東北タイにおける河川の流出特性と伝統的灌漑の技術様式

Abstract

Earthen weirs called thamnop have been widely used for rice irrigation in northeastern Thailand (NortheastThailand) and northwestern Cambodia. The most distinctive feature of thamnop irrigation is that it divertsthe entire river runoff into riverbanks. Complete blockage of river channels causes even floodwater to beflushed into riverbanks without any human control. This study discusses why thamnop can function withoutfrequent collapse and severe damage to human life and property and why such an irrigation method ispreferable, from a hydrological perspective. Results of a spectral analysis of fluctuation in daily runoff showthat even in the period immediately after rain, runoff of rivers in northeastern Thailand tends to fluctuatein a much more gradual manner than that in northern Thailand and Japan. Thamnop have been constructedon rivers that have no destructive flush runoff or storm flow. Analysis results with Filter SeparationAR Method show that runoff of rivers in northeastern Thailand consists mostly of surface runoff. Inother words, they do not have stable base-runoff that is usually used for weir irrigation. Because of instabilityand unpredictability of runoff, the locals use thamnop for maximizing the water intake. Such gradualbut unreliable river runoff is caused by the mountain-less terrain of northeastern Thailand. Therefore theuse of thamnop for irrigation is the optimum adaptation to the hydrology of northeastern Thailand.