Terrace cultivation, or terrace farming, is one of the oldest types of land and water resource management for large-scale farming. Essentially, the main purpose of terracing land for farming is to reduce the velocity of water runoff and thereby soil erosion by breaking the length of the slope that runoff has available. There are two main types of terracing, graded and level, and each comes with some advantages and disadvantages.
One of the principal advantages of terrace cultivation is that it can protect the terraced area's soil from overly rapid erosion. Erosion happens when moving water strips away soil from the area over which it is flowing. Terracing, by reducing the length of the slope the water has to run over (i.e. by creating a level terrace on an otherwise sloped face), slows the flow of water. This protects the soil in the area from being carried away in a deluge.
Level terraces, like those found in parts of Bali and China, can not only reduce the rate of soil erosion but can also trap and hold rainwater. This allows for the cultivation of water-intensive crops, such as rice, in these areas. Terracing creates flat spaces for crops and canals for water to flow between these areas. Water collected in the terraces can then be absorbed into the soil (if the soil is suitably permeable) and sustain crops.
One major disadvantage of terracing is rainwater saturation of the ground. This happens when terracing retains too much water, which is absorbed into the ground. The problem with ground saturation is that it can lead to water overflow during periods of heavy rains. This can end up causing more damaging runoff than in unterraced areas. Additionally, if not properly maintained, terraces can lead to greater soil erosion, often downslope from the terracing, than in non-terraced areas.
Terracing requires huge inputs of labour to construct and maintain, and when not properly maintained, the effects can be catastrophic. Unmaintained terraces can lead to mudslides, the creation of deep gulleys and increased soil erosion, particularly in sandy soils or on extremely steep terrains. Terracing also has been shown to reduce soil quality via the leaching of important nutrients from the soil in some areas.