Generic model codes are effective and efficient tools, because they are not limited to use for one specific geographic area, one specific time period, or one specific problem. They are developed on the basis of a conceptual understanding of the natural system in terms of narratives, equations, governing relationships, or natural laws.

Models become powerful and reliable tools for water managers, when they are applied correctly. The credibility of a model's output for decision-making depends both on its ability to represent the natural system and the user's ability to ensure that it represents the system correctly by going through specific procedures of model confirmation, verification, calibration and validation. These steps ensure that

• a correct understanding of the natural processes,

• a correct description of these in terms of mathematical equations,

• a correct set of parameters developed by comparing the model output with observed data, and

• a correct prediction capability through model validation are achieved.

Successful development and use of a model in IWRM depends on several factors, including:

• Awareness of, and access to, appropriate model software

• Availability of qualified staff to undertake the modelling studies

• Availability of reliable data to create a model which can provide good decision support

• Appropriate guidelines/agreements for accepting the model results.

It is important to emphasise that the results and findings to be used in an IWRM process must be accepted by all parties involved, i.e. the decision-makers and the stakeholders. This requires openness and transparency in all phases of the model development and application in which the modelling principles, procedures and criteria are established. Furthermore, it is important to assess the model responses in relation to the data quality and availability and the associated uncertainties.

(GWP 2013b)