Permaculture in Humid Landscapes - Permaculture Design Course by Bill Mollison

The category we are in now is humid landscapes, which means a rainfall of more than 30 inches [T.F.: 760 mm]. Our thesis is the storage of this water on the landscape. The important part is that America is not doing it.

The humid landscape is water controlled, and unless it is an extremely new landscape -- volcanic or newly faulted -- it has softly rounded outlines. When you are walking up the valley, or walking on the ridge, observe that there is a rounded 'S' shaped profile to the hills.

Where the landscape turns from convex to concave occurs a critical point that we call a keypoint.[2]
The main valley is the main flow, with many little creeks entering. At the valley head where these creeks start, we locate the major keypoint. From there on, the keyline starts to fall from one in 1,000 to one in 2,000 below contour. The dams we make in the lower valleys will be slightly lower at each point. They will not be at the keypoint.

Rain falling on the hilltop runs off. The paths described by single raindrops, wherever they fall, are similar in that they cross contours at right angles, because that is the shortest drop between two contours. Water takes the shortest path across the landscape from where it falls to where it hits the river line. It is along this path that raindrops are doing their thing. As soon as they are in the river valley, they are off to the sea.

It is possible to locate the keypoint from a contour map. Find where the contours start to spread. That is the keypoint.[3]

Having found the keypoint, we can now treat the whole landscape as if it were a roof and a tank.

Having found the keypoint, we can now treat the whole landscape as if it were a roof and a tank. In a fairly descending line, falling gently away from the horizontal, we put in a groove around the hill. This is the highest point at which we can work with mechanical tools. Above that, it is too steep. We make a little shelf around the hill leading to the keypoint. No matter where this water was going, we have now started to divert it, bringing it right around the hill to the keypoint. In effect, we have put a gutter around our roof, a very gently falling gutter. We started at the key point and extended a line that we lifted one foot at every 2,000 feet. We want to create a very, very gentle fall. Water just moves along it, and that is all. We have directed the water to our keypoint.

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