<p>Seasonality is ubiquitous in nature, and it is closely linked to water quality, ecology, hydrological extremes, and water resources management. Hydrological signatures aim at extracting relevant information about hydrological behaviour, and they can be used to better understand hydrological processes and to evaluate hydrological models. Commonly used seasonal hydro-climatological signatures consider climate or streamflow seasonality, but not how climate seasonality translates into streamflow seasonality. We propose and test hydrological signatures based on the attenuation of the seasonal climate input by a catchment. We approximate the seasonality in the input (precipitation minus potential evapotranspiration) and the output (streamflow) by sine waves. A catchment alters the input sine wave by reducing its amplitude and by shifting its phase. We use these quantities, the amplitude ratio and the phase shift, as seasonal hydrological signatures. We present analytical solutions describing the response of linear reservoirs to periodic forcing to interpret the seasonal signatures in terms of configurations of linear reservoirs. Using data from the UK and the US, we show that the seasonal signatures exhibit hydrologically interpretable patterns and that they are a function of both climate and catchment attributes. Wet, rather impermeable catchments hardly attenuate the seasonal climate input. Drier catchments, especially if underlain by a productive aquifer, strongly attenuate the input sine wave leading to phase shifts up to several months. Finally, we test whether two commonly used hydrological models (IHACRES, GR4J) can reproduce the observed ranges of seasonal signatures in the UK. The results show that the seasonal signatures can aid model building and evaluation.</p>