Flow dynamics at the continental scale: streamflow correlation and hydrological similarity Andrea Betterle 1,2,3,∗ , Mario Schirmer 1,2 , Gianluca Botter 3 1.EAWAG (Switzerland), 2.University of Neuchˆatel (Switzerland), 3.University of Padova (Italy) Abstract Streamflow variability in space and time critically affects anthropic water uses and ecosystem services. Unfortunately, spatiotemporal patterns of flow regimes are of- ten unknown, as discharge measurements are usually recorded at a limited number of hydrometric stations unevenly distributed along river networks. Advances in un- derstanding the physical processes that control the spatial patterns of river flows are therefore necessary to predict water availability at ungauged locations or to extrapolate pointwise streamflow observations. This work explores the use of the spatial correlation of river flows as a metric to quantify the similarity between hy- drological responses of two catchments. Following a stochastic framework, 340,000 cross-correlations between pairs of daily streamflows time series are predicted at a seasonal timescale across the contiguous United States using 413 catchments of the MOPEX dataset. Model predictions of streamflow correlation obtained in absence of runoff information are successfully used to identify catchment outlets sharing sim- ilar discharge dynamics and flow regimes across a broad range of geomorphoclimatic conditions, without relying on calibration. The selection of reference streamgauges based on predicted streamflow correlation generally outperforms the selection based on spatial proximity, especially as the density of available gauged sections decreases. Interestingly, correlated outlets share a broad spectrum of hydrological signatures (mean discharge, flow variability, recession properties), suggesting that catchments forced by analogous frequency and intensity of effective rainfall events might ex- hibit common geomorphoecological traits leading to similar hydrological responses. The proposed framework provides a physical basis to assist the regionalization of flow dynamics, and to interpret the spatial variability of flow regimes along stream networks. Keywords: Streamlfow Correlation, Hydrological Similarity, Stochastic Hydrology, Ungauged Catchments, PUB, Regionalization, Flow Patterns Preprint submitted to Hydrological Processes; *:corresponding author October 5, 2018 Accepted for publication in Hydrological Processes. Copyright (2019) Wiley. Further reproduction or electronic distribution is not permitted. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as an ‘Accepted Article’, https://doi.org/10.1002/hyp.13350 Published version available at: https://onlinelibrary.wiley.com/doi/abs/10.1002/hyp.13350 This document is the accepted manuscript version of the following article: Betterle, A., Schirmer, M., & Botter, G. (2019). Flow dynamics at the continental scale: streamflow correlation and hydrological similarity. Hydrological Processes. https://doi.org/10.1002/hyp.13350