When is uncertainty assessment required?

The modelling process

A modelling study will involve several phases and several actors. A typical WFD modelling study will involve the following four different types of actors:

·        The water manager, i.e. the person or organisation responsible for the management or protection of the water resources, and thus of the modelling study and the outcome (the problem owner).

·        The modeller, i.e. a person or an organisation that works with the model conducting the modelling study. If the modeller and the water manager belongs to different organisations, their roles will typically be denoted consultant and client, respectively.

·        The reviewer, i.e. a person that is conducting some kind of external review of a modelling study. The review may be more or less comprehensive depending on the requirements of the particular case. The reviewer is typically appointed by the water manager to support the water manager to match the modelling capability of the modeller.

·        The stakeholders/public. A stakeholder is an interested party with a stake in the water management issue, either in exploiting or protecting the resource. Stakeholders include the following different groups: (i) competent water resource authority (typically the water manager, cf. above); (ii) interest groups; and (iii) general public.

The WFD modelling process may, according to the HarmoniQuA project (Refsgaard et al., 2004; Scholten et al., 2004, www.harmoniqua.org), be decomposed into five major steps which again are decomposed into 45 tasks (Fig. 3). The contents of the five steps are:

·        STEP1 (Model Study Plan). This step aims to agree on a Model Study Plan comprising answers to the questions: Why is modelling required for this particular model study? What is the overall modelling approach and which work should be carried out? Who will do the modelling work? Who should do the technical reviews? Which stakeholders/public should be involved and to what degree? What are the resources available for the project? The water manager needs to describe the problem and its context as well as the available data. A very important task is then to analyse and determine what are the various requirements of the modelling study in terms of the expected accuracy of modelling results. The acceptable level of accuracy will vary from case to case and must be seen in a socio-economic context. It should, therefore, be defined through a dialogue between the modeller, water manager and stakeholders/public. In this respect an analysis of the key sources of uncertainty is crucial in order to focus the study on the elements that produce most information of relevance to the problem at hand.

·        STEP 2 (Data and Conceptualisation). In this step the modeller should gather all the relevant knowledge about the study basin and develop an overview of the processes and their interactions in order to conceptualise how the system should be modelled in sufficient detail to meet the requirements specified in the Model Study Plan. Consideration must be given to the spatial and temporal detail required of a model, to the system dynamics, to the boundary conditions and to how the model parameters can be determined from the available data. The need to model certain processes in alternative ways or to differing levels of detail in order to enable assessments of model structure uncertainty should be evaluated. The availability of existing computer codes that can address the model requirements should also be addressed.

·        STEP 3 (Model Set-up). Model Set-up implies transforming the conceptual model into a site-specific model that can be run in the selected model code. A major task in Model Set-up is the processing of data in order to prepare the input files necessary for executing the model. Usually, the model is run within a Graphical User Interface (GUI) where many tasks have been automated. The GUI speeds up the generation of input files, but it does not guarantee that the input files are error free. The modeller performs this work.

·        STEP 4 (Calibration and Validation). This step is concerned with the process of analysing the model that was constructed during the previous step, first by calibrating the model, and then by validating its performance against independent field data. Finally, the reliability of model simulations for the intended domain of applicability is assessed through uncertainty analyses. The results are described so that the scope of model use and its associated limitations are documented and made explicit. The modeller performs this work.

·        STEP 5 (Simulation and Evaluation). In this step the modeller uses the calibrated and validated model to make simulations to meet the objectives and requirements of the model study. Depending on the objectives of the study, these simulations may result in specific results that can be used in subsequent decision making (e.g. for planning or design purposes) or to improve understanding (e.g. of the hydrological/ecological regime of the study area). It is important to carry out suitable uncertainty assessments of the model predictions in order to arrive at a robust decision. As with the other steps, the quality of the results needs to be assessed through internal and external reviews.

Each of the last four steps is concluded with a reporting task followed by a review task. The review tasks include dialogues between water manager, modeller, reviewer and, often, stakeholders/public. The protocol includes many feedback possibilities (see Uncertainty aspects).