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FOURTH on the Keyline scale of the relative permanence of things agricultural is farm roads. The main farm roads are influenced in their location by the climate and the land shape, since our study of the patterns of water movement over various land shapes provides a basis for determining the sites of the roads. Increasing volumes of flow water are hazards to roads, so that their trafficability in wet weather and their cost in maintenance are vitally affected. The centre of main ridges, which form part of the boundaries of the secondary valleys, are neutral lines of no flow, and being high and dry are very suitable sites for main roads.
The influence of the water conservation drains of Keyline and the position of irrigation drains and irrigation areas determine the sites and general patterns for work roads and are arranged as follows: Those roads which run across the land follow the water conservation drain either above or below it; others which follow the irrigation drain are located above it so that irrigation water will not flow across the road. Again, other roads may follow the lower boundary of an irrigation area and another road on either side of the breaks of the land, namely the watercourses.
Some of these considerations have always influenced the siting of farm roads, but the relationships of the factors of the Keyline scale of permanence and the appreciation of the new significance agriculturally of land shape provides a guide so positive that all road sites are selected to advantage because all factors in their siting are understood. Farm roads, while serving their purpose, change the natural drainage pattern either towards destruction or preservation of land profiles. From the general planning of Keyline it will be seen that the position or sites of farm roads become natural, obvious, and constructive.