Trees excel at harvesting water. When you water a tree, there is evaporative cooling like an artificial cooler but during the night, the dew falls back on the leaves and the ground where some of it finds its way back to the trees (possibly via an invertebrate first). Also the reflectiveness of leaves helps. Then there's the soil where layers of dead leaves, wood and others accumulate, sequester CO2 and create a sponge. Finally, by virtue of making the region cooler, rain is more likely to fall. Humans can probably engineer something better but the bar is high.

There is something to be said for the parts about shading the surface too though. You're unlikely to cool an entire desert a significant amount with water you bring in but if that's something that happens as part of keeping the actual surfaces in the city cooler during peak heat times on top of the air cooling effects of evaporation then the sum result is greater than the parts in terms of effect.

Of course it's Vegas, I wouldn't be surprised if we decided to make the downtown completely indoors so we could just run AC in the streets too. It's not exactly the city of practicality.

  during the day, trees provide significant shade by intercepting solar radiation, reducing mean radiant temperature (up to 16 °C)

   during the day, trees provide significant shade by intercepting solar radiation, reducing mean radiant temperature (up to 16 °C)

In the abstract it discusses that most of the effect (16deg Celsius) is from reduced radiative heating and only a few degrees from evaporation.

Mostly the benefit is instead of having the concrete under you absorb and emit the sun, the leaves above you do.

This dramatically reduces the heat we feel at human height.

The increased water usage is tough because we're serious about water reclamation here in Vegas, but you can't reclaim water lost to evaporation, which is why there are policies (and serious fines) around excessive landscape watering. It might not be a worthwhile tradeoff, especially if there are alternate cooling methods that don't involve water loss.

It's confusing phrasing.

Increasing *evaporative* cooling by 0.5deg requires 3x more water, but shade alone is the *main* mechanism,it doesn't require water.

>during the day, trees provide significant shade by intercepting solar radiation, reducing mean radiant temperature (up to 16 °C)

The point of that article is that in many places the evaporative cooling is the main thing but in Vegas the water situation is such that it's more about the shade so the optimal tree is something that gives shade but doesn't need a lot of water.

Right where I am sitting now I have an LED strip above my desk and when I have my shirt off (right now) I can very definitely feel the radiant energy when it is on, so if it is really hot I either turn it off or switch it to green because the eye is most sensitive to green light. In fact, as I'm writing this, I just set the backlight on the 55-inch TV I use as a computer monitor down so I'd feel more comfortable.

Swamp coolers can't generally provide shade.

Trees acts for their little thermal mass and large surface exposed to air, essentially IR radiation from the Sun can't much reach the ground or humans under trees (if they are large and dense enough) and the part of the radiation touching trees get quickly dispersed in the air (climbing the atmosphere).

The limited effect is that cities are dense and can't be made as forest so trees can't do nothing for buildings taller them them.

Trees are also vertical structures. Any vertical structure will absorb some of the light, turning it into heat, then be cooled by rising air. This keeps the heat from getting to the ground, with or without evaporation. In other words, instead of the sidewalk getting hot, something 20+ feet in the air get hot. Hot air rises and the air near the ground stays cooler.

part of the cooling effect that trees produce is from photosynthesis, the percentage of light converted to plant matter can be as high as 1.5% more will be reflected, and the shaded area will of course,be shaded, and then there is transpiration of water, which varys greatly with species.other effects will be due to the built environment, where a lot of asphalt and concrete could mostly obliterate the real effects of a few trees. from wiki "The average rate of energy captured by global photosynthesis is approximately 130 terawatts, which is about eight times the total power consumption of human civilization"

https://en.wikipedia.org/wiki/Photosynthesis

There is literally no difference. Water is water, and its specific heat of evaporation doesn't depend on the way it's evaporated.

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Most places in the west, the water used for 'municipal' stuff is a small fraction of the total.

For instance where I live east of the Cascades, in the dry part of Oregon, only like 10% of the water used goes to the cities.

https://www.centraloregonlandwatch.org/update/2021/5/5/droug...

Street trees are hugely beneficial and if you want to cut something (ha ha), you want to look at things like lawns or golf courses.

Having lived in the desert, and not talking Vegas, but Nevada desert where folks bought cheap lots, dragged a single or double wide trailer into it, and started a life.

Trees were the first thing planted. Fast growing trees, placed to cast shade on the house.

After a few years, those dirt lots transformed into some very nice properties where sitting outside in the shade with the desert zephyrs rustling the leaves provide a very nice, idyllic place for conservation or reading.

Southern Nevada uses a tiny amount of water compared to most states, about 2% of the total that’s apportioned from the CO river, and recycles about 40% of it. For indoor usage, 99% of water is recycled.

I agree growing things in the desert may be inefficient, but speaking for the CO river, that problem is in California and Arizona.

https://www.snwa.com/water-resources/where-water-comes-from/...

https://lvgea.org/water/

Agriculture is the problem, not urban trees

There may be counter intuitive effects in there. Plants roots creates water buffer zone underground that can capture some of rainfall and make better use of it, allowing larger growth.

MIT should deploy their desert water tech in LV[0]

Since the point of the trees has been discovered to be just shade and not evaporative cooling, they just need to figure a way to reorient their panels?

[0] only sunlight needed

https://www.thebrighterside.news/post/mits-high-tech-hydroge...

Perhaps the desert will repent its desertness and accept that sand, minimal water and a massive diurnal temperature range will somehow become amical towards good old Plane trees.

OK, let's go full mad world: a vast web of PV for power. Is there a handy massive water resource deep underground? If not then moisture in the air will need redeploying. Tall towers and probably gobs of power are indicated for that.

There are proven methods for growing plants and trees in arid regions [0], but they have disadvantages which will become more evident as desertification expands with global warming. I agree that forcing non-native trees there is a losing battle in the long run.

If people were really serious about living in deserts in a sustainable way, they can't expect to have decorative greenery or classic architecture. A society as advanced as ours should be able to make compromises that allow modern comforts while adapting so well to their environment that the cities would look almost alien.

0. https://en.wikipedia.org/wiki/Desert_greening

what does he do?

I hope there would be enforced regulation around this kind of thing in cities in India. In residential areas, it's common that your house takes up almost all of your available plot space, and on top of that mostly constructed from concrete.

Air temperature is already high (e.g. 36C at my location just now), and radiant heat from sun and concrete can make the felt temperature more like 60C.

It's sad that new real-estate layouts continue to be approved, which will only be good for this type of dense concrete hell.