Hydrogeophysics: where do we go from here?

Hydrogeophysics is maturing as a scientific discipline and many challenges outlined in the past ten years have been addressed or are about to be solved in a satisfactory way. For example, we know how to deal with resolution-limitations of geophysical tomograms, uncertain petrophysical relationships, different geological scenarios, and fully stochastic and coupled inversion approaches can deal with increasingly complex models in a more and more refined manner. This is all fine, but the vast majority of geophysical applications in hydrology (academic or not; my own work included) ignore most of the crucial uncertainties (or consider only just a few of them). This results in predictions that sometimes are useful and sometimes just plain wrong (with corresponding views on the usefulness of geophysics by the hydrologists involved). False certainty is a serious enemy and hydrogeophysical studies should strive to include a realistic uncertainty assessment that is directly linked to the hydrological target of interest. Clearly, a correct uncertainty assessment is essentially impossible for realistic field problems, but the explicit recognition and the characterization of key uncertainties with respect to the problem of interest is necessary if hydrogeophysical techniques are going to be credible and widely used by hydrologists in the long run. A framework with examples is provided that identifies the main aspects to consider (e.g., petrophysics, resolution-limitations, and geological scenarios) and approaches are suggested to represent the resulting uncertainty to non-geophysicists.