Look mama. No hands!

When did hydrology become computer science?

There was once a time when all hydrology was field hydrology. Rating curves, pan evaporation, curve numbers, unit hydrographs were actually derived from field data. Run-off coefficients were obtained from paired catchment studies.

At some point (maybe in the 1970s?), people seem to have declared that the field of hydrology had become a mature field. There were no more discoveries left to be made. The next exciting phase for the field was to harness the massive computational power unleashed by computers of ever increasing power. Indeed, computers generated many exciting possibilities and made large scale river basin modelling possible.

In reality, modelling is more art than science. Models are highly dependent on assumptions or parameters. For instance, we know in theory, that stream flow depends on stream roughness or soil hydraulic parameters, in practice, we can never measure the roughness in every section of the stream or the soil types in every square inch of the watershed. Ultimately, we guess  these parameter values and then keep adjusting the guesses so that the stream flow values predicted by the model match what is observed in the real world, through model calibration.

Early on, models were calibrated manually by first running tests and then tweaking the parameters a bit. As computers became more powerful, inversion techniques made it possible to “auto-calibrate” models by simply trying every combination of numbers within the “parameter space” so that the simulated model output matched observations.

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“Don’t worry. I have plenty of time. The auto-calibrate model will figure out the soils, vegetation, climate, human water use and also the research question by tomorrow.”

The more dependent we become on computers, the less we rely on our intuition. One paper I reviewed the authors  (from a prestigious Indian university) had not even bothered to replace the SWAT model variables with their English names! The parameters resulting from the model auto-calibration were simply presented in a table with no reflection of whether they made sense. It was obvious the authors had never even visited that part of the country and in fact had no research question.

The objective of the paper was “to develop a model”, a goal which the paper obviously fulfilled! How do we change this culture of garbage-in-garbage-out modelling so that hydrologic research is firmly rooted in critical questions that matter to people? The stakes are non-trivial; a billion people’s lives and livelihoods!

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