One of the key features of nature-based solutions (NBSs) is their high effectiveness and economic and resource efficiency in solving problems compared to traditional grey interventions. These aspects, however, have so far attracted little attention in the literature but should be considered, as the evaluation of NbS projects before implementation provides an opportunity to use them as practical tools. This paper aims to present a new evaluation framework of NbS projects (at the stage of solution selection) based on the application of performance questions and the indicators approach. The framework consists of three main stages: (1) the formulation of the project aim(s), including the problem(s) to be solved, the scales of the expected effects and the thresholds to be achieved; (2) the elimination of solutions that do not fit the local conditions and setting requirements; and (3) the examination of the performance questions. To consider the whole spectrum of influencing factors, a systematic review was performed to indicate the main concepts related to the issue of NbS effectiveness, including (1) stakeholders' participation, (2) policy and management capability, (3) economic efficiency, (4) analysis of synergies and trade-offs, (5) adaptation to local conditions, (6) adequate spatial scale and (7) performance in the long term. In the current study around 130 indicators were identified for the evaluation of NbS projects. Regarding the impact on the state of knowledge, the present work contributes to the discussion on the conceptual sphere of NbS and its practical implementation by introducing multi-perspective standards and guidelines for the selection of possible NbS projects, and it deals with the complexity of synergies and the trade-offs between benefits.
