The concept behind the olla is simple: it's an unglazed clay jug or pot buried in the soil and filled with water. Ollas are an ingenious method of irrigation that slowly releases water through the container wall to a plant's roots via osmosis. They also cool the water by the thermal reaction of evaporation.
Note that evaporation is reduced since the opening to add more water is often small in comparison to the size of the container, and being buried affords some insulation from the heat. A cover or lid will also slow evaporation and keeps insects from laying eggs in the water. More water is added to the container as needed.
The depth of the pot (and therefore the length the passage needed to be refilled) is dependent on the depth of the root system of the plant being grown. Shallow-rooted plants benefit best from shallow ollas and deeper-rooted plants prefer deeper ollas.
Since most of the water is expressed at the root level and not on the surface, the top is allowed to remain drier than in other watering methods, which inhibits weed growth, and a higher percentage of the water used is distributed to the plant roots without loss from runoff.
Liquid nutrients can be added to the water in the pots to form a small nutrient solution reservoir. Mix nutrient solutions at half or one-quarter strength, as runoff is almost non-existent and water efficiency is very high.
One very nice thing about this method is that if the soil surrounding the pot is dry, it will absorb moisture from the surface of the pot, drawing more water from the interior of the pot. If the soil is moist, then this wicking action does not occur, and the water stays in the pot.
Since the pot creates a layer of moist soil around itself, roots are encouraged to spread and surround the pot. (Keep in mind that while most vegetables and garden herbs have soft roots that usually do little damage to the pot, woody plants like trees will often break the pots in order to get to the water within.)
While the olla method is especially popular in dry areas, it can be used in other climates as well with minor adjustments. I live in a pretty nice climate, and I have been using ollas in addition to my normal watering. I have found it extends the period of time between waterings, and it also helps the plants wilt less during particularly hot periods of summer heat.
So, systems that maximize rainwater use can collect the rain and divert it into the ollas, storing it for use as the soil dries out. In locations where temperatures get below freezing, the pots should be dug up and stored dry over winter. In humid areas, care should be taken to not allow the water in the pots to go stagnant; this can be done by occasionally overfilling the olla with fresh water to freshen the pots.
There are clay pots made specifically as ollas that have a wide bottom, narrower neck and sometimes a lid. The bottom portion is buried, and the top is used to refill the water. A stick can be used to measure the water level, and various sizes are available.
Get them locally if you can, as shipping can be expensive for them (they are clay pots after all). Another option is to simply use an unglazed, low-fired clay jug of reasonable thickness (the same stuff as regular cheap terracotta pots) obtained from wherever you can find one.
Finding a potter practicing making jugs would be ideal source, as ollas don't need to be pretty, just watertight. Garden and pottery shops sometimes have jugs that can be used without modification. Obviously the jug shouldn't be too fragile, and it has to be unglazed so it is porous enough for the water to seep through it.
And as a final option, one can improvise. Clay flowerpots can have their drain holes sealed shut and saucers can be used as lids (a little elbow grease and sandpaper can flatten edges for a better fit). A saucer can be glued to the top of the flower pot, turned upside down and the drain hole can be used to fill with water when buried. Two flowerpots can be glued lip to lip, and the hole in one plugged.
There are many alternatives, but the end goal is the same: to have a clay container that will hold the water. In dry areas, the exposed portion of the olla can be painted or glazed to reduce the amount lost through evaporation. (An interesting note, gourds have been used in a similar fashion; however, clay terra cotta remains the preferred material to use.)
This summer, I am trying an experimental container design. I took a strawberry pot, and inside placed a long tall flowerpot with a saucer glued to the bottom to plug the drain hole. A second saucer served as the lid. The remaining space in the pot was filled with a premium potting soil and vegetable seeds were planted in the strawberry cups.
The soil appears to be kept at a very nice moisture level, and so far the seeds have done well. My concern is that the olla takes up a fair amount of space, and I'm not sure there is enough left for the roots not to become root bound.
For those not so inclined to hand water such a primitive device, a drip-type system can be used with lines run to the ollas, allowing for them to be refilled on a timer. For those so inclined, if the ollas are sealed and connected to an automated system, the entire pot can be buried (although I admit, I prefer to be able to see the tops so I don't need a treasure map to remember where I put them).
Depending on the olla used, the area of moist soil surrounding it may range from a few inches to about a half foot or so. As a rule of thumb, the moisture will travel about the radius of the olla (about half the width of the pot) into the surrounding soil. Hand watering sprouts and young plants may be needed until the root systems are established enough to reach the olla.
Ollas can be used in the ground in raised beds or in containers. Ollas are especially well-suited for vegetables planted in hills such as melons and squashes.
Although the trend for many years was for older ollas to be replaced by newer and fancier irrigation systems, in recent years they are being rediscovered by folks who want to conserve water, extend time between waterings and make the most out of the rainwater they collect.
An olla has no batteries, no wires and no plastic parts. It is made from cheap materials and when properly used can still beat much more advanced and costly irrigation systems in water efficiency.
By either watering with ollas alone, or as part of a more conventional watering program, water can be saved, and plants can have some defence against wilting, which can be a big problem in some climates.