be damaged from excessively wet conditions, and the trees may need water sooner than expected. Having pressure chamber data in a year like this is more important than ever. Even in the past two drought years, trees we monitored in the Sacramento Valley didn’t reach 2-3 bars drier than baseline until mid-May.

You don’t own a pressure chamber. These devices cost north of $1,500, which can be a non-starter when folks are already operating in the red. Thankfully, many farm advisors have pressure chambers they can use during a visit to your orchard. You may also have a neighbor with a chamber, or you can look into getting a used pressure chamber through a second-hand source, local or online. You may be able to add pressure chamber monitoring to your PCA’s services for about $40/acre. Finally, in 2023 as an additional resource to growers without a pressure chamber, we will be posting on sacvalleyorchards.com and in the emailed ET reports, the progress of select Sacramento Valley walnut orchards with pressure chamber monitoring as they dry down to the 2-3 bar irrigation threshold.

The economics in actual orchards:

Example #1: A 100-acre walnut orchard irrigated with Nelson R10 mini-sprinklers. The orchard has been divided into three 33 +/- acre irrigation sets. The orchard is close to the Sacramento River so static groundwater level was 20 feet and pumping draw down was 30 feet. In this example once the pressure chamber was adopted, season-long irrigations declined from 26 to 13. This represented a 50 percent reduction in electricity demand and a total savings of $8,895 or $89 per acre.

Example #2: 200 acres of walnuts are irrigated with Nelson R10 mini sprinklers. The orchard is divided into four 50-acre irrigation sets. The orchard is located on westside Sacramento Valley terrace soils further from river or tributary influences. Static water table was 170 feet and pumping drawdown was about 230 feet. In this example once the pressure chamber was adopted, season-long irrigations declined from 29 to 23. This translated to a total cost savings for electricity of $13,320 or about $67 per acre.

For more details on the examples above, please follow this link.

These specific, concrete examples show how it is possible to save substantial electricity costs while sustaining healthy, productive orchards. In summary, it is realistic for irrigation scheduling with a pressure chamber to reduce power bills on the order of $50 to $100 per acre in the first season. If used across 50 or more acres, it is quite possible to pay for a pressure chamber in the first season and achieve substantial power cost savings from that season forward. Costs for subsequent years of using the pressure chamber to schedule irrigations should be limited to labor to acquire and evaluate the field data and maintenance of the pressure chamber which experience suggests should be on the order of $12 to $20 per acre depending on how intensely it is utilized. When using the pressure chamber, it is important to use it correctly and not exceed the management thresholds which could otherwise impact yield and quality unfavorably.

Bottom Line: To save money while helping ensure you have a healthy, and productive orchard this year – simply wait to irrigate until your trees are mildly stressed.