Where I'm located, the average temp is over 90 F for 5 months of the year, and for two of those the average temp. is 100 F. Rainfall can range from 7-12 inches a year, with the majority of the rain coming down halfway through the summer for about 2 1/2 months. Humidity when it's NOT raining, but IS hot, can be down to single digits during the hottest part of the day, and we have a very high evaporation rate, which is very hard on the plants, obviously. I grow both native species, drought resistant species, and also try my hand at veggie gardening as well.
My experiences with water, plants and drought conditions, good and bad, is the following.
Mulch based on plant matter -
The success of this depended a lot on water availability and amount of mulch. Because of the high evaporation rate, the ONLY time large amounts of mulch worked (4-6 inches deep) was when irrigation was involved. So around some of my fruit trees and garden veggies that I regularly water, it was tremendously useful. To not waste water, you have to have a set up that adds in the water UNDER the mulch, however. So a hose that goes into a section where the mulch is absent, or automatic watering systems that the mulch was put on top of, for example. The water gets to the soil this way, the mulch breaks down slowly, and it helps tremendously to keep the soil in the ground.
But if mulch from organic matter is used when there isn't irrigation, it can't be thick or it doesn't work well. Rain events can't penetrate the mulch and the water is simply absorbed by the mulch and released back into the air before it gets to the ground. And on top of that, any mulch that is wood based doesn't break down for a LONG time without a large addition of water. Things here desiccate rather than decay, with a little help from insects, typically - there's an old fallen tree that's been on a hiking trail for 10 years now, and it looks nearly the same as it did when it fell over, except for where termites have eaten it.
So for native plants, or drought hardy plants I don't water as often, I still add mulch, but it's typically a 1-2 inch layer, and the pieces have to be cut smaller if I want them to decay. This way, the rain can still penetrate, but it does help cover the soil and prevent some evaporation, and as long as I irrigate every once in a while, it usually will decay.
One solution for this particular issue that I do instead is, I guess I'd call it mulching with stones. Rock or sand mulch (covering the entire area with 1-2 inch layer of small rocks/gravel/sand) is really popular here, and is also terrible - no nutrients get to the soil, and it raises the temperature of the surrounding area quite a bit, making it more difficult for the plants as well.
However, once a med-large sized plant is established, and casts a bit of shade, I can go back and place some large stones underneath it, making sure they have full contact with the soil. I usually pick ones that are 4-12 inches wide. I'll put a few around the plant in question, with the spaces between the stones having that 1-2 inch mulch. It works fairly well, so far. It is kept moist underneath the stones, but the soil still has areas where it can absorb nutrients from decaying mulch.
This does not work, however, until there is shade. It is so hot here that if you put large stones around a smaller plant, in the sun, they'll heat up things so much they stress the plant and sometimes have killed it.
I have tried some homemade terra cotta ollas and they have done so-so, but I think that's more a case of my forgetting to add water to them than anything else. ^_^ One thing I did notice, however, was that with my high evaporation rate, the penetration of the water from the olla wasn't as far FROM the olla as expected, so there might be some experimentation needed with this with regards to one's evaporation rate and spacing, you know?
Oh! With re: to using plastic milk jugs as an olla for watering - I am highly jealous of the ability to do this, LOL. One CAN use plastic out here, but it is so hot and brutal that it destroys the plastic fairly rapidly, so you can't use it for anything that you want to last. I've tried milk jugs but within a few months, they are so brittle you can grab them and the plastic basically disintegrates in your hand.
drip lines -
I have had to avoid these, although they are popular here. But I live a little out in the boonies, and anything that might have water, the critters are usually desperate to go after during the worst of the heat, when it hasn't rained. I have had so many destroyed hoses and lines from these critters.
Although if anyone else has this problem, I have been having some success with putting out little shallow dishes of water near but also AWAY from the garden and watering equipment. That seems to cut down on the destruction of my equipment.
It had an unexpected benefit too. Animals here can REALLY find water, wherever you have it. But what that has meant is that even if I have a garden fairly well camouflaged with plants that mask the sight and smell, the critters will STILL find my garden by following the presence of the water. Putting out these little shallow dishes of water has cut down on that issue as well.
