A model–data intercomparison of simulated runoff in the contiguous United States: results from the North America Carbon Regional and Continental Interim-Synthesis

Abstract. Significant changes in the water cycle are expected under current global environmental change. Robust assessment of these changes at global scales is confounded by shortcomings in the observed record. Modeled assessments yield conflicting results which are linked to differences in model structure and simulation protocol. Here we compare simulated runoff from six terrestrial biosphere models (TBMs), five reanalysis products, and one gridded surface station product with observations from a network of stream gauges in the contiguous United States (CONUS) from 2001 to 2005. We evaluate the consistency of simulated runoff with stream gauge data at the CONUS and water resource region scale, as well as examining similarity across TBMs and reanalysis products at the grid cell scale. Mean runoff across all simulated products and regions varies widely (range: 71–356 mm yr^-1) relative to observed continental-scale runoff (209 mm yr^-1). Across all 12 products only two are within 10% of the observed value and only four exhibit Nash–Sutcliffe efficiency values in excess of 0.8. Region-level mismatch exhibits a weak pattern of overestimation in western and underestimation in eastern regions; although two products are systematically biased across all regions. In contrast, bias in a temporal sense, within region by water year, is highly consistent. Although gridded composite TBM and reanalysis runoff show some regional similarities for 2001–2005 with CONUS means, individual product values are highly variable. To further constrain simulated runoff and to link model-observation mismatch to model structural characteristics would require watershed-level simulation studies coupled with river routing schemes, standardized forcing data, and explicit consideration of water cycle management.