Modelling framework for flow, sediments and nutrient loads in a large transboundary river watershed: A climate change impact assessment of the Nemunas River watershed

Abstract

This study provides an in-depth analysis of the Nemunas River watershed, situated in the Baltic Sea basin, and possible future changes to the stream flow, hydrologic regime, sediment (SS), Total Nitrogen (TN) and Total Phosphorus (TP) load from the river to the Curonian Lagoon under different climate change scenarios using high-resolution modelling. The sub-regions of the watershed, represented by sub-basins of the Nemunas River tributaries and the main river branch, were modelled using the Soil and Water Assessment Tool (SWAT). The model setup was performed using the developed customizable MATLAB scripts for an advance Hydrologic Response Unit configuration and a hillslope delineation procedure. The modelling framework was used to assess the climate changes in the watershed under two future scenarios: RCP4.5 and RCP8.5, using the projected changes to precipitation, temperature, and carbon dioxide concentrations for the near-term (up to 2050) and long-term period (up to 2100) compared to the baseline period (1995–2010). The projections show that the annual long-term trends for flow, depending on the scenario, are small or not existing. Even if there is no visible change in trends over the years, inter-annual changes on flow, sediment and nutrient load will occur. The findings of the study suggest that the conditions of the RCP4.5 are likely to be more “wet”, whereas RCP8.5 will likely be “drier”. Most shifts will occur in the winter season, especially in January and February. A decrease in snow cover across the watershed, together with the greater frequency of soil freeze-thaw cycles can weaken the nutrient retention of soils and increase nitrogen and phosphorus losses. Coupled with the increased flows in winter, the projected nutrient load changes during winter season indicate a two-fold increase in sediment, up to 42% and 62% in TN and TP load to the Nemunas River and subsequently, to the Curonian lagoon.