Precipitation in Permaculture

Permaculture Designers Manual



Section 5.4 –

Precipitation in Permaculture

There are two basic inputs to precipitation:

That of rainfall, snow, and hail (WATER FALLING from the clouds), and that of CONDENSATION (water condensed or trapped from sometimes clear air or fogs by cool surfaces).

Although the latter may be of critical importance on seaward slopes and at higher altitudes (cloud forests), the only reliable and widespread measures we possess are of “rainfall”.

World rainfall averages about 86 cm (34 inches). While we may take 50 cm (20 inches) of rainfall or less as semi-arid, and 25 cm (10 inches) or less as arid and desert, we can locally experience seasonal or relative aridity due to long-term cycles and weather effects caused by periodic fluctuations in jet streams or oceanic currents in any climate.

Longer periods of increased aridity can also be caused by deforestation on a broad or local scale.

It is because of the potential for changes in precipitation that we give so much space in later chapters to water storage strategies and the conservation of water.

Water promises to be the main limiting factor for survival and growth, and the major future expense of food gardens and agriculture.

Thus, any strategy we can adopt to generate, conserve, or store water is critical to our design approach.

Any gardener knows that climatic averages are at best a very general guide to precipitation effects in the garden or orchard.

It is a much safer strategy to see to it that both the species chosen and water strategies developed ensure some yield in ‘”drier than usual‘” conditions.

After all, a fish population out of water for an hour is as dead as if a year-long drought were in effect.

Our annual gardens and crops are also susceptible to short-term changes in available water.

People live and garden, in average annual rainfalls of 10 cm (4 inches) or less, and they manage to both exist and produce crops.

Exotic (non-local) water enters dry ions as rivers and underground aquifers, and this enables us to make judicious use of that water and to implement a great variety of local strategies to cope with the lack of actual rainfall.

Rainfall averages are best used as broad indicators rather than as definable limiting factors.

Of far more use to us is the expected DISTRIBUTION of rainfall (including extremes such as 100 year flood records) and data on the INTENSITY of rains, as these factors are a limiting influence on the size of road culverts, dam spillways, and the storage capacity needed to see us over dry periods.

Flooding histories of sites and districts often indicate the real limits to the placement of plant systems, fences and buildings, so that attention to flood records avoids future costs and disappointment.

If flood data is omitted, life itself can be at risk in intense periods of rain.

As precipitation rises, available light decreases. Thus, in extremely cloudy industrial or fog-bound humid climates, light becomes the limiting factor for some plants to ripen or even flower.

At the dry end of the rainfall spectrum (as we reach 50 cm or 20 inches mean rainfall) sun is plentiful and evaporation in excess of precipitation becomes the limiting factor. That factor determines our arid-land storage strategies, just as the depth of seasonally frozen soils and ice cover determines water reticulation strategies in cold climates.


Rainfall is conveniently distinguished by the processes causing rain as:

OROCRAPHJC: the rooting of air as it rises over mountains or hills.

CYCLONIC or FRONTAL: the over-riding of cool and warm air masses of the polar circulation.

CONVECTIONAL: columns of hot air rising from deserts or oceans into cooler air.

Apart from rain, we have dew and fog. DEW is a common result of clear nights, rapid radiation loss, and a moist air mass over coasts and hills.

It occurs more frequently in clear-sky deserts than in cloudy areas, and a slight wind speed (1-5 km /h) assists the quantity deposited. Both still air and strong winds reduce dewfall.

Intensity of deposition is greatest 3-300 cm above ground level; the highest deposition due to areas of dry ground, the lower due to wet earth, which chills less quickly.

Not to be confused with dew (a radiation heat loss effect from earth with clear night skies) is the moisture found on leaves above warm damp ground on cloudy nights.

This is either GUTTATION (water exuded from the leaves) or DISTILLATION from rising ground vapor; it represents no net gain to total precipitation.

The waters of guttation cling to the tips of leaves, due to the whole leaf area.

Only in deserts is the 4-5 cm (1-2 inches) of dew per year of any significance in precipitation.

Dew in deserts can be regarded as an accessory to, rather than a replacement for, trickle irrigation.

Dew may be captured by building piles of loosely-stacked stones, where low night winds cool rock surfaces and dew can accumulate to dampen the ground below.

In the Negev desert and other dry areas, some plants are associated only with these dew condensers.

Each mound of stones may suffice to water one tree (Figure 5.5).

Very large radiation traps, such as those on Lanzarote in the Canary Islands (Figure 5.6) may grow one grape vine in each hole.

The most efficient dew collectors are free-standing shrubs of about 1-2 m (3-6 feet) in height.

Groups or solid stands of plants and grasses do less well in trapping dew and this may help to explain the discrete spacing of desert plants, where perhaps 40% more dew is trapped on scattered shrubs than would be caught in still air or on closed vegetation canopies.

It is possible to erect metallic mesh knees 1 m (3 feet) or so high and to use these as initial condensers in deserts, growing shrubs along the fence drip-line and moving the fence on after these plants are established.

In Morocco such fences are proposed for deforested coastal areas.

FOG forms where warm water or the vapor of warm rain evaporates into cool air, or where cold ground chills an airstream and condenses the moisture. Chang (1968) concisely differentiates between:

1. RADIATION GROUND FOG: where, on clear nights, hollows and plateaus cool rapidly and fog forms, often in much the same pattern as the frosts of winter.

2. ADVECTION FOG: where cold offshore currents condense the moisture in warm sea airstreams. These are the coastal and offshore fogs that plague many coasts such as that of Newfoundland and parts of northwest Europe.

3. UPSLOPE OR OROGRAPHIC FOG: where warm, humid airstream s are carried uphill slopes and condense as the air cools.

Unlike dew, fogs can provide a great quantity of moisture. Chang gives figures of 329 cm (128 inches) for Table Mountain, South Africa, and 127 cm (50 inches) for Lanal (Hawaii) from fog drip alone.

In such areas, even field crops may thrive without irrigation. Typically, bare rock and new soil surfaces are colonized with lichens and mosses on sea facing slopes, while rainforest develops on richer soils.

Much of New Zealand experiences upslope fog precipitation, and unless burned or cleared to tussock grasslands, dense forests will develop; the irregular canopy of such forests is an excellent fog condensers.

Even with no visible fog, trees will condense considerable moisture on sea-facing slopes with night winds moving in off warm seas over the land, and encountering the cool leaf laminae of forests.

In the very humid air of fog forests, giant trees may accommodate so much moisture and evapo-tranpiration is so ineffective if fogs and still air persist, that more large limbs fall in still air than in conditions of high winds (which tend to snap dry branches rather than living limbs).

It is an eerie experience, after a few days of quiet fogs, to hear a sudden “thump!” of trees in the quiet forests.

Almost permanent condensation fogs clothe the tops of high oceanic islands, and hanging mosses and epiphytes rapidly develop there, as they do at the base of waterfalls, for the same reasons (free moisture particles in the air).


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