The Differential Response of Surface Fluxes from Agro-Ecosystems in Response to Local Environmental Conditions

Meeting the competing demands for freshwater of the urban, industrial, and agricultural communities is increasingly challenging as the global population continues to grow and the need for potable water, food, fiber, and fuel grows with it. One of the keys to meeting these demands is maximizing the efficiency of water use in agricultural applications. Toward this end, a thorough understanding of the factors driving evapotranspiration and their response to spatiotemporal variations in local environmental conditions is needed for the development and validation of numerical and remote sensing-based models. Moreover, because these exchange processes are strongly nonlinear, scaling measurements collected at one scale to another remains a nontrivial task. In an effort to identify the key environmental drivers controlling the latent heat flux (lambda E) from agro-ecosystems and their potential impacts on upscaling in-situ flux measurements, eddy covariance and micrometeorological data collected over maize and soy at three distinct sites located in Maryland, Iowa, and Minnesota, respectively were evaluated for the years between 2007 and 2011. The magnitudes of the evaporative fluxes were comparable for measurements collected during clear-sky days with similar environmental conditions; on average, the measurements of lambda E agreed to within 50 W m(-2), or approximately 10%. When considered in terms of evaporative fraction (f(e)), however, there were marked differences among the sites. For example, while the magnitude and diurnal pattern of f(e) for mature maize at the Minnesota site was nearly constant (f(e) = 0.66) during the day, f(e) at both the Maryland and Iowa site increased steadily during the day from a minimum value near 0.68 at midmorning to peak value of 0.87 in the afternoon. These differences appear to be primarily linked to differences in soil moisture and vegetation density at the various sites. As such, this research underscores the impact of local environmental conditions in controlling land-atmosphere exchange processes. It also underscores the importance accurately describing local environmental conditions when modeling surface fluxes. (C) 2013 The Authors. Published by Elsevier B.V.