Characteristics of water reuse and its effects on paddy irrigation system water balance and the riceland ecosystem

Rapid industrial development in the rice-growing regions has increased competition for the scarce water resources. Water reuse (surface and subsurface agriculture drainage water, storm runoff, sewerage effluent and industrial wastewater recycling) is in widespread use as a method of supplementing the paddy water supply, therefore, there is a need to clarify its effects on the paddy system water balance and riceland ecosystem. Field data and simulation results from a complex runoff model (created on a daily basis), were used to estimate the water balance and assess the effects on the irrigation system of a water shortage area in Niigata Prefecture, Japan. For three years (1991–1993), the average water reuse component was within the range 14～ 15% of the total irrigation water supply. Apart from meeting the water needs at peak demand periods, water reuse is a quick-response water supply solution during dry spells, increasing both the water reliability and crop security. To understand the impact of water reuse on the riceland ecosystem, its effect on total phosphorus (T-P), total nitrogen (T-N), suspended solids (SS), and chemical oxygen demand (COD) was assessed. Blending of the drainage water was done to reduce the irrigation water concentrations, to approximate the traditional dual canal system and to increase farmer satisfaction with the water reuse system. Apart from the fact that the amount of return flow drained out of the irrigation system was reduced when drainage water was reused for irrigation, the effluent load reductions for SS and T-P owing to water reuse were consistently high. Hence, water reuse not only helps meet irrigation water needs, but also aids purification of the agricultural drainage water and preservation of the riceland ecosystem.