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Matthias Falk, R. D. Pyles, S. L. Ustin, K. T. Paw U, L. Xu, M. L. Whiting, B. L. Sanden, and P. H. Brown

Abstract

Among the uncertain consequences of climate change on agriculture are changes in timing and quantity of precipitation together with predicted higher temperatures and changes in length of growing season. The understanding of how these uncertainties will affect water use in semiarid irrigated agricultural regions depends on accurate simulations of the terrestrial water cycle and, especially, evapotranspiration. The authors test the hypothesis that the vertical canopy structure, coupled with horizontal variation in this vertical structure, which is associated with ecosystem type, has a strong impact on landscape evapotranspiration. The practical result of this hypothesis, if true, is validation that coupling the Advanced Canopy–Atmosphere–Soil Algorithm (ACASA) and the Weather Research and Forecasting (WRF) models provides a method for increased accuracy of regional evapotranspiration estimates.

ACASA–WRF was used to simulate regional evapotranspiration from irrigated almond orchards over an entire growing season. The ACASA model handles all surface and vegetation interactions within WRF. ACASA is a multilayer soil–vegetation–atmosphere transfer model that calculates energy fluxes, including evapotranspiration, within the atmospheric surface layer.

The model output was evaluated against independent evapotranspiration estimates based on eddy covariance. Results indicate the model accurately predicts evapotranspiration at the tower site while producing consistent regional maps of evapotranspiration (900–1100 mm) over a large area (1600 km2) at high spatial resolution (Δx = 0.5 km).

Modeled results were within observational uncertainties for hourly, daily, and seasonal estimates. These results further show the robustness of ACASA’s ability to simulate surface exchange processes accurately in a complex numerical atmospheric forecast model such as WRF.

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Dennis Baldocchi, Eva Falge, Lianhong Gu, Richard Olson, David Hollinger, Steve Running, Peter Anthoni, Ch. Bernhofer, Kenneth Davis, Robert Evans, Jose Fuentes, Allen Goldstein, Gabriel Katul, Beverly Law, Xuhui Lee, Yadvinder Malhi, Tilden Meyers, William Munger, Walt Oechel, K. T. Paw U, Kim Pilegaard, H. P. Schmid, Riccardo Valentini, Shashi Verma, Timo Vesala, Kell Wilson, and Steve Wofsy

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70°N to 30°S.

FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, and meteorological, plant, and soil data to the science community. (Data and site information are available online at the FLUXNET Web site, http://www-eosdis.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite.

Data being compiled by FLUXNET are being used to quantify and compare magnitudes and dynamics of annual ecosystem carbon and water balances, to quantify the response of stand-scale carbon dioxide and water vapor flux densities to controlling biotic and abiotic factors, and to validate a hierarchy of soil–plant–atmosphere trace gas exchange models. Findings so far include 1) net CO2 exchange of temperate broadleaved forests increases by about 5.7 g C m−2 day−1 for each additional day that the growing season is extended; 2) the sensitivity of net ecosystem CO2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO2 flux density exhibits peaks at timescales of days, weeks, and years, and a spectral gap exists at the month timescale; 4) the optimal temperature of net CO2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

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