Technical note: High Nash–Sutcliffe Efficiencies conceal poor simulations of interannual variance in seasonal regimes

In highly seasonal regimes hydrologic models generally achieve high scores on common performance metrics such as the Nash–Sutcliffe Efficiency (NSE) and the Kling–Gupta Efficiency (KGE). However, variance in streamflow time series is composed of seasonal, interannual, and irregular variance, and the NSE and KGE do not differentiate between these components. Differences in performance on these three components have not been evaluated across a broad spectrum of hydrologic models and regions. We analyse open-access simulations from 18 regional and global hydrologic models. We find that these models consistently achieve the highest NSE and KGE in highly seasonal catchments where they are worse at simulating interannual variability, compared to less seasonal catchments. Simulated year-to-year changes in ecologically relevant hydrologic signatures are less accurate in highly seasonal catchments, and the NSE of the interannual variance component is usually lower. This suggests that these hydrologic models may struggle to predict long-term responses to climate change, especially in highly seasonal tropical, alpine, and polar regions, which are some of the most vulnerable to climate change. We encourage hydrologic modellers to explicitly evaluate skill at simulating interannual variability, rather than relying only on aggregate measures such as the NSE and KGE.