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