President Kuni Takeuchi propagated the suggestion that the IAHS should take the lead on a decade-long initiative to further the cause of hydrological science in currently ungauged catchments. This is a major challenge, and subsequent discussion of this proposal among the community has so far revealed a lack of consensus on how best to proceed. Broadly speaking, one portion of the scientific community has suggested placing emphasis on developing remote-sensing techniques to estimate the missing data, while another portion is sceptical about the feasibility and reliability of this suggestion, and believe there is greater opportunity in extrapolating aspects of the data available for gauged catchments to ungauged catchments. Here, I argue that, at least at the scale of small catchments, neither of these approaches are sufficient on their own, and preferably both approaches should be used in a complementary setting.
One thing is obvious. If a catchment is currently ungauged, and gauging is needed, the preferred response is to introduce proven in-catchment monitoring systems and methods. Experience shows that the most effective way to obtain and retain deployment of in-catchment gauging is to demonstrate that sustained economic and/or social value results from it. Thus, the IAHS can aid the introduction of new, or the retention of existing monitoring, by continuing to lobby for this. However, it might be more effective to foster research and publish results that demonstrate the usefulness of gauging to stakeholders in the catchment. Symposia and workshops on the results of the stakeholder-driven research undertaken under the Hydrology for the Environment, Life, and Policy (HELP) programme is one way to do this. The IAHS can also contribute to new and sustained monitoring by stimulating, through symposia and workshops, the development and deployment of new, reliable, inexpensive in-catchment monitoring technologies, and simpler and less-expensive data collection methods.
If in-catchment monitoring using traditional methods is not feasible or fundable, what can we do then? The only way forward is to estimate the missing measurements in some way. One option is to estimate hydrometeorological forcing variables, or at least to estimate their statistical properties, by assuming similarity with actual measurements from a gauged catchment in the same general climatic region. Given an additional assumption that hydrological models can be made simple but robust enough to estimate the water balance of