Groundwater HIA post edit - FreshwaterLife

The process of developing a conceptual model is as follows:
• Start with initial ideas, such as observations, hypotheses and areas of
uncertainty, and write them down.
• Test the model, by for example doing some crude water balance
calculations with long-term average values for the various water balance
components.
• Based on the results of the testing, re-evaluate the model, rejecting some
hypotheses, keeping some and developing some new ones, as necessary.
• Test the improved model, and then continue the cycle of re-evaluation and
testing until the initial ideas become the best available conceptual model,
as appropriate for the problem being addressed.
It is worth repeating the point that conceptual modelling is continuous and cyclical; it is
a process, not a finished product. It is also important to realise that the degree of
development of a conceptual model is determined by the availability of data, and the
sophistication of the tools that have been used to test the model. Bredehoeft (2005)
introduces the phrase 'hydrogeologic surprise', which he defines as the collection of
new information that renders one's original conceptual model invalid. From limited
empirical data, he estimates that such surprises occur in 20-30 per cent of model
analyses. Rushton (2003) goes further, saying that it is his experience that in each
modelling study at least one fundamental feature was not identified in the initial
conceptual model (but became clear in subsequent modelling cycles).
Superimposed on the continuous cycle of model development and testing is a
hierarchical or tiered approach, with basic, intermediate and detailed levels of model.
These tiers can be described as follows:
Tier 1 (Basic): Tested using lumped long-term average water balances
and simple analytical equations, to arrive at a ‘best basic’ conceptual
model.
Tier 2 (Intermediate): Tested using more detailed data, such as timevariant
heads and flows, and more sophisticated tools, such as seasonal or
sub-catchment water balances (semi-distributed), analytical solutions (to
investigate the impact of abstraction on river flows, for example), or twodimensional
steady-state groundwater models.
Tier 3 (Detailed): Likely to be tested using a spatially-distributed and timevariant
numerical groundwater model, calibrated and validated against
historical data.
The tiered approach to conceptual modelling is illustrated in Figure 2.2, from which it
can be seen that the conceptual model is refined within each tier from an initial
understanding to the best available model. The diagram also illustrates that associated
with each tier is an assessment of the risk involved in the decision being made.
As the investigation progresses through the tiers, the cost increases, but so does the
confidence in the model. As confidence increases, so the uncertainty decreases, and
the investigation should continue up the tiers until the uncertainty (and therefore the
risk) has been reduced to an acceptable level. The level that is considered acceptable
depends of course on what the conceptual model is being used for. Common sense
must be used, and in general, decisions should be made with the simplest model
possible, with refinement of the model required only if a decision cannot be made
because the uncertainty is still too great.
8 Science Report – Hydrogeological impact appraisal for groundwater abstractions