Journal of Japan Society on Water Environment

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The effluent load generated from paddy field situated along Lake Kasumigaura during the puddling and transplanting period was investigated at an irrigation and drainage pump station. When the pump was in operation, effluent water from the area was pumped up from the bank-based water canal with lake water and circulated as irrigation water. However, the effluent flowed to the lake at a velocity of about 0.01 m·s^-1 when the pump was stopped. The net effluent loadings of total nitrogen (T-N) and total phosphorus (T-P) during the puddling and transplanting period were -0.06 kg·ha^-1·d^-1 and -0.003 kg·ha^-1·d^-1, respectively. The loads were smaller than those for other regions where the irrigation and drainage systems were different. The cyclic irrigation and drainage system in this area contributed to the reduction in load. When effluent water from the area was not circulated, the kasumigaura outflow discharge was estimated to increase by 5.7 times, and T-P load was suggested that Kasumigaura outflow load became bigger than Kasumigaura inflow load.

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