# Temperature for 3 or 4ft underground

## Main Question or Discussion Point

There are often talks of ground temperature being a constant 55F at 6ft or so. But some also said that in very hot and dry climate, say Arizona desert, temperature is much higher at the same depth.

Anyone has any idea of underground (4ft? under) temperature in dry hot desert?

Thanks

## Answers and Replies

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DEvens
Gold Member
It is strongly dependent on surface conditions, precipitation, ground cover, etc. A depth of 4 feet is not sufficient to insulate from surface effects. There will be only a limited insulation, and some delay and averaging of surface temperatures.

So, for example: If there is a tree growing there, or some other source of shade, the shade will make a big difference. If it has rained recently it will make a big difference. If it is winter or summer will make a big difference.

jim mcnamara
Mentor
Here is a graph: http://www.builditsolar.com/Projects/Cooling/time-lag-vs-depth.gif

This is for the Northern Hemispshere.

The graph shows average temperature as it changes throughout the year (number of days at the bottom) - for depths of: surface, 2ft, 5ft, and 12ft. Notice the peaks and valleys grow smaller as you go deeper. 12ft never gets really warm or really cold. Unlike the surface.

It shows the following features of subsoil temps:
1. deep soil temperatures "follow" the seasons with a lag. Example:12FT is warmest Oct 29. 5 months after the greatest sunlight intensity.
2. There are lots of diurnal (daily) temperaturechanges on the surface, but the temperatures futher down change slowly. Like over weeks or months.

With sandy dry soil, in the Northern Hemispshere (35N) at about 40C on a sunny day at midsummer the surface temperature can exceed 90C.
https://en.wikipedia.org/wiki/List_of_weather_records#Measuring_conditions - see the paragrpah discussing soil temps.

Desert ant species, for example, evade the high noonday heat by digging into the sand a few inches. At 3-4 feet the temperatures vary slowly over the year with insolation angle changes, but they are always substantially lower: 25C-30C. This I suppose is the answer you want. Large mammals take advantage of this and dig deep burrows, and only come out to forage at night.

Baluncore
2019 Award
Anyone has any idea of underground (4ft? under) temperature in dry hot desert?
A hot desert during the day can become very cold at night. This is an averaging and a heat diffusion problem.

As deeper underground measurements are made, the daily and then the seasonal variations are progressively attenuated and phase shifted. Those daily and seasonal cyclic changes are impressed upon the diffusion of heat up through the Earth's crust from the mantle to the atmosphere.

The temperature four feet down will vary slightly throughout the year, but almost not at all over the period of a day. The average temperature at points near the surface will be the average temperature at the surface over recent times. Greater depths average over greater times.

Thanks guys, this is what I am trying to achieve: I am trying to grow organic vege with hydroponic here in the desert. One of the problems would be the water temperature, plants simply do not like warm feet. From July - Sept, day time temp could reach 115F (46C) and even night temp could be 85F (30C). To have cooler water, one can run a water chiller, but some suggested to install an underground water storage tank. Say a 2500 gallons tank underground, with the top of the tank say 2ft under and the bottom of the tank 5ft under to take advantage of the ground temp.

Not sure the water tank idea would work, but sure would help to know what kind of coolin' effect at say 2-5ft under.

Baluncore
2019 Award
Work out what the average surface temperature is for each month of the year. That will give you the average temperature of the water if your reservoir capacity is sufficient and it is deep enough underground. Beware evaporation of water as it will result in salination of the reservoir.

DEvens
Gold Member
One wonders what it is you are growing in hydroponic tanks in the desert. And why you cannot select a crop that will thrive in easily produced conditions rather than setting up this complex system.

jim mcnamara
Mentor
Now things are clearer. You do not need a tank. You can do a search on "geothermal cooling" to see what I describe. Forget 3 or 4 feet down.

There is a point in your soil profile that is well below the caliche (or lime) line in your native soil that remains pretty much constant all year long. In Albuquerque it is about 12' down. What you do is to create a long, continuous ditch that keeps turning back and forth so it fits in a small area. You place a long pipe in the ditch - it is connected to the surface for input and output. The ditch can also be vertical going straight down - like a well. This works better in an urban envrionment with limited yard space. Connect input to your water source, connect output to irrigation or house water input. In effect you are detouring the warm water through cool ground.

This shows well-water temperatures for the USA:
http://www.builditsolar.com/Projects/Cooling/US-ground-temps.gif

Get hold of someone in your area who does geothermal cooling installs, and learn about options, required burial depth in your area.
Assuming irrigation at the rate of 2500 gallons per day, consider that the pipe must be able to contain more than that amount of water. Local experts can tell you how much more capacity (more than 2500 gallons) you need. The flip side of this is that the water you get is the same temperature all year long, which may or may not be useful in cool seasons.`

This is far more efficient at reducing temperature than a large tank at 3-4 feet down. Because you have a huge pipe surface area in contact with soil compared with the tank.

billy_joule