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 aiming at coordinating data acquisition and hypothesis testing to facilitate an efficient cross-site synthesis of findings. To this end, a common vision and practical data management solutions need to be developed. This opinion paper provocatively discusses two potential end members of a future hydrological observatory (HO) network, based on a given hypothesized community budget: a comprehensive set of moderately instrumented observatories or, alternatively, a small number of highly instrumented super-sites. A network of moderately instrumented monitoring sites would provide a broad spatial coverage across the major pedoclimatic regions by supporting cross-site synthesis of the lumped hydrological response (e.g., rainfall-runoff relationship, Budyko analysis) across diverse continental landscapes. However, the moderate instrumentation at each site may hamper an in-depth understanding of complex hydrological processes. In contrast, a small number of extensively instrumented research sites would enable community-based experiments in an unprecedented manner, thereby facilitating a deeper understanding of complex, non-linear processes modulated by scale-dependent feedback and multiscale spatiotemporal heterogeneity. Lumping resources has proven to be an effective strategy in other geosciences, e.g. research vessels in oceanography and drilling programs in geology. On the downside, a potential limitation of this approach is that a few catchments will not be representative of all pedoclimatic regions, necessitating the consideration of generalization issues. A discussion on the relative merits and limitations of these two visions on HOs is presented to build consensus on the optimal path for the hydrological community to address the UPH in the coming decades. A final synthesis proposes the potential for integrating the two end members into a flexible management strategy.