Changes in monthly baseflow across the U.S. Midwest

Characterizing streamflow changes in the agricultural U.S. Midwest is critical for effective planning and management of water resources throughout the region. The objective of this study is to determine if and how baseflow has responded to land alteration and climate changes across the study area during the 50‐year study period by exploring hydrologic variations based on long‐term stream gage data. This study evaluates monthly contributions to annual baseflow along with possible trends over the 1966–2016 period for 458 U.S. Geological Survey streamflow gages within 12 different Midwestern states. It also examines the influence of climate and land use factors on the observed baseflow trends. Monthly contribution breakdowns demonstrate how the majority of baseflow is discharged into streams during the spring months (March, April, and May) and is overall more substantial throughout the spring (especially in April) and summer (June, July, and August). Baseflow has not remained constant over the study period, and the results of the trend detection from the Mann–Kendall test reveal that baseflows have increased and are the strongest from May to September. This analysis is confirmed by quantile regression, which suggests that for most of the year, the largest changes are detected in the central part of the distribution. Although increasing baseflow trends are widespread throughout the region, decreasing trends are few and limited to Kansas and Nebraska. Further analysis reveals that baseflow changes are being driven by both climate and land use change across the region. Increasing trends in baseflow are linked to increases in precipitation throughout the year and are most prominent during May and June. Changes in agricultural intensity (in terms of harvested corn and soybean acreage) are linked to increasing trends in the central and western Midwest, whereas increasing temperatures may lead to decreasing baseflow trends in spring and summer in northern Wisconsin, Kansas, and Nebraska.     

Thymol as a Biocide in Japanese Rainwater

Rainwater samples (wet-only; event samples) collected in Niigata in late autumn 1996 and springtime 1997 were used to assess the effectiveness of thymol as a biocide in Japanese rainwater. Upon collection each rainwater sample was divided into sub-samples, with thymol added to one sub-sample. Sub-samples with and without thymol were shipped to CSIRO, Australia, for chemical analysis. Comparison of analytical results for each pair of sub-samples proved the effectiveness of thymol in preventing biological action in this region where effects of rainwater microflaura and fauna on rainwater composition have not before been studied. Sub-samples without thymol exhibited lowered electrical conductivity, loss of the cations H⁺ and NH ₄ ^- , and loss of the anions HCOO^-, CH₃COO^-, C₂O ₄ ^2- , CH₃SO ₃ ^- and PO ₄ ^3- . Nitrate showed no change in all but one of the samples, indicating that ammonia was the preferred source of nitrogen for the biological processes that consumed the rainwater organic acids and phosphate. These results suggest that thymol is a suitable rainwater biocide for use under Japanese conditions.     

The origin and response of zircon in eclogite to metamorphism during the multi-stage evolution of the Huwan Shear Zone,China: Insights from Lu–Hf and U–Pb isotopic and trace element geochemistry

The Huwan Shear Zone (HSZ) is an eclogite bearing transpressive wrench zone located along the Shangdan Suture that juxtaposes the Paleozoic Qinling and Mesozoic Hong'an–Dabie orogenic terrains. The region preserves a complex history that bridges the gap between adjacent orogenic terrains. Simultaneous in-situ trace element, U–Th–Pb and Lu–Hf‐isotope analysis of zircon grains from samples of the Xiongdian and Sujiahe eclogite identify a late Carboniferous to early Permian period of high pressure metamorphism, ca. 283 to 306 Ma. Zircon grains are observed to respond to metamorphic overprint via a two stage process: (1) An initial prograde stage of fluid catalyzed interface coupled dissolution–reprecipitation, involving exsolution of a non-ideal solid solution thorite (ThSiO₄) end member and loss of highly incompatible components (LREE and Pb), (2) A second stage of coupled zircon dissolution, coarsening, and new rim growth in equilibrium with garnet at high pressure conditions.We identify Proterozoic whole rock Sm–Nd and zircon grain Lu–Hf isotopic evidence which challenges the traditional interpretation that the Xiongdian and Sujiahe eclogite formed in response to early Paleozoic mantle melting and oceanic crust generation. We argue the Huwan Shear Zone contains no conclusive evidence of early/middle Paleozoic oceanic crust, but rather Proterozoic crustal components analogous to those found in the Northern Qinling Terrain and associated with formation of the Shangdan Suture. We present a simpler geodynamic model involving continuous convergence and accretion of terrains onto the southern margin of the North China Block during the Paleozoic Qinling and Mesozoic Dabie orogenies.     

