Water management balance of the Ros River basin: optimization problems and water use conditions during low water periods

The main components of the water balances for the Ros River basin as a whole, its water management areas (WMAs), and individual sub-basins were analyzed. This included determining the volumes of surface runoff from the main river and lateral tributaries, groundwater withdrawal, and return water. Calculations were also performed for water losses due to increased evaporation from water bodies and filtration from them. Furthermore, water use volumes and minimum ecological flow requirements within each WMA and sub-basin were quantified. The developed water management balances allowed for an assessment of water resource availability and usability within the basin, considering their quantity and development level under various hydrological conditions. To perform a detailed water management balance, observation series from 4 hydrological stations of the Ukrainian Hydrometeorological Center’s stationary network were utilized. Additionally, data from 7 hydrological stations of the Regional Office of Water Resources of the Ros River (ROWR of the Ros River) were used, referred to gauges located at the mouths of the main tributaries of the Ros River and at the Kosiv Reservoir Dam. Since the hydrological observation series from the ROWR of the Ros River were rather short, a procedure was undertaken to extend the statistical parameters of the average monthly and annual water flows to a multi-year period using analog rivers. The established correlation relationships facilitated the restoration of observation series for the Ros River Regional Water Resources Monitoring Center’s hydrological cross-sections, allowing for the construction of flow duration curves and the identification of years with exceedance probabilities close to 50%, 75%, and 95%. Results from the developed water management balances indicate that in years of average flow (50% exceedance probability), the water resource deficit period will last from 2 to 6 months for 50% of the analyzed sections. In low-flow years (75% exceedance probability), this applies to 75% of the sections, with a deficit duration of 2 to 10 months. During severe low-flow years (95% exceedance probability), a water resource deficit will be experienced throughout the year for 4 studied sections, while for others, the deficit period will range from 2 to 8 months. These findings are valuable for optimizing the water resources management process. Calculating the water management balance of the Ros River basin (with details for individual WMAs and selected sub-basins) for specified exceedance probabilities (50%, 75%, and 95%) on a monthly basis, considering current water use levels, will enhance the information resources of the State Water Agency of Ukraine and the Regional Office of Water Resources of the Ros River. This will ultimately contribute to saving specialists’ working time when making decisions on issuing special water use permits, regulating water consumption and discharge, and assessing water quality indicators.