Intercomparison of global ERA reanalysis products for streamflow simulations at the high-resolution continental scale

Abstract

Watershed-scale streamflow routing models play a key role in real-time flood forecasting, flood risk evaluation, and water resource management, but the accuracy of modeled streamflow depends on both the spatial resolution and the accuracy of the runoff data used as the model forcing. The production of new, higher-resolution runoff datasets therefore motivates an evaluation of the relative improvement in streamflow model performance. This study evaluates streamflow routing output for the Mississippi River Basin, with the simulations driven by three different runoff products from the European Centre for Medium-Range Weather Forecasts. The first runoff product is the ERA-Interim surface and subsurface runoff dataset (ERAI), which was projected to a grid with a resolution of ∼39 km at the equator. Second, this study considers the ERA5 reanalysis dataset (ERA5-25km) placed on a quarter degree grid (∼25 km resolution at the equator). The simulations which generated ERA5-25km included land–atmosphere coupling and land data assimilation. The third runoff product is the ERA5-Land dataset (ERA5L-9km), which was produced in an offline simulation at 9-km resolution without land–atmosphere coupling or land data assimilation, forced by the coarser ERA5 meteorological input. For each runoff input, streamflow was simulated by applying the Routing Application for Parallel computation of Discharge (RAPID) model to the vectorized NHDPlus stream network, which comprised 1.2 million reaches within the study area.Comparison of modeled and measured daily and monthly mean discharge from 60 USGS gages reveals several broad trends. Streamflow simulated using the ERAI runoff tends to generate an earlier peak monthly discharge, while peak discharge from the ERA5L-9km runoff systematically exceeds peak discharge from the ERA5-25km runoff. Basin-aggregated model performance varies by metric, with discharge simulated using the ERA5-25km runoff having the smallest median root-mean-square error even as the discharge modeled using ERA5L-9km runoff has the smallest median Kling-Gupta efficiency. Model performance also varies spatially, with all three simulations exhibiting the least accurate results in the western region of the Mississippi River Basin, and by drainage area, with more accurate results generally obtained for larger drainages. These results suggest that streamflow model accuracy will benefit from the use of either ERA5-25km or ERA5L-9km runoff in place of ERAI. However, the selection of ERA5-25km versus ERA5L-9km depends on regional considerationsas well as which components of model performance are most critical to the user.