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Sun, G; Caldwell, P; Noormets, A; McNulty, SG; Cohen, E; Myers, JM; Domec, JC; Treasure, E; Mu, QZ; Xiao, JF; John, R; Chen, JQ (2011). Upscaling key ecosystem functions across the conterminous United States by a water-centric ecosystem model. JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES, 116, G00J05.

[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(-1)) compared well to Moderate Resolution Imaging Spectroradiometer (MODIS) based (527 +/- 251 mm yr(-1)) and watershed water balance based ET (571 +/- 242 mm yr(-1)). Our mean annual GEP estimates (1362 +/- 688 g C m(-2) yr(-1)) compared well (R(2) = 0.83) to estimates (1194 +/- 649 g C m(-2) 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(-2) 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(-2) yr(-1)) predicted by this study was 60% higher than EC-MOD's estimate (-220 +/- 225 g C m(-2) yr(-1)) 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.



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