The El Teniente porphyry Cu–Mo deposit from a hydrothermal rutile perspective

Mineralogical, textural, and chemical analyses (EPMA and PIXE) of hydrothermal rutile in the El Teniente porphyry Cu–Mo deposit help to better constrain ore formation processes. Rutile formed from igneous Ti-rich phases (sphene, biotite, Ti-magnetite, and ilmenite) by re-equilibration and/or breakdown under hydrothermal conditions at temperatures ranging between 400°C and 700°C. Most rutile nucleate and grow at the original textural position of its Ti-rich igneous parent mineral phase. The distribution of Mo content in rutile indicates that low-temperature (∼400–550°C), Mo-poor rutile (5.4 ± 1.1 ppm) is dominantly in the Mo-rich mafic wallrocks (high-grade ore), while high-temperature (∼550-700°C), Mo-rich rutile (186 ± 20 ppm) is found in the Mo-poor felsic porphyries (low-grade ore). Rutile from late dacite ring dikes is a notable exception to this distribution pattern. The Sb content in rutile from the high-temperature potassic core of the deposit to its low-temperature propylitic fringe remains relatively constant (35 ± 3 ppm). Temperature and Mo content of the hydrothermal fluids in addition to Mo/Ti ratio, modal abundance and stability of Ti-rich parental phases are key factors constraining Mo content and provenance in high-temperature (≥550°C) rutile. The initial Mo content of parent mineral phases is controlled by melt composition and oxygen fugacity as well as timing and efficiency of fluid–melt separation. Enhanced reduction of SO₂-rich fluids and sulfide deposition in the Fe-rich mafic wallrocks influences the low-temperature (≤550°C) rutile chemistry. The data are consistent with a model of fluid circulation of hot (>550°C), oxidized (ƒO₂ ≥ NNO + 1.3), SO₂-rich and Mo-bearing fluids, likely exsolved from deeper crystallizing parts of the porphyry system and fluxed through the upper dacite porphyries and related structures, with metal deposition dominantly in the Fe-rich mafic wallrocks.     

Regional estimation of base recharge to ground water using water balance and a base-flow index

Naturally occurring long-term mean annual base recharge to ground water in Nebraska was estimated with the help of a water-balance approach and an objective automated technique for base-flow separation involving minimal parameter-optimization requirements. Base recharge is equal to total recharge minus the amount of evapotranspiration coming directly from ground water. The estimation of evapotranspiration in the water-balance equation avoids the need to specify a contributing drainage area for ground water, which in certain cases may be considerably different from the drainage area for surface runoff. Evapotranspiration was calculated by the WREVAP model at the Solar and Meteorological Surface Observation Network (SAMSON) sites. Long-term mean annual base recharge was derived by determining the product of estimated long-term mean annual runoff (the difference between precipitation and evapotranspiration) and the base-flow index (BFI). The BFI was calculated from discharge data obtained from the U.S. Geological Survey's gauging stations in Nebraska. Mapping was achieved by using geographic information systems (GIS) and geostatistics. This approach is best suited for regional-scale applications. It does not require complex hydrogeologic modeling nor detailed knowledge of soil characteristics, vegetation cover, or land-use practices. Long-term mean annual base recharge rates in excess of 110 mm/year resulted in the extreme eastern part of Nebraska. The western portion of the state expressed rates of only 15 to 20 mm annually, while the Sandhills region of north-central Nebraska was estimated to receive twice as much base recharge (40 to 50 mm/year) as areas south of it.     

Wet deposition of excess sulfate at Macquarie Island, 54° S

Annual wet deposition of excess sulfate at Macquarie Island has been estimated from 5 months of rainwater composition data covering the Austral summer of 1985/86. The resulting figure of 2.1±0.6 mmol/m²/yr is at the low end of previous estimates of maritime excess sulfate deposition by precipitation. Within estimated uncertainty limits this figure is consistent with the DMS emission flux which would be predicted for latitude 50°–60° S, based solely on available Northern Hemispheric DMS measurements.Temporarily at the International Meteorological Institute, Stockholm University, S-106 91, Stockholm, Sweden.     

