Upscaling key ecosystem functions across the conterminous United States by a water‐centric ecosystem model

[1] We developed a water‐centric monthly scale simulation model (WaSSI‐C) by integrating empirical water and carbon flux measurements from the FLUXNET network and an existing water supply and demand accounting model (WaSSI). The WaSSI‐C model was evaluated with basin‐scale evapotranspiration (ET), gross ecosystem productivity (GEP), and net ecosystem exchange (NEE) estimates by multiple independent methods across 2103 eight‐digit Hydrologic Unit Code watersheds in the conterminous United States from 2001 to 2006. Our results indicate that WaSSI‐C captured the spatial and temporal variability and the effects of large droughts on key ecosystem fluxes. Our modeled mean (±standard deviation in space) ET (556 ± 228 mm yr) compared well to Moderate Resolution Imaging Spectroradiometer (MODIS) based (527 ± 251 mm yr) and watershed water balance based ET (571 ± 242 mm yr). Our mean annual GEP estimates (1362 ± 688 g C m yr) compared well (R = 0.83) to estimates (1194 ± 649 g C m yr−1) by eddy flux‐based EC‐MOD model, but both methods led significantly higher (25–30%) values than the standard MODIS product (904 ± 467 g C m yr−1). Among the 18 water resource regions, the southeast ranked the highest in terms of its water yield and carbon sequestration capacity. When all ecosystems were considered, the mean NEE (−353 ± 298 g C m yr) predicted by this study was 60% higher than EC‐MOD’s estimate (−220 ± 225 g C m yr) in absolute magnitude, suggesting overall high uncertainty in quantifying NEE at a large scale. Our water‐centric model offers a new tool for examining the trade‐offs between regional water and carbon resources under a changing environment.