Valleys start or head where a portion of a slope near the top of a watershed or divide becomes steeper than the general slope on either side. Thus the first part of a valley formation is steeper than the ridges or shoulders on each side that form the valley. At some point down the valley--the Keypoint--the valley slope flattens to such an extent that it becomes flatter than the ridges or shoulders on each side in the same vertical interval.
The prime purpose of the lines of cultivation on the Keyline principle is to counteract the natural rapid concentration of rainfall into the valleys by an induced drift out of the valleys. Keyline development first controls the usual water run off into valleys from the higher land, by tremendously increasing the absorption capacity of this area and diffusing the excess water, thus greatly retarding and delaying its time of concentrationThe time needed for a drop of water to reach the outlet of a catchment from the most remote location within the catchment..
From the Keyline both up the slope and down the slope of the land, timber strips are left (or planted) on the contour at regular vertical intervals apart. The important guide for determining this vertical interval between timber strips is related to the height of the trees. If trees are 45 feet high the timber strips could be 40 feet apart vertically. This provides some overall wind protection for all the land and locates the timber strips closer together in the steep country and farther apart as the country flattens.
Even in very flat country of low scrub or mallee only 10 to 15 feet high this formula for clearing will provide greatly improved farm conditions.