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By Daniel Collins

When I see ads for New Zealand’s Next Top Model, I sometimes share a quiet chuckle with myself. I’m notorious for puns, I’m afraid, and the show’s name is ripe for the picking.

To many hydrologists, TOPMODEL is the name of a hydrological model developed by Keith Beven and Mike Kirkby in 1974. It describes how the wetness of ground in rolling hill country relates to upslope area and local slope (the “TOP” refers to topography). Back in 1997, my colleague, Ross Woods, and another of his colleagues visiting NZ at the time, took the idea of TOPMODEL and applied it across a river network. They christened the new model TopNet. TopNet has since become our primary catchment hydrology model, used for all sorts of studies across New Zealand, and as such it already is New Zealand’s next TOPMODEL.

Back in May I was filmed for the new season of ‘Ever Wondered?’ and I talked about TopNet and hydrological modelling in general. (You’ll have to wait until at least August to see the programme.) In a nutshell, TopNet is a computer model, comprised of umpteen mathematical equations, that simulates the movement of water through a landscape, from rain to river discharge. If you’re interested, I’ll explain my philosophical approach to modelling in another post.

As for its applications, we’ve used TopNet to make projections of river flow under climate change, and of flooding after land use change. We’ve used it to estimate the water balance of the regions, as seen in the Statistics NZ Water Stock Accounts mentioned previously. I am hoping that we can also use it to infer what rivers were like about the time when Polynesians arrived, and Europeans. It’s a great tool to help us understand the hydrology we have today, and how it can change.