Contactless River Discharge From Unoccupied Aerial Systems Hydrometry: A Multi‐River Evaluation Across a Wide Range of Near‐Natural Cross‐Sectional Conditions

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Abstract
Discharge is a fundamental hydrological variable, yet often unavailable under hard‐to‐access or critical conditions, hindering understanding of ungauged catchment hydrology. Unoccupied Aerial Systems (UAS) offer promising contactless approaches for monitoring inland waters under such conditions. Numerous studies have assessed UAS discharge estimation, but few have focused on evaluating fully contactless approaches across diverse river conditions. To address this gap, we present a UAS‐based framework for estimating discharge using contactless measurements of water surface velocity, water surface elevation, water surface slope and river bathymetry, supplemented by satellite observations and external DEMs where needed. Four surface‐to‐bulk velocity conversion methods: 0.85 coefficient, log law, power law and entropy method were applied. UAS discharge estimation accuracy was benchmarked at 46 cross‐sections across four river systems with gauge records. We quantified uncertainty propagation from each input variable, assessed their relative contributions, and performed an error propagation experiment to simulate high‐bias measurement scenarios. To identify factors controlling performance, we calculated correlations between discharge error, uncertainty metrics, and cross‐sectional properties. Across most cross‐sections, at least one method achieved discharge error less than 30%, though no single method consistently outperformed the others. The 0.85 method was robust with respect to input uncertainty, with performance patterns across methods linked to morphology: 0.85 and entropy methods performed generally better at larger rivers, while high water surface slopes reduced the accuracy of the log law and power law methods. Correlation analysis revealed that surface velocity uncertainty dominated discharge uncertainty in deep rivers, whereas water surface elevation was more influential in shallow ones.

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