@article{22510,
  abstract     = {This study advances mechanistic interpretation of predictability challenges in hydro-geomorphology related to the role of soil moisture spatial variability. Using model formulations describing the physics of overland flow, variably saturated subsurface flow, and erosion and sediment transport, this study explores (1) why a basin with the same mean soil moisture can exhibit distinctly different spatial moisture distributions, (2) whether these varying distributions lead to non-unique hydro-geomorphic responses, and (3) what controls non-uniqueness in relation to the response type. Two sets of numerical experiments are carried out with two physically-based models, HYDRUS and tRIBS+VEGGIE+FEaST, and their outputs are analyzed with respect to pre-storm moisture state. The results demonstrate that distinct spatial moisture distributions for the same mean wetness arise because near-surface soil moisture dynamics exhibit different degrees of coupling with deeper-soil moisture and the process of subsurface drainage. The consequences of such variations are different depending on the type of hydrological response. Specifically, if the predominant runoff response is of infiltration excess type, the degree of non-uniqueness is related to the spatial distribution of near-surface moisture. If runoff is governed by subsurface stormflow, the extent of deep moisture contributing area and its “readiness to drain” determine the response characteristics. Because the processes of erosion and sediment transport superimpose additional controls over factors governing runoff generation and overland flow, non-uniqueness of the geomorphic response can be highly dampened or enhanced. The explanation is sediment composed by multi-size particles can alternate states of mobilization or surface shielding and the transient behavior is inherently intertwined with the availability of mobile particles. We conclude that complex nonlinear dynamics of hydro-geomorphic processes are inherent expressions of physical interactions. As complete knowledge of watershed properties, states, or forcings will always present the ultimate, if ever resolvable, challenge, deterministic predictability will remain handicapped. Coupling of uncertainty quantification methods and space-time physics-based approaches will need to evolve to facilitate mechanistic interpretations and informed practical applications.},
  author       = {Kim, Jongho and Dwelle, M. Chase and Kampf, Stephanie K. and Fatichi, Simone and Ivanov, Valeriy Y.},
  issn         = {0309-1708},
  journal      = {Advances in Water Resources},
  keywords     = {Soil moisture, Spatial heterogeneity, Hydrological response, Geomorphic response, Non-uniqueness, Hydraulic connectivity},
  pages        = {73--89},
  publisher    = {Elsevier},
  title        = {{On the non-uniqueness of the hydro-geomorphic responses in a zero-order catchment with respect to soil moisture}},
  doi          = {10.1016/j.advwatres.2016.03.019},
  volume       = {92},
  year         = {2016},
}

@article{22536,
  abstract     = {Increasing concentrations of atmospheric carbon dioxide are expected to affect carbon assimilation and evapotranspiration (ET), ultimately driving changes in plant growth, hydrology, and the global carbon balance. Direct leaf biochemical effects have been widely investigated, whereas indirect effects, although documented, elude explicit quantification in experiments. Here, we used a mechanistic model to investigate the relative contributions of direct (through carbon assimilation) and indirect (via soil moisture savings due to stomatal closure, and changes in leaf area index) effects of elevated CO2 across a variety of ecosystems. We specifically determined which ecosystems and climatic conditions maximize the indirect effects of elevated CO2. The simulations suggest that the indirect effects of elevated CO2 on net primary productivity are large and variable, ranging from less than 10% to more than 100% of the size of direct effects. For ET, indirect effects were, on average, 65% of the size of direct effects. Indirect effects tended to be considerably larger in water-limited ecosystems. As a consequence, the total CO2 effect had a significant, inverse relationship with the wetness index and was directly related to vapor pressure deficit. These results have major implications for our understanding of the CO2 response of ecosystems and for global projections of CO2 fertilization, because, although direct effects are typically understood and easily reproducible in models, simulations of indirect effects are far more challenging and difficult to constrain. Our findings also provide an explanation for the discrepancies between experiments in the total CO2 effect on net primary productivity.},
  author       = {Fatichi, Simone and Leuzinger, Sebastian and Paschalis, Athanasios and Langley, J. Adam and Donnellan Barraclough, Alicia and Hovenden, Mark J.},
  issn         = {1091-6490},
  journal      = {Proceedings of the National Academy of Sciences},
  keywords     = {carbon dioxide, modeling, FACE, soil moisture, evapotranspiration},
  number       = {45},
  pages        = {12757--12762},
  publisher    = {National Academy of Sciences},
  title        = {{Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO}},
  doi          = {10.1073/pnas.1605036113},
  volume       = {113},
  year         = {2016},
}