Isoprene emissions from vegetation and hydrocarbon emissions from bushfires in tropical Australia

Information from a variety of sources, including an airborne field expedition in November 1985, is used to produce estimates of the annual emissions of some hydrocarbons from bushfires, and isoprene from trees, in tropical Australia. For the continent north of 23° S the annual bushfires (biomass burning) input was estimated, in units of Tg carbon, to be 2 TgC (uncertainty range 0.8–5 TgC), emitted predominantly during the May to October dryseason. Isoprene emissions during this period were estimated also to be 2 TgC (uncertainty range 0.5–8 TgC), but were estimated to be an order of magnitude higher during the November to April wet season, at a level of 23 TgC (uncertainty range 6–100 TgC).The large annual emission of isoprene over the tropical part of the Australian continent yields ppbv levels of isoprene measured at the surface in summertime. Isoprene reactivity with hydroxyl radical is such that at these concentrations isoprene must be a dominant factor in controlling the concentration of OH radical in the convective boundary layer. Simple arguments based on the convective velocity scale suggest that the shape of the isoprene vertical profile in November 1985 would be consistent with available data on the OH-isoprene reaction rate if OH concentration in the boundary layer averaged about 2.5×10⁶ cm^-3 over the middle part of the day.Temporarily at the International Meteorological Institute, Stockholm University, S-106 91, Stockholm, Sweden.     

Numerical study of droplet size dependent chemistry in oceanic,wintertime stratus cloud at southern mid-latitudes

Cloud droplet chemistry is modelled for the first 150 m of rise in a wintertime, mid-latitude, marine stratus cloud using observations made at and near the Cape Grim Baseline Station as a source of input parameters. The emphasis in this work was to study the variation in droplet chemistry as a function of both droplet size and nucleus composition, with a particular focus on the way in which oxidation of dissolved sulfur dioxide varied.At 150 m above the condensation level, solute concentration as a function of droplet size was found to increase by as much as 2 to 3 orders of magnitude for only a factor of 2 increase in droplet radius, primarily as a consequence of the 1/r dependence in the droplet growth equation. This type of size dependence exists at all levels in the model cloud, and has a significant influence on oxidation rate of sulfur dioxide in droplets growing on sulfate nuclei, oxidation by ozone being favoured in the smallest droplets, but oxidation by hydrogen peroxide being favoured in the larger droplets. Oxidation by ozone is favoured at all sizes in droplets formed on sea-salt nuclei as a result of the initially high alkalinity of these droplets, and in the cloud overall is calculated to be the more important oxidation pathway. Although based on a simplified chemical scheme, these results suggest that both size-dependent and nucleus-dependent chemistry of cloud droplets may need to be considered explicitly in cloud modelling work.Volume-weighted mean pH values in the range 5 to 6 were predicted from sensitivity studies in which input variables were varied over reasonable ranges, in agreement with two sets of bulk cloud-water pH data obtained by aircraft near Cape Grim.     

Chloride and Bromide Loss from Sea-Salt Particles in Southern Ocean Air

Datasets on aerosol composition in Southern Ocean air at Cape Grim and Macquarie Island, and rainwater composition at Cape Grim, have been analysed for sea-salt components in order to test the validity of the multiphase autocatalytic halogen activation process proposed initially by Sander and Crutzen (1996) and developed fully for clean marine air by Vogt et al. (1996). Four distinct datasets from the two locations were analysed. All four datasets provided consistent evidence in support of three predictions of the autocatalytic model: (1) overall Cl- deficits in sea-salt aerosol were small, difficult to quantify against analytical uncertainty and at most a few percent; (2) Br- deficits were large, averaging –30% to –50% on an annual basis, with strong seasonality ranging from about –10% in some winter samples to –80% or more in some summer samples; and (3) the Br- and Cl- deficits were clearly linked to the availability of strong, S-acidity in the aerosol, confirming the importance of acid catalysis to the dehalogenation process.