Modern Practices, Worldwide Methods
The word terrace is applied to geological formations, architecture such as a housing complex built on a slope, or an island between two paved roads. However, the act of terracing specifies an agricultural method of cultivating on steeply graded land. This form of conservation tillage breaks a hill into a series of steplike benches. These individual flat structures prevent rainwater from taking arable topsoil downhill with it. The spacing of terraces is figured mathematically by comparing the gradient of the land, the average rainfall, and the amount of topsoil which must be preserved.
Different forms of terracing are required, depending upon how steep the ground is that is intended for cultivation. The bench is the oldest type, used on very steep territory. A little dam called a riser marks off each bench, and can slow down rainwater runoff on slopes as extreme as 30%. Just the way a steplike or "switchback" layout of railroad tracks prevent trains from having to go up one steep grade, the effect of gravity is lessened by bench terracing. Climate as well as soil condition and farming methods must be taken into account, so the Zingg conservation bench is a type of flat-channel terrace constructed in semiarid climates. Slopes between each bench store runoff water after each rain.
Newer formats were developed to accommodate for mechanized farm equipment, so now variations such as the narrow-base ridge terrace and the broadbase terrace are used for less extreme gradients. Two approaches to broadbase terracing are used, depending upon the conditions of the topsoil and its vulnerability. The Nichols or channel variation is a graded broadbase terrace for which the soil is cultivated from above. Water is channeled off by this construction at a steady rate. The Mangum or ridge type is a level broadbase terrace used in permeable soil. This type shores up the topsoil from above and below. A less ambitious form than the broadbase is the steep-backslope terrace, which takes soil from the downhill side of a ridge, but this backslope cannot be used as cropland.