I think that one of the problems is that a model is restricted in its outcomes by the accuracy of that model, if you have not thought out the parameters fully, then the model will not accurately reflect reality.
There was a similar issue a few years back with climate change models, they did not include the global water currents, for example the gulf stream that keeps the UK warmer than other areas at the same latitude. now it was known that in the past the great atlantic conveyor (a cold current of water at the bottom of the atlantic) had ceased to flow, it it this stream that generates just about every other ocean current, but it was not included in any of the climate modelling - why?
now although the modelling for the broads must be simpler than those models, is there anything that has been overlooked in the models, as until all the factors are correctly in place, no model will be accurate, I mean does the model account for the fact that Morris Mynah washes his socks on a thursday when on his boat, and this creates a side flow of the currents around the pleasure boat, although this may have a negligible effect upon the flow of the rivers, who are we to know what may or may not count and in what way.