PS 37-30 - CANCELLED - The use of the eutrophication risk index as a tool for managing water resources in the São Francisco River basin in Brazil.

Abstract: Human activities have altered biogeochemical cycles around the world. Developing practices that promote the efficient use of nutrients for food and energy production can reduce threats to social and economic well-being. Many Latin American countries need to focus on improving nutrient management, particularly in reducing nutrient water pollution.The most significant effect of high nutrient transport rates into aquatic systems is eutrophication. This work aims to evaluate nutrient use practices in the São Francisco River basin (SFRB), to develop a Eutrophication Risk Index (ERI) using a methodological approach that considers specialized soil P balance and nutrient loading from urban wastewater. The ERI is given by the contribution of P to aquatic environments due to the amount of effluent from urban areas and the excess of nutrients applied to the soil calculated in kilograms per hectare per year (kg ha^-1 year^-1). Therefore, the ERI for the SFRB is calculated using an equation with two main components: one related to nutrient loss from agricultural systems and another related to nutrient loss in wastewater. We show that most nutrient exported to waterbodies comes from agricultural practices in Uppe, Middle, Sub-Middle, and Extended sub-basins (65 to 98%). In contrast, the Lower sub-basin waterbodies receive more nutrients from urban areas (54%). We observed considerable heterogeneity in the P exported between the sub-basins in the SFRB. The Upper SFR sub-basin corresponds to an area of 111.8 km² and exports 0.51 kg P ha^-1 year^-1 on average. Due to the prevalence of highly intensive agricultural and pastoral lands, the Medium SFR sub-basin (339.8 km²) exported more P than any other region on average (1.83 kg P ha^-1). The sub-medium SFR sub-basin (155.6 km²) exported a total of 0.46 kg P ha^-1, while the low SFR sub-basin (32 km²) exported 0.22 kg P ha^-1. Although agricultural areas represent the main source o P in the SFRB, peaks of P export are observed in highly urbanized municipalities, reaching maximum values of 27.9 kg P ha^-1 year^-1 in the Upper SFR sub-basin. The proposed index showed to be a good indicator of regions where human activities can bring a risk of water impairments due to the increase of phosphorus concentration. Understanding the contribution of P losses to the water quality impairment at a drainage basin scale is of fundamental importance to guide measures aimed at protecting water resources and identifying areas of greatest risk to the integrity of ecosystem services.