Exploring Nutrient Dynamics in the rivers Rhine and Pearl using the IMAGE-GNM model for the period 1900 – 2000

Abstract

Nitrogen (N) and phosphorus (P) dynamics in the Rhine River and the Pearl River from 1900 to 2000 are modelled using the IMAGE_GNM model. The modelling results illustrate the spatio-temporal changes of N and P delivery to rivers, exports to mouth, the contributions of nutrient sources and the in-stream retention. The modelled TN and TP exports to the sea from the Rhine River increased from 190 to 388 Gg yr-1 and from 22 to 51 Gg yr-1, respectively, between 1900 and 1970. With policies and measures carried out to limit the nutrient load, N and P from agricultural source and sewage wastewater decreased distinctly during 1970 - 2000. The effect of improving wastewater facilities has a quick effect on cutting down the nutrient load. But the legacy effect of the N and P in soil and groundwater delays the visible effect of agricultural reduction measures. Without regulatory on agricultural fertilizer use and proper wastewater treatment, the nutrient transport in the Pearl River increased over the entire 20th century. N and P exports to mouth increased up to 960 Gg yr-1 and 75 Gg yr-1 till 2000. Measures were taken to reduce nutrient load in the 21st century in the Pearl River basin, mainly focusing on improving the wastewater facilities. The modelling results show that agricultural N and P account for 80% and 89% of the total loads to the Pearl River, respectively. Therefore, we recommended that regulating agricultural N and P loads should be the top priority for the Pearl River.