Abstract
Various types of models are being used to quantitatively assess the transfers of plant nutrients (nitrogen, N; phosphorus, P; silicon, Si) from land to streams and via rivers to coastal waters. Two model categories can be distinguished. One category is that of lumped river-basin scale models. The second category consists
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of distributed approaches. Lumped river-basin scale approaches are statistical models that relate lumped or aggregated river basin characteristics to river export to coastal waters. However, these lumped models do not help to improve our understanding of the changing biogeochemistry within river basins. The distributed approaches consider all components of landscapes (soil, vegetation, land types, riparian zones, wetlands, streams, lakes and reservoirs) to simulate nutrient transport and processing through various hydrological transport pathways (surface runoff transport, leaching, groundwater transport, direct discharge from sewage and atmospheric deposition). In contrast to the lumped approaches, the distributed models improve our understanding of the interaction of multiple processes within different landscapes and predict the biogeochemistry and temporary storage of nutrients along the river basin as a result of long travel times.
This thesis presents simulations of the N, P and Si transport from land to sea based on the distributed, spatially explicit, mechanistic Integrated Model to Assess the Global Environment-Dynamic Global Nutrient Model (IMAGE-DGNM). A new extended version of the DGNM wastewater model and its application to compute nitrogen (N) and phosphorus (P) emissions in wastewater and their fates in rural and urban areas in China, covering 1970-2015. A new model and applications to calculate silicon transport and biogeochemical processing in the Rhine and Yangtze River basins and the eventual export to the coastal seas for the period 1900-2010. The model is, by using long-term changes in the drivers of the Si cycle, and coupling climate, land use and hydrology with in-stream processing, a major step forward compared with regression models, basin-scale box models and reservoir box models that all produce snapshots for individual years and lack the capability to simulate legacies. The results demonstrate that the current nutrient load being exported from the Yangtze River basin toward the East China Sea and the Yellow Sea is much larger (almost twice) than previously estimated based on statistical models such as Global NEWS. Our revised nutrient export estimates are important, as the Yangtze River is one of the largest rivers in the world with a significant portion of the Chinese agricultural production and population. The thesis explores nutrient dynamics in four river basins within four climate zones during the past century, the rivers Rhine, Mississippi, Yangtze and Pearl. The experience of nutrients reduction policies obtained in the Rhine and Mississippi river basins can provide valuable lessons to other polluted river basins worldwide, such as those in China.
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