Summary Chapter 6 in Permaculture Designers Manual

Permaculture Designers Manual




Section 6.7 –

Summary Chapter 6 in Permaculture Designers Manual

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Let us now be clear about how trees affect total precipitation.

The case taken is where winds blow inland from an ocean or large lake:

1. The water in the air is that evaporated from the surface of the sea or lake. It contains a few salt particles but is “clean“.

A small proportion may fall as rain (15-20%), but most of this water is CONDENSED out of clear night air or fogs by the cool surfaces of leaves (80-85%).

Of this condensate, 15% evaporates by day and 50% is transpired. The rest enters the ground water.

Thus, trees are responsible for more water in streams than the rainfall alone provides. (Figure 6.4)

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2. Of the rain that falls, 25% again re-evaporates from crown leaves, and 50% is transpired.

This moisture is added to clouds, which are now at least 50% “tree water”.

These clouds travel on inland to rain again.

Thus, trees may double or multiply rainfal1 itself by this process, which can be repeated many times over extensive forested plains or foothills.


3. As the air rises inland, the precipitation and condensation increases and moss forests plus standing clouds may form in mountains, adding considerably to total precipitation and infiltration to the lower slopes and streams.


4. Whenever winds pass over tree lines or forest edges of 12m (40 feet) or more in height.

Ekman spirals develop, adding 40% or so to rainfall in bands which roughly parallel the tree lines.


5. Within the forest, 40% of the incident air mass may enter and either lose water or be re-humidified.


6. And, in every case, rain is more likely to fall as a result or organic particles forming nuclei for condensation, whereas industrial aerosols are too small to cause rain and instead produce dry, cloudy conditions.


Thus, if we clear the forest, what is left but dust?

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