59 / 2016-04-20 09:07:50

Impact of paddy field operation on river flow rate focusing on the farming calendar: a case study from the Eai River watershed, Japan

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In Japan, irrigated paddy fields, supplied by water intake from rivers, constitute the dominant farming method. Paddy field operation is determined by the farming calendar, which is a rice production manual. The farming calendar is used to control ponding water depth for each stage of rice growth, and the associated changes in the quantities of irrigation and drainage water, in detail. In addition, local meteorological conditions also affect paddy field operation. Therefore, for better description of the hydrological characteristics of paddy fields and their effects on river flow rate, it is necessary to define some rules, such as controlling water depth, and model the paddy field response to weather conditions, while considering the farming calendar.
In this study, field measurements were conducted in the Eai River watershed, which was one of the richer grain-yielding areas in Northeast Japan during 2014. Based on the measurements and the results of a water balance analysis, a water control model was built by specifying the values of water depth, the speed and timing of irrigation and drainage. Water depth, quantities of irrigation and drainage water in the paddy field, and variations in river flow rate were simulated under the actual weather conditions for each stage of farming calendar. The simulated water depth and river flow rate show good agreement with the measurement results. This result proved that the water control model can clearly explain the influence of paddy field operation on longitudinal variations in river flow rate during the irrigation period. Finally, a decreased range of river flow rates was calculated under drought conditions; these results suggest that water shortages may occur only during certain periods.