What is uncertainty?

Sources of uncertainty

Walker et al. (2003) describes the uncertainty as manifesting itself at different locations in the model based water management process. These locations, or sources, may be characterised as follows:

·        Context, i.e. at the boundaries of the system to be modelled. The model context is typically determined at the initial stage of the study where the problem is identified and the focus of the model study selected as a confined part of the overall problem. This includes, for example, the external economic, environmental, political, social and technological circumstances that form the context of problem.

·        Input uncertainty in terms of external driving forces (within or outside the control of the water manager) and system data that drive the model such as land use maps, pollution sources and climate data.

·        Model structure uncertainty is the conceptual uncertainty due to incomplete understanding and simplified descriptions of processes as compared to nature.

·        Parameter uncertainty, i.e. the uncertainties related to parameter values.

·        Model technical uncertainty is the uncertainty arising from computer implementation of the model, e.g. due to numerical approximations and bugs in the software.

·        Model output uncertainty, i.e. the total uncertainty on the model simulations taken all the above sources into account, e.g. by uncertainty propagation.