QUALILOGICAL 2.0: a revolutionarily simple river water quality model ideal for planning restoration at river basin scale

The European Water Framework Directive (WFD) set the ambitious goal of achieving “good status” for all European waters by 2015. However, more than a decade later, this objective remains largely unmet. One of the main reasons is the difficulty in selecting appropriate and effective measures. Programs of Measures established under the WFD often reduce to mere lists of actions, known as Key Types of Measures (KTM), without being supported by quantitative analyses of cause–effect relationships. This gap is primarily due to the scarcity of reliable water quality (WQ) models that can be applied with reasonable effort and cost. Focusing on rivers, this paper first examines the structural and operational challenges that make the development of conventional WQ models complex, resource-intensive, and often unsuitable for standard planning. We then introduce a revised version of an alternative model, QUALILOGICAL, based on a variational approach. Unlike traditional models, QUALILOGICAL requires no calibration, yet it remains suitable for planning purposes. We outline its conceptual foundations, algorithms, and capacity to reproduce the desired system behaviour. Its strengths and constraints are plainly pointed out. The novelty and added value of this approach lie in its simplicity and low cost, combined with robustness, making it a practical tool for building rational and verifiable WQ management plans. While QUALILOGICAL provides only a simplified representation of reality, we argue that such a model, with clearly acknowledged limitations, may be more useful than highly sophisticated, but fragile models that are difficult to implement and validate.