Development of the transboundary large river watershed model for hydrology and water quality using modified SWAT setup procedure

The aim of the study is to assess a different approach of SWAT model setup with enhanced HRU definition procedure. The method uses a set of GIS process models based on ArcPy and customizable MATLAB scripts (hence, SWAT-LAB) to produce hydrological response units (HRUs), as defined in SWAT, from a combination of topographic, soil, landuse and administrative unit rasterized datasets. Moreover, the applied approach uses a combination of subbasin and hillslope discretization procedures, with the aim of creating a model setup, which is flexible, can be used for large scale models with substantial amount of subbasins and HRUs. The model input data set assembled, therefore, produce a physically more sound model setup than the subbasin delineation alone would have at an acceptably low increase of the number of land elements (subbasins and HRUs) within the watershed. This method was used to setup, run and calibrate a large-scale transboundary Nemunas River model, consisting of eleven sub-models and link them from upstream to downstream. Nemunas River is the major contributory that discharges into the Curonian Lagoon, which is the largest European coastal lagoon. Nemunas River basin is shared by Belarus, Lithuania, Poland and the Russian Federation Kaliningrad Oblast. The river basin is under nutrient load pressure from different sources in the riparian countries, nevertheless, the burden of improving the water quality of the river falls mainly on Lithuania (as defined by the HELCOM Nutrient Reduction Scheme). Currently, a good measure of confidence was achieved in several calibrated sub-models, representing upstream sub-watershed of the Nemunas River basin (48% of the total watershed area), which are situated outside the territory of Lithuania, for hydrology, sediments and nutrients (TN and TP), thus providing insight of the possible hydrology change and nutrient loads to Lithuania from neighboring countries. This ongoing research will further develop the modeling system and use it for assessing the hydrology, sediments and nutrients of the entire Nemunas River watershed under different land management and climate change scenarios, as well as assessing the best management practices for nutrient load reduction.