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Despite the fact that Global Climate Model (GCM) outputs have been used to project hydrologic impacts of climate change using off-line hydrologic models for two decades, many of these efforts have been disjointed-applications or at least calibrations have been focused on individual river basins and using a few of the available GCMs. This study improves upon earlier attempts by systematically projecting hydrologic impacts for the entire conterminous United States (US), using outputs from ten GCMs from the latest Coupled Model Intercomparison Project phase 5 (CMIP5) archive, with seamless hydrologic model calibration and validation techniques to produce a spatially and temporally consistent set of current hydrologic projections. The Variable Infiltration Capacity (VIC) model was forced with ten-member ensemble projections of precipitation and air temperature that were dynamically downscaled using a regional climate model (RegCM4) and bias-corrected to 1/24° (~4 km) grid resolution for the baseline (1966-2005) and future (2011-2050) periods under the Representative Concentration Pathway 8.5. Based on regional analysis, the VIC model projections indicate an increase in winter and spring total runoff due to increases in winter precipitation of up to 20% in most regions of the US. However, decreases in snow water equivalent (SWE) and snowcovered days will lead to significant decreases in summer runoff with more pronounced shifts in the time of occurrence of annual peak runoff projected over the eastern and western US. In contrast, the central US will experience year-round increases in total runoff, mostly associated with increases in both extreme high and low runoff. The projected hydrological changes described in this study have implications for various 3 aspects of future water resource management, including water supply, flood and drought preparation, and reservoir operation.

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X-ray absorption spectroscopy (XAS) has been used to characterize the local structural environment of the Mn in the resting (S1) state and two different reduced derivatives of the photosynthetic oxygen-evolving complex (OEC). Short-term incubation with NH2OH gives a state with minimal structural rearrangement relative to the S1 state, consistent with the small shift in X-ray absorption edge energy for the NH2OH reduced sample. In contrast, hydroquinone reduced samples show significant structural rearrangements, including the appearance of a new Mn−O shell at 2.17 Å and a decrease in the amplitude of the 2.7 Å Mn···Mn interaction. These changes are consistent with hydroquinone producing a reduced state consisting of ca. 2 Mn(II) and a single, oxidized Mn2(μ-O)2 core. The interaction assigned to Mn···Mn or Mn···Ca scattering at ca. 3.3 Å is not present in the hydroquinone reduced sample, but is present in the EXAFS of the NH2OH reduced sample. The effects of both NH2OH and hydroquinone are reversed by illumination and dark adaptation, indicating that the reductant induced changes are not the result of sample decomposition. Long-term incubation with NH2OH and short-term incubation with higher concentrations of NH2OH both result in greater reduction, more extensive structural change, and loss of activity. There is a linear correlation between the activity of these highly reduced samples and the Mn content of the samples. However, the activity per Mn atom remains constant, demonstrating that all of the Mn detected by XAS is present in active OEC centers. This demonstrates that highly reduced centers can be produced not only by hydroquinone but also by NH2OH. However, when NH2OH is used to produce highly reduced centers, Mn loss competes with reduction. Based on differences in the reactivity of these reduced states and on their very different structural properties, a refined mechanism for reduction of Mn in the OEC is proposed.

X-ray absorption spectroscopy (XAS) has been used to characterize the structural consequences of Ca2+ replacement in the reaction center complex of the photosynthetic oxygen-evolving complex (OEC). EPR and activity measurements demonstrate that, in the absence of the 17 and 23 kDa extrinsic polypeptides, it is not necessary to use either low pH or Ca chelators to effect complete replacement of the active site Ca2+ by Sr2+, Dy3+, or La3+. The extended X-ray absorption fine structure (EXAFS) spectra for the OEC show evidence for a Mn···M interaction at ca. 3.3 Å that could arise either from Mn···Mn scattering within the Mn cluster or Mn···Ca scattering between the Mn cluster and the inorganic Ca2+ cofactor. There is no significant change in the either the amplitude or the phase of this feature when Ca2+ is replaced by Sr2+ or Dy3+, thus demonstrating that there is no EXAFS-detectable Mn···Ca contribution at ca. 3.3 Å in these samples. The only significant consequence of Ca2+ replacement is a small change in the ca. 2.7 Å Mn···Mn distance. The average Mn···Mn distance decreases 0.014 Å when Ca2+ is replaced by Sr2+ and increases 0.012 Å when Ca2+ is replaced by Dy3+. A structural model which can account both for the variation in Mn···Mn distance and for the known properties of Ca2+-substituted samples is one in which there is a hydrogen bond between a Ca2+-bound water and a Mn2(μ-O)2 unit. This scheme suggests that an important role for the Ca2+ may be to modulate the protonation state, and thus the redox potential, of the Mn cluster.