The variation of dissolved nitrogen discharge at a flooded river in a suburban catchment

A river contains self-purification mechanisms such as denitrification and assimilation by plants. Although these mechanisms have extensively been studied, during a rainfall event when a river's flow differs from its normal condition, it is not clear what effect this has on its self-purification mechanisms. Therefore, to clarify the transformation and self-purification process of nitrogen, we focused on attention on variations in the discharge and nitrogen flux following three rainfall events in the Kurose River in Hiroshima Prefecture. Our observation site was put established 17 km from the riverhead. Water reaching the site flows through both urban and agricultural area. During three rainfall events, we sampled the river water at 2-hours intervals using an automatic water sampler and measured the water level in the river by means of level indicator. Water samples were analyzed for dissolved nitrogen (DN), NO₃^--N, NO₂ ^--N, and NH₄⁺-N. The discharge was calculated from measured water level data and H-Q curve, and the flux from the product of concentration and discharge. Our results showed that DN flux varied inversely to variations in the discharge, and that its main factor was a decrease in the NO₃^--N flux. This indicates that the variations in upstream conditions involved changes in the discharge influenced the nitrogen discharge in the river.