Linking Water Vapor and Carbon Dioxide Fluxes to Quantify Ecosystem-Available Water in Dryland Regions (Invited Presentation)

The combined measurement of water vapor and carbon dioxide fluxes is a powerful tool for understanding linkages between ecosystem water and carbon cycling. As eddy covariance data become available for more sites and years, there is a tremendous opportunity for synthesis research to improve understanding of these land-atmosphere exchanges around the world. Particularly in dryland regions, ecosystem carbon exchanges are tightly coupled with water availability. In this talk, I present how multiple years of site data and multiple sites across drylands of the Southwest are used to infer the temporal and spatial response of ecosystem net and gross carbon fluxes to changes in water availability. Here, I show that evapotranspiration (ET) represents water available for ecosystem processes more accurately than often-used precipitation because ET accounts for storage and losses. Furthermore, I use the tight linkage between water vapor and carbon dioxide fluxes at four summer-rainfall semiarid sites to demonstrate a novel method of partitioning ET into transpiration (T) and abiotic evaporation (E). At four summer-rainfall semiarid sites, T/ET increases to a peak coincident with maximum photosynthesis and remains elevated as the growing season progresses, consistent with previous, direct measurements. The seasonal course of T/ET is related to increasing leaf area index and declining frequency of rainy days— a proxy for the wet surface conditions which promote E—suggesting both surface and climatic controls on ET partitioning. These analyses with long-term, multi-site flux data signal the advent of an exciting, data-rich era in which to develop new insights on the linkages between these formerly invisible fluxes, allowing us to better predict the response of these cycles to a rapidly changing climate.