Redistribution of dissolved inorganic nitrogen loading and transport in global rivers via surface water regulation

Surface water (SW) regulation, including reservoir regulation and surface water use, alters the soil-river hydrological processes and then influences the dissolved inorganic nitrogen (DIN) transport from rivers to oceans. However, global response of the DIN transfer to such human activity has not been well investigated. Therefore, in this study, we have taken advantage of a recently-developed land surface model to show the effects of SW regulation on DIN loading and transport in global major rivers. Three simulations were conducted at the global scale over the period of 1991-2000. Results show that, acceleration on riverine N loading caused by the SW regulation was more noticeable. With the reservoir regulation only, removal rates of DIN in most rivers were reduced by 5 %, up to 20 %, while the increase of removal rate in some reaches occurred when the DIN was transferred back to soil through SW withdrawal and use. Influenced by the SW regulation, DIN flow has decreased obviously by 100-1000 t N yr^-1. Mean DIN flow was most significantly reduced in the Yellow River basin. Yangtze River, Mississippi River, and Nelson River suffered from the strong effect of such water regulation. The DIN exported to oceans was reduced as well. Mean reduction of 40-50 % occurred for those exported to the Pacific Ocean, the Atlantic Ocean and the Indian Ocean, while slight effect for the estuaries into the Arctic Ocean. The impact on the DIN exported to the Atlantic Ocean was lasting decreasing from 50 % to 40 %, while the trend of the impact on that exported to the Pacific Ocean was opposite. There was no evident temporal change in the impacts on the DIN transferred to the Arctic Ocean and the Indian Ocean. The findings of this study could provide new insights for the researches on nitrogen change in rivers under the impacts of human water use.