Here is everything you wanted to know (or didn't!) about making ollas for dryland irrigation in our geothermal greenhouse. Feel free to ignore this post and go to this more general summary post instead (or read that post first, which links back to this page)!
In Wisconsin, when we were trying to maintain a garden while working full time, Chip and I were known for both forgetting to water, and then forgetting to turn off the water. It makes taking on a 90'x17' garden in the high desert a little daunting! One idea for improving in this area is to use ollas for watering.
An irrigation olla is a terracotta pot that you bury in your garden and fill with water. Your garden plants are placed next to the pot. The water seeps through the porous sides of the pot into dry soil. If the soil is damp, the water stops seeping. So as roots take up the water, drying the soil, the water in the olla can replenish the moisture. This can keep plants watered if you head out of town, and also keeps you from over-watering. It decreases your water usage considerably, since the plant roots determine the rate of water extraction -- you are giving the plant just what it needs, and no more.
There are a lot of websites with more detailed information on using terracotta ollas for irrigation. Here are a few that I have relied on:
(Note that "olla" just means "pot" in Spanish, so don't ask a Spanish-speaker where to buy an "olla" and expect them to know you meant an irrigation olla!)
Ollas made for gardening are expensive. A cheap and functional substitute is to make them out of plain terracotta pots. First you block the drain hole. I used Gorilla Glue to glue in plugs. I tried discs that I cut from a broken water bag. They worked OK, but were a pain to hold in place with appropriate clamping pressure, so I switched to plastic bottle caps for later pots.
I had to dig around to find the right profile "press" that would be narrow enough to fit into the bottom of the pot, and thick enough to provide pressure. I did five pots at once and stacked them until the glue set.
Once the hole plug is dried, each plugged pot needs an unplugged mate. You glue the two pots together at the rims.
I made gallon pots and quart pots. Most of the instructions that I see online use silicone caulk -- I did that as a test, but it stayed sticky and gooey, and I preferred the stronger bond of the Gorilla Glue, even though it requires the clamping. I coated the plugged bottom inside and out with silicone sealer for the larger pots, but switched to gorilla glue for the smaller ones, again to avoid the gooiness, The point of coating the bottom is to avoid water seeping out directly down to the ever-thirsty water table -- you want the water to seep through the sides of the pot, to the surrounding roots.
These will be buried plug-side down in a row in the garden bed, with the tops peeking out of the dirt an inch or two. We'll fill them through the open hole and cover it with a lid to avoid evaporation and discourage mosquitos. Time will tell how this works for us.