Factors controlling seasonal groundwater and solute flux from snow‐dominated basins

Critical zone influences on hydrologic partitioning, subsurface flow paths and reactions along these flow paths dictate the timing and magnitude of groundwater and solute flux to streams. To isolate first‐order controls on seasonal streamflow generation within highly heterogeneous, snow‐dominated basins of the Colorado River, we employ a multivariate statistical approach of end‐member mixing analysis using a suite of daily chemical and isotopic observations. Mixing models are developed across 11 nested basins (0.4 to 85 km²) spanning a gradient of climatological, physical, and geological characteristics. Hydrograph separation using rain, snow, and groundwater as end‐members indicates that seasonal contributions of groundwater to streams is significant. Mean annual groundwater flux ranges from 12% to 33% whereas maximum groundwater contributions of 17% to 50% occur during baseflow. The direct relationship between snow water equivalent and groundwater flux to streams is scale dependent with a trend toward self‐similarity when basins exceed 5.5 km². We find groundwater recharge increases in basins of high relief and within the upper subalpine where maximum snow accumulation is coincident with reduced conifer cover and lower canopy densities. The mixing model developed for the furthest downstream site did not transfer to upstream basins. The resulting error in predicted stream concentrations points toward weathering reactions as a function of source rock and seasonal shifts in flow path. Additionally, the potential for microbial sulfate reduction in floodplain sediments along a low‐gradient, meandering portion of the river is sufficient to modify hillslope contributions and alter mixing ratios in the analysis. Soil flushing in response to snowmelt is not included as an end‐member but is identified as an important mechanism for release of solutes from these mountainous watersheds. End‐member mixing analysis used in combination with high‐frequency observations reveals important aspects of catchment hydrodynamics across scale.     

Web 2.0 collaborations address uncertainty in climate reconstructions of the past millennium

Reducing uncertainty in global temperature reconstructions of the past millennium remains the key issue in applying this record to society’s pressing climate change problem. Reconstructions are collaborative, built on the research of hundreds of scientists who apply their diverse scientific expertise and field and laboratory skill to create the individual proxy reconstructions that underlie the multi-proxy, global average temperature time series. Web 2.0 features have enabled collaborative efforts that improve the characterization of uncertainty. Raw data shared via a repository (the World Data Center for Paleoclimatology) enable new reconstructions from the collection of user-generated data. Standards propagated by expert communities facilitate quality control and interoperability. Open access to data and computer code promote transparency and make the science accessible to a broader audience. Blogs, wikis, and listservs share background information and highlight contentious as well as unique aspects of paleo science. A novel approach now underway, titled the Paleoclimate Reconstruction Challenge, and based on the sharing of simulated data (pseudo-proxies) and reconstruction results, seeks to facilitate method development, further reducing uncertainty. Broadly-useful aspects of the Challenge may find application in other fields.     

Analytical methods to estimate the hydraulic conductivity of jointed rocks

Hydrogeology Journal - The hydraulic conductivity of jointed rocks is one of the main input parameters to predict water inflow to engineering structures that are located in the jointed rocks....     

Soil moisture response to white ash mortality following emerald ash borer invasion

Emerald ash borer (EAB) (Agrilus planipennis Fairmaire), an invasive forest insect first identified in southeastern Michigan in 2002, is established in at least 32 US states and three Canadian provinces. Ash (Fraxinus spp.) mortality rates in some forested areas exceed 90%, but to date, little is known about the potential effects of EAB-caused ash mortality on hydrological processes. More broadly, there is a need for information on the timing and magnitude of soil moisture response to species-specific mortality of overstory vegetation in deciduous forest systems. Soil moisture was examined in 28 forested sites where 0–100% of the white ash basal area (Fraxinus americana L.) was killed by EAB. Synoptic measurements of near-surface (0–6 cm depth) soil moisture were collected from 112 plots (18 m radius) within the sites. Three plots were also instrumented with soil moisture sensors at 10 and 25 cm depth to log hourly measurements from May to October. Synoptic data showing white ash mortality and soil moisture were positively correlated in the 34 plots with ≥?5% mortality (by total basal area). In the intensively monitored plots, volumetric soil moisture declined from 37 to 16% between July and September where white ash mortality was low (0.6% of basal area killed), but remained near field capacity (~?30%) throughout the monitoring period in the high mortality plot (8.6% of basal area killed), meriting further investigation to assess effects of white ash mortality on evapotranspiration and soil moisture dynamics in heterogeneous upland forests. Altered soil moisture may have implications for regrowth dynamics, infiltration/runoff partitioning, and nutrient cycling, but additional study to quantify the extent and duration of EAB-related ash mortality on hydrology at the plot and watershed scale is necessary.     

Formation process of a large paleolandslide-dammed lake at Xuelongnang in the upper Jinsha River,SE Tibetan Plateau: constraints from OSL and <Superscript>14</Superscript>C dating

A large number of the landslide dams located on the major rivers at the southeastern margin of the Tibetan Plateau have been previously identified through remote sensing analysis and field investigations. The Xuelongnang paleolake was one of the lakes formed by these landslide dams in the upper Jinsha River, where the association of a relict landslide dam, lacustrine sediment, and outburst sediment is well preserved. This preservation provides an opportunity to better understand the formation, evolution, and longevity of a large landslide-dammed lake in the upper Jinsha River. It was inferred that the Xuelongnang dammed lake may have been formed by an earthquake-induced paleoavalanche. The surface area of the lake at its peak was estimated at 7.0?×?10⁶ m², and the corresponding volume was approximately 3.1?×?10⁸ m³. Two outburst flood events were determined to have occurred during the life span of the lake. Based on the 18 ages obtained from optically stimulated luminescence (OSL) and carbon-14 (¹⁴C) dating combined with stratigraphic sequences observed in the field, the paleolandslide-dammed lake was formed at approximately 2.1 ka and subsequently breached locally. The dammed lake was sustained for a period of some 900 years based on the chronological constraining. This study confirms that a major landslide-dammed lake can be sustained for at least hundreds of years and breached by several dam breaks in multiple periods, which contributed to the preservation of the knickpoints at millennial scale along the major rivers in the study area.     

Silica grain catalysis of methanol formation

The specific catalytic effect of a silica grain on the formation of methanol via the sequential addition of H atoms to CO adsorbed on the surface is investigated. A negatively charged defect on a siliceous edingtonite surface is found to reduce the gas phase barriers for the H + CO_ads and H + H₂C=O_ads reactions by 770 and 399 K, respectively, when compared to the same reactions in the gas phase. The catalytic effect of negatively charged surface sites could also be applicable to the hydrogenation of other adsorbed unsaturated species. However, the activation energies on the surface defect are still too large (1150 and 2230 K) for CH₃OH to form efficiently at 10–20 K in the interstellar medium via a classical mechanism. It is therefore suggested that quantum mechanical tunnelling through the activation barrier is required for these hydrogen addition reactions to proceed at such temperatures. The calculations show that because the adsorption energies of CO and H₂C=O on the negatively charged defect are substantial, CH₃OH may form efficiently during the warm-up period in star-forming regions.