Chestnut Orchard Irrigation Design

Chestnut Orchard Irrigation Design

The two most important things that we need to do in the orchard before we plant the trees is prep the ground and install the irrigation system. Both of these are critical for the establishment and long term health of the trees. Both are very difficult to redo once the chestnuts are planted. This post will share the rough design of the irrigation system and the math behind the design.

 

Water Source

We're very fortunate that the previous owners of our property drilled a couple of wells and constructed a large reservoir. Some of the public records we found suggest that one of the wells may be ~40 years old. Drilling even a small well can cost $25,000 or more in our area so we're grateful to have two working wells already in place! These wells will pump water into the reservoir as needed. We'll use a booster pump to move water from the reservoir into the orchard. 

 

Determining Crop Need & Application

Designing an irrigation system is very much an exercise of beginning with the end in mind. It all starts with how much water we expect the trees to need at maturity. There is tons of information about how much water other permanent crops in our area need (almonds, pistachios, walnuts, grapes, etc.), but almost nothing for chestnuts. Our expectation is that chestnuts will have water requirements most similar to walnuts, so that's going to be our starting point.

UC ANR studies suggest that in the Sacramento Valley, mature walnuts need a total of about 3.5 Acre-Feet of water per year with about half of that in June, July, and August. We need to design an irrigation system that can deliver 8 Acre-Inches of water in each of those three months. Of course, the trees will need far less water than that in the first 5-8 years. In fact, they may never need that much since chestnuts aren't walnuts and our soils may store water better than the UC average. However, we'll design the system that can deliver that much water if needed.

"Where do those monthly numbers come from?" you ask. Great question!

Walnut ETc is the area's base evapotranspiration, ETo adjusted by Kc for walnuts, which is their crop coefficient. The table below is also from UC ANR and shows the per tree ETc through the growing season in half-month increments. This shows that in the heat of the summer, the trees need 4" to 4.5" of water every 15 days. Aka 8"  to 9" per month.

  

Most of our orchard will be planted on 20 ft x 20 ft spacing. That means we'll have 109 trees per acre that may need up to 4.5" every two weeks in the summer. Now some math....

4.5" of water every two weeks is 0.32" of water per day (4.5" / 14 days = 0.32")

0.32" of water per day is 0.2 gallons per day (0.32" x 0.62 gallons per inch of water = 0.2)

0.2 gallons of water per day across 20 x 20 square feet is 80 gallons per day per tree (0.2 gallons x 400 sqft = 80)

 

We're going to use drip irrigation due to it's low maintenance, high efficiency, and distribution uniformity, so we need to have enough emitters with high enough flow  rates to deliver that peak 80 gallons per tree per day. We've selected drip tube with a pressure compensating 1/2 Gallon Per Hour (GPH) emitter placed every 24". A single line of this tube will give each tree 10 emitters, 5 on either side of the tree, extending 10 feet in each direction down the tree row away from the trunk. Those 10 emitters will deliver 5 GPH to each tree. We'd have to irrigate for 16 hours a day to deliver the 80 gallons the trees may one day need (80 gallons total need / 5 GPH = 16 hours).

16 hours is a longer irrigation set than we'd like to tend, so we can cut that time in half by doubling the drip lines. Placing a second drip line doubles the system's output per hour. Another options is to increase the emitter flow rate to 1 GPH or faster, but we are concerned with over saturating our loam soils. We'll place the two 1/2 GPH drip lines 3-4 feet apart (on either side of the tree row) so that the water is applied in a larger area, reducing the risk of erosion or water logging and encouraging the trees' lateral root growth.

 

Sizing the Pump & Mainlines

We now know we'll have 2 lines of drip tube with 1/2 GPH emitters every 24" on every row. The next step is determining how big of a pump and mainlines we'll need to deliver the water into all these drip lines.

Since we have an irrigation reservoir from which we can pump at very high Gallons Per Minute (GPM) with a relatively low-cost pump, we're going to plan on irrigating the orchard in a single set. If we didn't have the reservoir, we'd break the orchard into several sets so that we could irrigate directly from our low-GPM wells.

Below is a table of the math that we used to determine how many GPM we need to irrigate the whole orchard at once.

 

 

Only about 2/3rds of our rows are a full 700 feet long, so we won't need all the 292 GPM this math suggests. However, we'll get a pump that can deliver 290+ GPM to be on the safe side as there is always loss in the system.

We also need to make sure that the mainline and submain lines that deliver water to the drip tube are not limiting the flow. We'll use 3" schedule 40 PVC for the mainline from the pump to the middle of the field. 3" PVC can move >500 GPM even at low pressures, like 30 PSI. In the middle of the field, we'll have 2" schedule 40 PVC extending east and west off the 3" mainline with a tee into the drip on each tree row. These 2" submains can move ~200 GPM at low pressures, which is fine since each submain will only be serving half the orchard.

That's a lot of words and numbers. Here's a picture that'll make more sense.

Back to blog

2 comments

The short answer is that they require more maintenance than drip. They would increase the wetting pattern, help incorporate dry spread fertilizers or compost, and better support cover crops. But, they spaghetti hoses get caught in mowers or eaten by animals. The sprinklers get knocked over and soak a small spot. Etc. The dual line drip will make a wet spot every 24" in lines that are 4-6 feet apart. Our bet is that that is a sufficient wetting pattern for the trees and we’ll rely on winter rains to soak in compost & support cover crops.

Sawyer

Why not go with Micro sprinklers to wet a larger area?

bill

Leave a comment

    1 out of ...