Permaculture Designers Manual
CHAPTER 11 –
Section 11.2 –
Dryland Temperature in Permaculture
The air temperature (up to 60m in height) over deserts approximates the daily pattern given in Figure 11.1.
I have not given the 6 a.m. temperatures, which can range from 8°-30°C, but show a generalized graph which holds true for most cases and indicates a rise of about 10°C to 25°C over the day, with a peak at from 12-3 p.m..
Soil temperatures follow this general curve, but peak about an hour earlier.
The soil is also colder just before dawn, at about 5-6 a.m. and the effect of temperature falls off as we go deeper in the soil.
There is little effect of daily changes at 30cm (12 inches) depth. The peak surface temperature reduces about 2°C for every 5cm depth (or 15°C in 30cm), so that soils at 30cm deep may have fairly constant dally temperatures some 5°C or so higher than the lowest surface temperature and 15°C or so lower than the highest surface temperature.
Another way to look at this is that soil evens out the heating that affects air for a considerable distance above the ground.
Both air and soil follow a slower cycle of annual range depending on latitude and radiation received in season. This affects soils to about 2m depth, with about 5°C annual fluctuation at this depth or about the same fluctuation that we experience in underground houses and cellars.
Soils also can gain and hold much higher levels of heat than air and at 5cm depth, in favorable sun slopes and in poorly conducting soils, about 60°-70°C can be reached at peak! While this effect is good for solar hot water collectors, it is lethal for young plants.
Surface air temperatures frequently reach 30°C in deserts and there are isolated records of 52°C (125°F). Layers of super-heated air just above the soil may become unstable and form rapid convectional currents, especially on sun-facing slopes.
Cumuliform clouds can arise from this effect as thermals or strong up draughts or as clear-air turbulence in very hot dry periods.
In order to escape the extreme surface soil heat, desert animals commonly burrow to below 30cm in depth or in some cases; seek out high refuges in low shrubs or on stumps or posts in hot weather.
The other way that animals escape soil and air heat extremes is to seek shade, become nocturnal and to develop efficient cooling strategies and water economy.
In larger species, shade may be the limiting factor in survival (e.g., the amount of cave shelters in the desert may limit the numbers of large desert marsupials).
Consequently, deserts have a far greater proportion of their fauna as subterranean and nocturnal species.
Soil humidity also rises steeply, especially in dune sands. Given an average 4% of water in the top one meter, we can reach 10-20% water content at 2 meters and at 20m-60m; there may be quite saturated soils.
For both plants and animals, it is preferable to draw from these deeper, cooler and more humid layers of soil.