東北タイにおけるタムノップ灌漑と天水田の発生 <i>Thamnop</i> Irrigation and the Evolution of “Rain-fed” Rice in Northeast Thailand

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Abstract

Northeast Thailand is known as a typical “rain-fed” rice area. Archival materials and interviews with village elders, however, indicate that “rain-fed” rice land began to appear only around the mid-20th century, and until then, most rice lands had been irrigated by a kind of earthen weir diversion system. The system consists of a higher-than-bank earthen bund, called <i>thamnop</i> (probably of Khmer origin), across a stream, which diverts the whole volume of the stream flow into adjacent rice fields with or without canals and ditches. The diverted water irrigates the rice, and the excess flows through paddy fields and eventually returns to the original stream downstream. The region is an erosional plain under the hot and humid climate. Base rock beneath the plain is chemically weathered at a great depth, leaving only fine materials near the surface. The <i>thamnop</i> system can be seen as an adaptation to an environment void of stone materials.<br>Today, <i>thamnop</i> are disappearing. The rice area of the region increased by about eight times in the first half of the 20th century, and two and half times in the second. This staggering expansion was made possible by the drainage of lowland on the one hand, and reclamation toward higher ground that cannot be irrigated by the <i>thamnop</i> on the other—that is, the evolution of “rain-fed” rice fields. The expansion toward higher ground has affected the <i>thamnop</i> system adversely. First, the stream flow has been greatly reduced due to the declining run-off ratio caused by a greater retention of water upstream by bunded “rain-fed” rice plots. Second, the bottom land to be irrigated by the <i>thamnop</i> system has been reduced to a small portion of the total area under the rice at the regional as well as the village level. The changes in villages' economic structure due to the commercialization of rice and other export-oriented crops, and off-farm jobs have caused a shift in farmers' priority from food security to profitability. This has made the assured harvest by the <i>thamnop</i> less relevant.<br>Around the Tonle Sap in Cambodia, the rectangular earthen bund with its opening towards the bank, called <i>tnup</i> (a corruption of <i>thamnop</i>), is used for the retention of receding water towards the end of the rainy season for irrigating dry season rice. Although the erosional plain and the fans between the lake and Kulen hills are often described as “rain-fed” areas, there actually exist numerous <i>thamnop</i> apparently still playing an important role in village society. Although the antiquity of the <i>thamnop</i> is not known, it appears likely that the <i>thamnop</i>-irrigated rice production was one of the irrigational forms existed during Angkor times. Scattered records in Vietnam, Burma, and southern Thailand suggest that the system might have been distributed throughout Southeast Asia.

Journal

  • Southeast Asia: History and Culture

    Southeast Asia: History and Culture 2006(35), 53-73, 2006

    Japan Society for Southeast Asian Studies

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