Irrigation water amount for a market garden

Hi,
some friends who are opening a retreat center would like to offer their guests their own produced food during retreats, meaning they may have 100+ guests each time, various times a year during several days.

How can I figure out how much water they may need to grow veggies, especially in the hottest time of the year in order to understand if growing them is even viable?

The garden space itself is in Mediterranean climate, with some small rainfall even in July and August.
So any water storage should be able to store the amount of water needed to compensate for the lack of rainfall (they don't have any consistent water source, like a creek or a good well) in those drier months
I know it is not a straightforward task and can depend on many factors, such as:

soil type/texture

evapotranspiration rates,

temperatures

amount of rainfall

gardening technique used

mulching (or not)

amount of surface to be cultivated

and maybe other factors as well.

But given a specific type of soil - in order to simplify the task I may assume that they have very good soil with good amounts of organic matter, 3-4%  for example - a specific ETo (to be determined)
a gardening technique (I may assume for example that they will use the biointensive method) and they mulch a lot, so whatever water there is won't evaporate immediately
I know the amount of land that they will want to put into production and that they will not produce monocrops.

How can I figure out a sort of water consumption benchmark for the garden, so that if they don't even meet this optimal (minimal) figure already (because they are already storing some water) they will have to start considering how to catch and store more water and what they may need to do to become viable in terms of water  (or decide it's not worth the effort).

In my fantasy I have hoped that there could be tables out there, for different climates, synthesizing various situations so that one can have an initial idea on what to expect, but couldn't find any so far.

Most vegetables want a total of at least one inch of rain per week.  Surface water is typically measured in acre-feet, which is 325,851 gallons.  An acre-inch would be about 27,154 gallons.  So to irrigate an acre of vegetables you might need 27,154 gallons of water per week, possibly less, possibly more, depending on the soil, temperature, and how much it rains.

If you were able to shift most of your guest influx to the 'wet season' that would help.
You can grow most of your oil, olives and nuts, without worrying about water during the dry season.
Alot of fruits ripen in the dry season and taste better without the extra moisture so again less worries. They can also be fermented/water kefir/alcohol/vinegar, dehydrated/soaked in alcohol.
Honey can also be made in the wet season and used in the dry season, you can even cheat and feed the bees fruit juice, as a way to preserve the fruits and the pulp can be dehydrated.
Growing your own grain sounds like a bit much, but that too can be grown in the wet season.

Aquaponic systems are said to use 90% less water, so you could take that route, water tank=fish pond, same pump+irrigation pipes, a bit more electricity because the water will be recirculating every hour vs just at night. Grow pipes can be placed in raised beds, and hidden for a more natural look and ease of harvest.  https://docs.google.com/presentation/d/1DaQgq7raA2srDWJirBhkymfejWRtEhYHVuzFGAjJSGo/embed?start=false&loop=false&delayms=5000&slide=id.g68452b3ff_0_37

Wicking beds are one of the most water-efficient methods of growing vegetables, and less expensive than aquaponics.

https://www.youtube.com/watch?v=sxXfa0YGQvM

As Tyler said, the general rule of thumb for market gardening is that vegetables need an inch of water every week.  Obviously good or bad water conservation techniques will adjust the numbers, but it's a good rule of thumb.  I've never seen a table looking at it with the level of detail you're asking for - it would depend on so many factors, not just soil type and organic matter content, but the slope, depth to bedrock, plant life growing, temperature, and more. I would calculate out a "standard worst case" length of drought (maybe a 20-year drought?), plan on watering an inch a week during that time, and figure out your water needs from there.

Hi all
thanks for all your inputs and ideas. I like the idea of trying to match the veggies production and the retreat dates with the times of higher rainfall.
The wicking beds idea seems interesting as well.

Tyler, this amount of 1 acre-inch doesn't seem to take into account how many plants are growing at the same time in the same acre of land, right, or it does?
Can it be that as long as the soil is saturated with that amount of water it doesn't really matter how many plants are there?
For example the grow-biontensive method uses a very thick spacing of plants compared to other cultivation techniques, does it mean that if your cultivation method is less intensive
you can actually irrigate less than an inch of water?...I am probably a bit confused on this.

Is there any piece of literature you can recommend on this subject?

also when you guys say "an inch of water per plant"...what do you actually mean in terms of volume?
How many liters (or gallons) per plant per week are we talking about?

Hey Antonio, the "inch of water" is phrased that way as it's often rainfall - if you get an inch of rain over the course of the week, you don't need to worry about irrigation. If you look at it in terms of irrigation, however, it's probably roughly accurate if you look at the size of the root zone you're actually trying to irrigate, which will depend on the plant - a tomato is obviously pulling water from a bigger area than a carrot. Tyler's figure of 27,000 gallons per acre is about right if you're growing tightly on a whole acre, but if, for instance, you're growing an acre of zucchini in a more conventional row pattern, and irrigating via drip tape, you may only end up needing to irrigate a third or less of that acre - you won't bother irrigating the walking paths.

So yes, to some degree if you're growing less in a space, you don't need as much water (especially if you can only water near the plants, not indiscriminately over the whole area) However, you do need to either mulch heavily or do a lot of weeding - if you have a lot of weeds they will compete with your plants for water., and if you have bare soil it will speed up evaporation. If you do a very dense planting you can shade out the weeds, and prevent water from evaporating off the soil surface as quickly, but you will need somewhat more water per unit area than a less intense planting.

Antonio Scotti wrote:
Tyler, this amount of 1 acre-inch doesn't seem to take into account how many plants are growing at the same time in the same acre of land, right, or it does?

It's a general guideline, and like all such guidelines "it depends."  Some gardeners swear by close plant spacing (John Jeavons) and others by wide spacing (Steve Solomon).  I think it might be a mistake to commit to one technique without experimenting under the specific conditions.  The only method I would expect to be foolproof would be wicking beds, but they require substantial infrastructure.  I advise starting small with a couple of techniques and see how they do.  Keep accurate records of water used as well as yield of produce, so that when you decide to expand, you will know what the enlarged area will require.

A series of wicking beds close to an eating or gathering area could be made very attractive and provide guests with a view of their food being grown. Beyond these beds could be an area of in-ground beds for experimenting with soil improvement and plant spacing.