The value of hydrologic observatories for large sample hydrology and vice versa

Hydrologic observatories have been a cornerstone of hydrologic science for many decades, advancing hydrologic process understanding with focused field observations and targeted experiments. Observatories present our key opportunity for achieving great depth of hydrologic investigation, most often at the headwater catchment scale. We address two main aspects concerning hydrologic observatories in this contribution: (1) While reviews of individual hydrologic observatories and observatory networks exist, no study has investigated the diversity of observatories to understand whether common aspects increase the likelihood of scientific success. We synthesise information from 80 hydrologic observatories and conduct 25 interviews with observatory leads to fill this gap. We find that scientific outcomes are most enhanced by involving scientific and stakeholder communities throughout observatory inception, design, and operation; by enabling infrastructure to be adjustable to changing ideas and conditions; and by facilitating widespread data use for analysis. (2) While observatories are key for advancing local hypotheses, the transferability of knowledge gained locally to other places or scales has often been difficult or even remained elusive. Headwater catchments in particular show a wide range of process controls often only understood if viewed in a wider regional context of climatic, topographic, or other gradients. We therefore must place observatories into the wider tapestry of hydrologic variability, for example through comparison with large samples of catchments, even though significantly less information is available to characterise these diverse systems. We provide some thoughts on how this connection could be improved through digital infrastructure, mobile observational infrastructure and a renewed focus on gradients and contrasts of controlling processes. We believe that there is a significant opportunity to enhance transferrable knowledge creation in hydrology.