Terrace Irrigation | Research & Encyclopedia Articles

In much of Asia, a shortage of natural lowland areas suitable for food cultivation has led to extensive terracing along the edges of the uplands. This was done in order to increase the acreage of flooded fields, which are required for wet rice cultivation. From India and China, where terracing first originated, to many parts of Java and Bali in Indonesia, in North Vietnam, and in Luzon in the Philippines, terraced irrigation works extend like huge flights of steps far up along mountainsides. These terraced fields have been built since antiquity and maintained at a prodigious cost in human labor and toil.

The rice terraces provide the most spectacular agricultural landscapes in Asia. Here, permanent irrigation has allowed the conversion of slopes along uplands into rice fields and fields of other crops with which the rice crop is alternated. While much of irrigation technology on terraced land is somewhat crude, dependent on mud and brush dams and short distribution canals, some groups of farmers, such as the Balinese and the northern Vietnamese, have created complex hydraulic systems. These societies have, since an early time, developed elaborate systems for carrying as well as regularizing the water supply to fields used for crop cultivation. The famous Ifugao rice terraces of northern Luzon are one example, although they contribute a relatively small proportion of the national total of rice production and are not representative of typical Filipino rice culture.

Terrace building and water control, reputedly the "eighth wonder of the world," is a distinctive cultural phenomenon of Asia. Terracing, as done in many parts of Asia, consists of building stone walls, often without mortar, along the slope and contours of uplands being converted into cultivated land. The walls might be given a slight batter (receding upward slope) or inward lean at the top. These walls often exceeded six meters in height and were topped with sluices for water drainage. Earth materials would then be used to fill each unit to a level just below the top of the wall. By then staggering the field units in overlapping manner at different levels, a whole mountain slope might be terraced toward the top such that it resembled a sculptured system of fields covered with rice and other types of crops. The field units may be as narrow as 2.5 meters but as long as 90 meters on steep slopes. They are, however, commonly much wider and not so long. For irrigation, water is usually introduced at the top of a terrace series or sometimes as a complementary supply at an intermediate point. The water that is supplied then moves through sluices in each field by gravity downward across each level of the terrace. Finally, the water empties into a stream channel at the bottom of the valley.

Water control, which is crucial in the use of terraces, consists of careful adjustment of the volume of water in the beginning and, hence, its distribution throughout the system of field units making up the terraced series. The system must therefore be able to bring in the required volume of water at any time during the growing season. The system must also be able to divert excess water at any point at any time.

The types of terrace walls built and the materials used vary across Asia. While the Ifugao in northern Luzon build terrace walls of stone with a slight batter, the Bontoc people in the same region build stone walls with almost no batter. By contrast, the Kallnga, also in northern Luzon, use a stone base wall and upper walls of earth with a high batter. Some groups build a few high-quality terrace walls with only a few levels, while others are known for terracing that is relatively sophisticated.

In Bali, both terracing and irrigation practices are elaborately organized through a remarkable system of aqueducts, and in some cases water is carried via tunnels through hillsides in order to distribute it from reservoirs to the cultivated fields.

Tremendous amounts of labor and toil go into the construction of the terrace walls, from the gathering of the earth fill for each field unit to the maintenance and repair that ensures the soundness of the terrace walls. Skilled water control, however, is the most important factor in maintaining good soil as well and high productivity of the field units.

With the possible exception of Java in Indonesia, all the main rice-growing areas of Southeast Asia acquired the basic method of wet-rice cultivation by cultural borrowing from India or China. The skills of some Southeast Asian wet-rice cultivators have declined rather than improved within recent times. The Thai farmers who had first learned of irrigated cultivation from the Chinese farmers of Yunnan, and who would have been familiar with the art of hillside terracing for irrigation, apparently lost this art in the course of migrating to the Menam lowlands. Similarly, in peninsular Malaysia, the Malays, who it is assumed learned their wet-farming techniques from the upland peoples of Sumatra in Indonesia—where, again, terracing was in widespread use for irrigated cultivation— also show no knowledge of the practice.

Terracing of the fields not only served irrigation purposes but also provided a high degree of protection against the ever-present threat of soil erosion as well as loss of soil fertility. In many areas of rice and other crop cultivation using terraced irrigation and such traditional methods of farming, cultivation has continued for hundreds and even thousands of years without any serious erosion or loss of soil fertility. This has been the case particularly in areas where irrigation or floodwaters bring abundant mineral matter and where there is deposited a seasonal layer of fertile silt.

The existing area of wet-field terrace systems has grown probably because of population pressures on land as well as land scarcity in areas such as mountainous parts of Asia. Other crops apart from rice that are cultivated in terraced fields include corn, cabbages, potatoes, beans, bananas, and even coffee trees.

Yue Choong Kog

Dutt, Ashok K. (1996) Southeast Asia: A Ten-Nation Region. Dordrecht, Netherlands: Kluwer Academic Publishers.

Fischer, Charles A. (1964) Southeast Asia. London: Methuen and Co.

Wernstedt, Frederick L., and J. E. Spencer. (1967) The Philippine Island World. Berkeley and Los Angeles: The University of California Press.

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