Browsing by Author "Harman, CJ"

Now showing 1 - 4 of 4
  • Thumbnail Image
    ItemOpen Access
    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.
  • Thumbnail Image
    ItemOpen Access
    Spatial scale dependence of ecohydrologically mediated water balance partitioning: A synthesis framework for catchment ecohydrology
    (2011-05-11) Thompson, SE; Harman, CJ; Troch, PA; Brooks, PD; Sivapalan, M
    The difficulties in predicting whole catchment water balance from observations at patch scales motivate a search for theories that can account for the complexity of interactions in catchments. In this paper we suggest that the spatial patterns of vegetation may offer a lens through which to investigate scale dependence of hydrology within catchments. Vegetation patterns are attractive because they are observable drivers of evapotranspiration, often a dominant component in catchment water balance, and because the spatial distribution of vegetation is often driven by patterns of water availability. We propose that nontrivial, scale‐dependent spatial patterns in both vegetation distribution and catchment water balance are generated by the presence of a convergent network of flow paths and a two‐way feedback between vegetation as a driver of evapotranspiration and vegetation distribution as a signature of water availability. Implementing this hypothesis via a simple network model demonstrated that such organization was controlled by catchment properties related to aridity, the network topology, the sensitivity of the vegetation response to water availability, and the point‐scale controls on partitioning between evapotranspiration and lateral drainage. The resulting self‐organization generated spatial dependence in areally averaged hydrologic variables, water balance, and parameters describing hydrological partitioning. This spatial scale dependence provides a theoretical approach to connect water balance at patch and catchment scales. Theoretical and empirical studies for understanding the controls of vegetation spatial distribution, point‐scale hydrological partitioning, and the implications of complex flow network topologies on the spatial scale dependence of catchment water balance are proposed as a research agenda for catchment ecohydrology.
  • Thumbnail Image
    ItemOpen Access
    Spatiotemporal scaling of hydrological and agrochemical export dynamics in a tile‐drained Midwestern watershed
    (2011-05-11) Guan, K; Thompson, SE; Harman, CJ; Basu, NB; Rao, PSC; Sivapalan, M; Packman, AI; Kalita, PK
    Conceptualizing catchments as physicochemical filters is an appealing way to link streamflow discharge and concentration time series to hydrological and biogeochemical processing in hillslopes and drainage networks. Making these links explicit is challenging in complex watersheds but may be possible in highly modified catchments where hydrological and biogeochemical processes are simplified. Linking hydrological and biogeochemical filtering in highly modified watersheds is appealing from a water quality perspective in order to identify the major controls on chemical export at different spatial and temporal scales. This study investigates filtering using a 10 year data set of hydrological and biogeochemical export from a small (<500 km2) agricultural watershed in Illinois, the Little Vermilion River (LVR) Watershed. A number of distinct scaling regimes were identified in the Fourier power spectrum of discharge and nitrate, phosphate, and atrazine concentrations. These scaling regimes were related to different runoff pathways and spatial scales throughout the catchment (surface drainage, tile drains, and channel flow in the river). Wavelet analysis indicated increased coupling between discharge and in‐stream concentrations at seasonal‐annual time scales. Using a multiresolution analysis, nitrate, phosphate, and atrazine loads exported at annual scales were found to exhibit near‐linear scaling with annual streamflow, suggesting that at these scales the export dynamics could be approximated as chemostatic responses. This behavior was pronounced for nitrate and less so for phosphate and atrazine. The analysis suggests that biogeochemical inputs built up legacy loads, leading to the emergence of chemostatic behavior at annual time scales, even at the relatively small scale of the LVR.
  • Thumbnail Image
    ItemOpen Access
    Vegetation-infiltration relationships across climatic and soil type gradients
    (Journal of Geophysical Research-Biogeosciences, 2010) Thompson, SE; Harman, CJ; Heine, P; Katul, GG