The function of subsurface drainage system depends not only on the drainpipe but also on the condition of the filler material and the soil structure. This study aims to present the feasibility of functional diagnosis of the system for clayey paddy field by measuring the pressure head distribution along the water pathway from the plowed layer to the drainage canal. Three measuring sites (DA: Drainage canal side of Field A; IA: Irrigation canal side of Field A; and IB: Irrigation canal side of

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... ld B) were chosen very above the drainpipes in the two neighboring clayey paddy fields. The pressure transducers were installed at the bottom of the plowed soil, inside the drainpipes and the drainage canal. The tensiometers were installed at the plowsole and upper part and the bottom of the backfilled trench filled with rice husk filler. These points are on the route of the major water flow during the drainage of excess water in the clayey paddy field. At DA and IA, total head loss for the entry of water from the filler to the drainpipe was very high, while it was negligible at IB, where total head loss in the filler layer of the backfilled trench was the highest. The flow resistance, defined as the head loss divided by water flow rate, in the plowsole: R plowsole , was about 60 h (hours) at DA and DI, while it was higher at IA (275 h). The flow resistance in the filler (R filler ), was as high as 564 h at IB, while that between the filler and drainpipe (R entry ) was more than 200h at DA and IA (333 and 232 h). These results were consistent with the visual observations and the analysis of the husk filler taken at the field. Although the measurement of hydraulic head above the drainpipes is a promising method to analyze the cause of functional degradation, further studies on the spatial distribution of the vertical hydraulic head profiles are required to develop a procedure to know the averaged state of the subsurface drainage system of each field.