Safeguarding hydrological regimes appropriate for aquatic and water dependent ecosystems is necessary in order to achieve or maintain good ecological status (or good ecological potential) of water bodies, being an important task of sustainable water management. Water requirements of ecosystems are mainly represented by minimum required or ecological flows, which constitute a constraint to the use of water resources. The main objective of this paper is a comparison, in terms of economic effects, of the new ecological flow assessment method against the currently operational RZGW Szczecin hydro-biological method (2014), and the simplified hydro-biological method by Kostrzewa (1977). Another issue is to evaluate whether the ecological flow method is useful and applicable to water balance planning analysis. Computer simulation approach has been used. Calculated hydro-biological and ecological flow volumes were presented along the Wieprza River longitudinal profile and in the selected cross-sections. The time reliability of maintaining the flows as well as changes in volume of available water resources have been used to evaluate economic effects of the new method implementation. It has been proven that the application of ecological flow requirements causes a decrease in the available water resources for both existing and potential water users. Based on the results, selected issues have been discussed, related to the methodology of ecological flow determination, especially gaps and inconsistencies calling for additional assumptions.