Ecosystem Carbon Balance During the Early 21st-Century Drought: Synthesis of Flux Observations from Semiarid Southwestern North America and Comparison with Models and Remote Sensing

Global modeling studies suggest semiarid regions dominate the increasing global land CO2 sink and interannual variation of atmospheric CO2, due primarily to variations in water availability. However, direct flux measurements are limited in these areas as compared to wetter regions. Here we gather ~150 site-years of eddy covariance measurements of CO2, water, and energy across 25 semiarid sites in southwestern North America. This regional gradient includes shrublands, grasslands, savannas and forests. Site annual precipitation spans 200 – 900 mm and annual temperature ranges from 5 – 25 ⁰C, a climate space that is underrepresented in flux databases and publications. Measurements represent a period of prolonged regional drought (1998-2014). We evaluate observed evapotranspiration (ET) as an improved metric of ecosystem-available water following hydrologic losses of precipitation. We compare measured fluxes against MODIS remote sensing-based estimates and MsTMIP models representing current best regional estimates. We find that 65% of annual net ecosystem CO2 production (NEP) was explained by ET. Across diverse semiarid ecosystems, a change of ±100 mm in annual ET produced an average NEP change of ±62 gCm2y-1. This relationship held for both temporal variations at a given site and climatic differences across sites, suggesting a common response to short- and long-term differences in water availability. Seven of the 25 sites were, on average, CO2 sources to the atmosphere. Therefore we are testing the hypothesis that semiarid ecosystems function as net CO2 sources during drought, until productivity and respiration equilibrate with current hydroclimate. Remote sensing estimates of ET and productivity and an ensemble of five models from the MsTMIP model inter-comparison reproduced patterns of measured annual fluxes spatially across the region. However, measured ET and productivity showed 200-400% greater interannual variability than model estimates. In ongoing work, we are evaluating the capacity of the MODIS and MsTMIP products to represent measured ecosystem seasonal dynamics across five climatically distinct Southwest sub-regions.