Permaculture Designers Manual
CHAPTER 11 –
Section 11.2 –
Dryland Precipitation in Permaculture
Rain occurs in deserts as a lesser part of the normal cyclonic, convectional, or orographic rains occurring elsewhere.
Both warm, unstable tropical air masses and cold westerly rain may be entrained into air cell circulation over deserts, but there is a significant proportion of convectional rain due to local heating over sands, rocks, and bare soils.
Only in some deserts does rain fall, fairly reliably, in a seasonal distribution, in areas affected by monsoon borders or westerly coastal belts.
Elsewhere, rain is episodic and averages are meaningless in that many years may pass at any one place without rain.
There are the natural deserts (the Namibian coastal areas and the Atacama) where offshore winds and cold sea currents ensure that rain is rare or virtually absent. Such areas are treeless, and depend on fogs and dew alone for plant growth.
As dew may be critical to plant survival, dew traps of stone, scattered shrubs or even vertical metal screens to 1m high are strategies to catch moisture.
Moroccan foresters are contemplating such metal screens to condense dew and to establish shrubs, which in turn will become moisture condensers.
OPPORTUNISTIC RESPONSES TO PRECIPITATION
A rainstorm 8-12 mm produces run-off sufficient to start headwater stream flow and only the occasional downpour of torrential convectional rain or the heavy rains of monsoons and trade winds over headwater streams, remote from the desert, produce river flow down the braided or sandy water networks that thread through the desert.
For tribes people and the mobile fauna of the desert, distant rain is the trigger that sets off a whole sequence of migration and perhaps a subsequent Intensive breeding program.
Just as we can write a dissertation on the phrase Om mani padme hum, so we can write essays on such simple concept – words as “walkabout” for Central Australian tribes.
Rather than being an arbitrary, willful, or unpredictable movement that the calendar-regulated Europeans see it to be, walkabout translates as something like:
Our scouts saw thunderstorms to the far north: If we go now we can arrive in time to harvest some of the birds, animals and plants that will respond to rain and to follow the water to the waterholes that will fill with freshwater and give us fish, turtle and frogs for a few months. We must go now. In time to celebrate, the cycle of plenty that comes from the rain.
Thus, we can see such movements as being sensible, planned and appropriate to an environment, which presents rare opportunities to harvest the varied resources provided by rain, to visit newly regenerated country and to lighten the burden of resource pressure on favored home areas, so that they can regenerate as reserves for hard times.
The response of the desert to rain is truly remarkable. Ephemeral plants carpet the ground and flowers and seeds are abundant.
Buried tubers throw out great patches of melon, bean and yam vines. Shrubs or trees may produce numerous seedlings in areas where few young trees existed.
In the streams and pools, frogs and fish appear from mud-packed retreats, or invade from the rare permanent pools.
Fish may breed and grow in a few months and sea birds migrate inland and nest to take advantage of this. Flocks of pelicans, flamingos, herons, terns, gulls, waders and ducks occupy the waters and for a few months life is riotous and breeding unconfined.
Antelope, kangaroo and the desert quail, pigeon, finches and parrots flock to waterholes and spread their ranges over new forage areas and animal numbers build rapidly.
The longer-term collapse of this explosion of life is slower but usually inevitable. Some new tree generations may have established, but the waters turn salty, dry up and life scatters, dies or returns to rest in mud and sand.
Seeds are blown and buried and the desert waits again. Birds disperse, as do the migratory animals and the desert fringes experience a sudden irruption or migrating species. The party is over.
It is part of our strategy to capture some of the estimated 88% of water that will either evaporates or rush unused across the land at these times and to safely store it below ground for a prolongation of the growth period and a re-humidification of the desert air itself via transpiration from trees and shrubs.
In natural conditions as little as 0.8% of total rainfall infiltrates the soil to recharge desert aquifers.