SWAT Runoff Modeling and Salinity Estimation in the Odra River Catchment

Abstract: In 2022, a significant fish die-off happened in the Odra River due to a bloomof the toxic gold algae species Prymnesium parvum, which occurs in saline andwarm waters. This event is seen as a major ecological disaster in recent Europeanriver incident history. Understanding the hydro-climatic factors and basin characteristicsthat led to this disaster is crucial to prevent similar occurrences inthe future. Multiple national and international research groups have been activelystudying this event, reconstructing its chronology and analyzing the toxicalgal bloom. However, the specific relationship between the algal occurrence andhydro-climatic conditions, as well as the potential for recurrence, remains largelyunexplored. This project utilizes the semi-distributed hydrological model Soiland Water Assessment Tool (SWAT) on a daily time scale to investigate runoffbehavior during the disaster year. The main goal is to assess SWATs capability tosimulate runoff in a large-scale watershed. The obtained results are then analyzedin relation to water quality parameters and climatic conditions. The focus is onunderstanding the relationship between runoff and salinity, which is further examinedthrough a climate change scenario analysis to evaluate the potential riskof increased river salinity due to a change in the flow regime in the future. Byimplementing two Shared Socioeconomic Pathway (SSP) scenarios in the calibratedand validated SWAT model, a relative trend of runoff behavior and salinityin the Odra River is examined. The findings indicate that SWAT effectively simulatesrunoff in a large-scale catchment, despite limited information and computationalresources. The analysis reveals a significant relationship between salinityand runoff. The climate change scenario analysis predicts a decrease in runoffunder the pessimistic scenario, while the optimistic scenario suggests a more balancedrunoff trend. Additionally, the estimation of future river salinity suggestsan increase under reduced runoff conditions, and a subsequent recovery in salinitylevels for high discharge conditions. This thesis provides insights into the2022 disaster event in the Odra River and highlights the interplay between hydroclimaticfactors, catchment conditions, and toxic algae blooms. The findings contributeto a better understanding of potential risks and can give information foreffective water management strategies to protect the river ecosystem.