Modelling the Impact of Climate and Socio-Economic Changes on Nutrient Dynamics in the Catchment of Lake Vomb

Abstract: Climate change and socio-economic development are greatly affecting the quality of freshwater, especially the excessive accumulation of nutrients (N and P), which eventually leads to the occurrence of eutrophication. Lake Vomb is one of the main sources of drinking water in southern Sweden and the nutrient load from the catchment makes it a eutrophic lake with recurring algal blooms. This project developed a hydrological model by Soil and Water Assessment Tool (SWAT) to model the impact of climate and socio-economic changes on nutrient dynamics on Lake Vomb. Two combinations of future scenarios were selected as combinations of Representative Concentration Pathways (RCP) and Shared Socioeconomic Pathways (SSP): 1) SSP1/RCP4.5 and 2) SSP5/RCP8.5. For the calibration and validation period, satisfactory results were obtained for monthly flow (R2 and NSE were 0.92 and 0.82, respectively) and yearly nutrient load simulation obtained overall convincing results (R2, NSE and PBIAS were 0.67, 0, 17% for total nitrogen (TN) and 0.78, -0.11, -1% for total phosphorus (TP) at the main inflow and R2, NSE and PBIAS were 0.46, 0.27, 15% for TN and 0.62, -0.01, -0.06% for TP at the second inflow). Simulation results of increased future flow scenarios showed higher peaks under the RCP8.5 scenario than under RCP4.5. Also, there was an increasing trend that flow will continuously rise during the simulation period. Results from future nutrient load simulation showed that the TN load was below the baseline in most scenarios and the TP load was all above the baseline. Besides, nutrient loading is more sensitive to the combination of SSPs and RCPs and got the highest loads under RCP4.5/SSP1. The challenge of reducing nutrient load increases over time, especially for P because the simulated load was higher for 2090-2100 than the load for 2040-2050. This project has limitations such as uncertain agriculture activities input and nutrient observations. This project provides projections of flows and nutrient loads into Lake Vomb for a combination of possible future scenarios and more different scenarios such as different RCPs and different designed SSPs deserve to be studied in the future.