Those are the evapotranspiration numbers. That is the amount of water lost to the atmosphere daily. The soil type doesn't impact those numbers. What the soil type does effect is how much water the soil can hold in the root zone. So a plant that transpires 20 gallons of water a day will dry out wet sand faster than wet clay because wet clay holds more water. However the tree is still only pulling 20 gallons of water from the soil each day regardless. If you want to make sure your plant has enough water but not too much, you just replace the water lost each day with irrigation water. Soil type will effect how you water. Deep loamey soil can take deep watering infrequently whereas shallow sandy soil will need to be watered more frequently. Percolation rate matters and if you water sand deeply, the water moves out of the root zone before it can be used and you've just wasted a bunch of water.

but yeah, Avocados need a lot of water.

When we lived in Watsonville our property came with a mature avocado. We lived there 30 years and it was never watered. Here in Calistoga we also inherited a mature avocado in our dry farmed orchard. Both trees were healthy without anything but winter rainfall, about a 30 inch annual average. Soil in both locations was/is a clay/loam. Al

Our native soil isn't loamy, it's clay.

The problem with the avocado tree is that the landscape contractor jumped the gun and planted it before I could discuss with him what I wanted done with the soil. This was back in 2005. The guy who did the planting was fired by the contractor for an unrelated screwup, so I never got to ask him what was done. There was agricultural sand, compost, and loamy backfill dirt on site at the time. Maybe they made a mixture, maybe they used native soil + compost only. I'm not sure except that the top 1-2" looks like native soil.

The author of the article linked above says this:

Rather, the
frequency of irrigation is determined
by feel of the soil (when a ball of soil
starts to crumble in the hand) or by a
tensiometer reading (usually 20cb to
25cb from a tensiometer set at 8âÂÂ
below to the soil surface in the wetted
pattern of the sprinkler). For
instance, if the tensiometer indicates
that the soil reaches critical dryness in
five days, and the chart indicates that
trees are using about 30 gallons of
water per tree per day, the grower
would apply 5 x 30 gallons = 150
gallons per tree during the irrigation
event. Recent research findings have
indicated that the water requirement
for maximum production of avocados
in a grove is probably about 30-50%
higher than that presented in this
chart. However, in home garden
situations, avocados often are able to
steal water from neighboring plants.

I'm not clear if the above recommendations were written with loamy soil and San Diego County growers in mind, or if it applies universally?

We've had poor yields in years past because of insufficient irrigation. On the other hand, I don't want to water too heavily and cause root rot.

Additionally, if it's even possible to use drip or low-flow irrigation for the Avocado tree, the amount of water needed is going to have a major effect on the system design.

That's why I'm stuck at the moment...

Our Avocados are on drip, and get nowhere near 200 gallons a month, let alone a week. We only get maybe 100-150 avocados a year per tree, but that is plenty for our needs. The trees are healthy and fully foliaged.

You are looking at documents for commercial production? Do you want commercial production levels? You allow the fallen foliage to form a thick mulch over the root system?

What soil type do you have, and how big are the avocados when you harvest them?

The table and information mentioned above come from a document entitled: Avocado Production in Home Gardens.

We get rid of the old leaves because they frequently show signs of disease or mold (spots, usually) and use pine bark and compost as mulch.

What soil type do you have, and how big are the avocados when you harvest them?

Silty loam, and huge.

I water mine quite a bit, but the leaves are the best mulch under the trees, because they form a thick puffy layer that keeps the ground cool. I would err on the side of less water with clay.

you can use drip, but personally I find it to be a PITA. I only use drip in awkwardly shaped beds that are impossible to fit with rotors and only with netafim 1/2" tube not that stupid spaghetti tube system nonsense (which is only good for patio containers). If at all possible I use MP Rotators. They are low precipitation rate finger spray rotors that were designed to avoid runoff in hills and not be too affected by wind.

Let me elaborate on why I asked about the amount of water.

Our landscape architect was not the sharpest knife in the drawer. He put all of the backyard planters on the same automatic valve without inquiring what types of fruit trees we intended to plant.

Most of the shrubs and trees in the same zone are low-water types, so without extensive plumbing work, the avocado tree is going to require dozens of drippers, which, like you said, would be a major PITA.

Instead of drip, if I use low-flow heads (10-15 GPH) to water the avocado tree, the wide difference in flow rates may cause the drippers on the other shrubs and trees to quit working altogether. Remember, it's all on the same automatic valve.

I may be forced to combine the drippers on this valve with those on another, freeing up a valve for the exclusive watering of the avocado tree. I dread having to go this route because there are shrubs and a tree already established where I would need to do underground plumbing work.