HESS Opinions: Towards a common vision for the future of hydrological observatories

The Unsolved Problems in Hydrology (UPH) initiative has emphasized the need to establish networks of multi-decadal hydrological observatories to gain a deep understanding of the complex hydrologic processes occurring within diverse environmental conditions. The already existing monitoring infrastructures have provided an enormous amount of hydrometeorological data, facilitating detailed insights into the causal mechanisms of hydrological processes, the testing of scientific theories and hypotheses, and the revelation of the physical laws governing catchment behavior. Yet, hydrological monitoring programs have often produced limited outcomes due to the intermittent availability of financial resources and the substantial efforts required to operate observatories and conduct comparative studies to advance previous findings. Recently, some initiatives have emerged that aim to coordinate data acquisition and hypothesis testing to facilitate an efficient cross-site synthesis of findings.To this end, a common vision and practical data managementsolutions need to be developed. This opinion paper provocatively discusses two potential endmembers of a future hydro-logical observatory (HO) network based on a given hypothesized community budget: a comprehensive set of moderatelyinstrumented observatories or, alternatively, a small numberof highly instrumented supersites.A network of moderately instrumented monitoring siteswould provide a broad spatial coverage across the majorpedoclimatic regions by supporting cross-site synthesis ofthe lumped hydrological response (e.g., rainfall–runoff relationship, Budyko analysis) across diverse continental land-scapes. However, the moderate instrumentation at each sitemay hamper an in-depth understanding of complex hydrological processes. In contrast, a small number of extensivelyinstrumented research sites would enable community-basedexperiments in an unprecedented manner, thereby facilitating a deeper understanding of complex, non-linear processesmodulated by scale-dependent feedback and multiscale spatiotemporal heterogeneity. Lumping resources has proven tobe an effective strategy in other geosciences, e.g., researchvessels in oceanography and drilling programs in geology.On the downside, a potential limitation of this approach isthat a few catchments will not be representative of all pedoclimatic regions, necessitating the consideration of general-ization issues.A discussion on the relative merits and limitations of thesetwo visions regarding HOs is presented to build consensus onthe optimal path for the hydrological community to addressthe UPH in the coming decades. A final synthesis proposesthe potential for integrating the two endmembers into a flexible management strategy.