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Characteristics of Drainage Water Quality and Loading from Paddy Field under Cyclic Irrigation and Its Management Options

In Japan, the cyclic irrigation system is encouraged through the national subsidy for system installation and pump operation and has been practiced for reducing pollutant outflow loadings from paddy fields. The cyclic irrigation pumps drainage water and reuses it as irrigation water to the farmland. Quantification and assessment of the effects of the cyclic irrigation are needed to identify management options for maximizing the benefits of cyclic irrigation. The study was aimed at assessing loading characteristics from paddy field area under the cyclic irrigation and developing a model for simulating water and material flow in paddy field area that can be used as a management tool.

The study was carried out in a paddy field in the Asagoi District, Oumihatiman city in Shiga Prefecture, Japan. The field data were collected from April 2010 to September 2012. And during the monitoring period, four sampling points were set up; three in the main drainage canal and one at the Biwa Lake Pumping Station. The automatic water level sensors were installed at each sampling point in main drainage canal. And during the normal irrigation period, water samplings and flow measurements were conducted once every two weeks; during the land preparation period, samplings and measurements were carried out every day owing to the high amount of drainage loadings and variations of water quality/quantity. The concentrations of total nitrogen (T-N) and total phosphorus (T-P) were analyzed by using the Absorption Spectrophotometry method. In addition, cyclic irrigation model was developed using System Dynamics Model (Vensim Professional).

The result indicated a higher nutrient absorption rate in the cyclic irrigation site than that in the non-cyclic irrigation site. The developed cyclic irrigation model showed good agreements between observed and simulated drainage volumes and nitrogen loadings. The scenario analysis by application of the model showed a potential of reducing the loading amount by increasing the cyclic irrigation ratio and reducing the amount of fertilizer application without affecting the rice yield.

Article by Nobumasa Hatcho, et al, from Japan.

Full access: http://mrw.so/1OLPQv
Image by Prakash Shankaran, from Flickr-cc.

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