Because of the intensity of the sun here, it's extremely rare to have any issues with a plant that can't get enough sun, but the sun itself causes a lot of water loss IN the plants. So finding ways to add dappled or partial shade has been helpful for keeping water with the plants during the driest parts of the year. Usually, it does well enough with slightly bushy trees to the west/ south west of the plants, to help them the most during the summer months. Mesquite trees are good for this with native plants I don't need to water. Pomegranate and texas persimmon have worked well for more garden oriented plants, and shade cloth in a pinch.
Also, a sort of 'personal' shade for some plants can be helpful as well. I planted some greek oregano in a spot that DOES get shade during the heat of the day due to a wall, but I thought it had died, and just let native grasses grow over it for a year as I forgot about it. Come back later, when dead clump grass has fallen over it, and the oregano used the dead plant as shade for its base and was happily growing through it. I have a clump now that's probably 3x3 feet, and I might water it 2-3 times a year. If I try to weed it, it gets dry and struggles. If I let native weeds partially take it over, so it has to creep up through them, it flourishes. The ground is always noticeably more damp underneath the shaded, weeded oregano than the surrounding area.
That is something I have noticed in this area - shade is king. I'm sure this is not the same in areas without such intense heat/sun, but for me, it's a vital part of planning with water and plants.
pit-type gardens -
The past few years I have used a Zuni style pit, the kind of 'waffle garden.' This does all right here, but while we get little rain, when it DOES rain, it comes down so hard that it tends to eventually wash away the walls of the waffle gardens, no matter how hard you make them, even with stone mulch on top trying to keep them safe. I'm sure with more maintenance I could keep them going, but I'm hoping that zai pits will be better for this climate and need less maintenance, so we'll be trying those this year.
Keyhole gardening -
Does not work so well here if you are looking for water conservation. Again, it's mostly because of our intense heat and high evaporation rate. This is an issue with raised beds of any kind here - the sun significantly heats up the soil down to about 2 feet. So anything that is raised up tends to get much hotter soil, and the plants need a lot more water, and even compost piles here have to be covered or they tend to dry out too fast. I mean, you CAN get it to work, don't get me wrong. I know people who have done keyhole gardening here, but it's a lot more difficult, and it's not good from a water conservation standpoint, and most of them had to build a more complicated set up to keep the compost more enclosed above the ground so it didn't lose too much water.
Dryland farming -
Also does not work so well here, except during one tiny part of the year...and even that's iffy. While we are often within the range of dryland farming when it comes to rainfall, the heat and low humidity are so brutal that it makes it nearly impossible. The rain also comes so seldom that most plants cannot sustain themselves long enough, no matter how awesome they are (native plants are the exception, so I'll talk about that in a sec). The one time it might work is during the middle of the summer, when the monsoon rains come.
Midsummer IS a growing season here - there is the possibility for setting up a dryland farming scenario during this time. I know that the native tribe here, in the past, made garden beds in these small areas that flooded during the rains, and vegetable varieties were used with short growing seasons. But the flooding areas collected water from a number of arroyos, and it's nearly impossible to get that amount of water from your yard, even when you are trying to collect it, you know?
That said - there are a lot of native plants that are sources of food that do perfectly well in an environment like this. They don't need watering - I have a lot of plants that I don't water in the slightest and can get food from every year. A lot of cactus and native trees that are legumes, mostly. Some native herbs and greens that are seasonal. It's not the most exciting diet, but it would do, in a pinch. One just has to adjust your thoughts away from 'what do I want to eat' and shift it a bit toward 'what can grow here.'
And...ha, yeah, didn't mean to go on so much! This is such an interesting topic, though! And in part I wanted to share what my own experiences are because many of these were ideas I looked at when I first started out, and they kept failing and confused the heck out of me. I finally realized that while many of these methods discussed rainfall conditions, most didn't really talk about heat or humidity, or any other condition that might impact water usage and retention. When I investigated, most of the methods that did not work here were from areas where the heat was lower, the humidity was higher, or the method in question had more water due to irrigation or rainfall.