Browsing by Author "Konings, AG"

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    Comparative hydrology across AmeriFlux sites: The variable roles of climate, vegetation, and groundwater
    (2011-07-15) Thompson, SE; Harman, CJ; Konings, AG; Sivapalan, M; Neal, A; Troch, PA
    Watersheds can be characterized as complex space‐time filters that transform incoming fluxes of energy, water, and nutrients into variable output signals. The behavior of these filters is driven by climate, geomorphology, and ecology and, accordingly, varies from site to site. We investigated this variation by exploring the behavior of evapotranspiration signals from 14 different AmeriFlux sites. Evapotranspiration is driven by water and energetic forcing and is mediated by ecology and internal redistribution of water and energy. As such, it integrates biological and physical controls, making it an ideal signature to target when investigating watershed filtering. We adopted a paradigmatic approach (referred to as the null model) that couples the Penman‐Monteith equation to a soil moisture model and explored the deviations between the predictions of the null model and the observed AmeriFlux data across the sites in order to identify the controls on these deviations and their commonalities and differences across the sites. The null model reproduced evapotranspiration fluxes reasonably well for arid, shallow‐rooted systems but overestimated the effects of water limitation and could not reproduce seasonal variation in evapotranspiration at other sites. Accounting for plant access to groundwater (or deep soil moisture) reserves and for the effects of soil temperature on limiting evapotranspiration resolved these discrepancies and greatly improved prediction of evapotranspiration at multiple time scales. The results indicate that site‐specific hydrology and climatic factors pose important controls on biosphere‐hydrosphere interactions and suggest that plant–water table interactions and early season phenological controls need to be incorporated into even simple models to reproduce the seasonality in evapotranspiration.
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    The rainfall-no rainfall transition in a coupled land-convective atmosphere system
    (Geophysical Research Letters, 2010-07-01) Konings, AG; Katul, GG; Porporato, A
    A one-dimensional representation of the atmospheric boundary layer (ABL) depth is coupled to a soil moisture bucket model to dynamically explore the relative roles of surface and free atmospheric conditions on convective precipitation occurrence and resulting soil moisture states. This occurrence is taken to depend on the crossing of the ABL height and the lifting condensation level in the presence of pure convective instability. If rainfall occurs (unrealistically) whenever these conditions are met, and free atmospheric conditions are constant, the resulting system state evolves towards a limit cycle with precipitation every day or every few days, or to a completely dry state. The free atmospheric humidity profile has a larger effect on determining the stationary soil moisture state than the temperature profile. The effect of dry air entrainment on surface energy partitioning decreases soil moisture sensitivity to free atmospheric conditions. © 2010 by the American Geophysical Union.