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Inversion of chemical records archived in ice cores to atmospheric concentrations requires a detailed understanding of atmosphere-to-snow-to-ice transfer processes. A unique year-round series of surface snow samples, collected from November, 1994 through January, 1996 at South Pole and analyzed for H202, were used to test a physically based model for the atmosphere-to-snow component of the overall transfer function. A comparison of photochemical model estimates of atmospheric H202, which are in general agreement with the first measurements of atmospheric H202 at South Pole, with the inverted atmospheric record (1) demonstrate that the surface snow acts as an excellent archive of atmospheric H202 and (2) suggest that snow temperature is the dominant factor determining atmosphere-to-surface snow transfer at South Pole. The estimated annual cycle in atmospheric H202 concentration is approximately symmetric about the summer solstice, with a peak value of •0280 pptv and a minimum around the winter solstice of •01 pptv, although some asymmetry results from the springtime stratospheric ozone hole over Antarctica.

Environmental flows (e‐flows) are powerful tools for sustaining freshwater biodiversity and ecosystem services, but their widespread implementation faces numerous social, political, and economic barriers. These barriers are amplified in water‐limited systems where strong trade‐offs exist between human water needs and freshwater ecosystem protection. We synthesize the complex, multidisciplinary challenges that exist in these systems to help identify targeted solutions to accelerate the adoption and implementation of environmental flows initiatives. We present case studies from three water‐limited systems in North America and synthesize the major barriers to implementing environmental flows. We identify four common barriers: (a) lack of authority to implement e‐flows in water governance structures, (b) fragmented water governance in transboundary water systems, (c) declining water availability and increasing variability under climate change, and (d) lack of consideration of non‐biophysical factors. We then formulate actionable recommendations for decision makers facing these barriers when working towards implementing environmental flows: (a) modify or establish a water governance framework to recognize or allow e‐flows, (b) strive for collaboration across political jurisdictions and social, economic, and environmental sectors, and (c) manage adaptively for climate change in e‐flows planning and recommendations. This article is categorized under: Water and Life > Conservation, Management, and Awareness Human Water > Water Governance Engineering Water > Planning Water

Abstract. Of the main atmospheric oxidants, only hydrogen peroxide (H202)isfrom snow-pit and ice-core records and (2) to invert snow-pit and ice-core H202 profiles to obtain estimates of the seasonal or annual accumulation time series. In the first case, an independent estimate of snow accumulation is needed, and in the second application, an independent estimate of the annual H202 atmospheric cycle is needed.

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A fully worked example of decision-support-scale uncertainty quantification (UQ) and parameter estimation (PE) is presented. The analyses are implemented for an existing groundwater flow model of the Edwards aquifer, Texas, USA, and are completed in a script-based workflow that strives to be transparent and reproducible. High-dimensional PE is used to history-match simulated outputs to corresponding state observations of spring flow and groundwater level. Then a hindcast of a historical drought is made. Using available state observations recorded during drought conditions, the combined UQ and PE analyses are shown to yield an ensemble of model results that bracket the observed hydrologic responses. All files and scripts used for the analyses are placed in the public domain to serve as a template for other practitioners who are interested in undertaking these types of analyses.

[1] The chloride mass balance (CMB) method for estimating groundwater recharge is economic and effective, provided that the hydrological conditions for its applications are met and the modeling parameters are known. However, modeling parameters such as precipitation and Cl À deposition rates vary temporally, most notably as a result of the climatic changes from late Pleistocene to Holocene. The temporal variability of atmospheric Cl À input and annual precipitation were considered in this study by using a discrete steady state CMB model with different parameters for late Pleistocene and Holocene. Cl À deposition rates, estimated from 36 Cl data, were lower in late Pleistocene than Holocene at Yucca Mountain, Nevada, but higher in late Pleistocene than Holocene at Black Mesa, Arizona. Paleoclimate proxies at both Yucca Mountain and Black Mesa point to higher precipitation rates in late Pleistocene than Holocene. The resulting average recharge estimates for Black Mesa are 9 ± 5 mm/yr for Holocene and 35 ± 22 mm/yr for late Pleistocene. Local recharge rates at Yucca Mountain were estimated from the 36 Cl/Cl ratios and Cl À concentrations in perched waters. The estimated recharge for Yucca Mountain is 5 ± 1 mm/yr for Holocene and 15 ± 5 mm/yr for late Pleistocene. These recharge rates are comparable to results of independent numerical groundwater flow models and watershed-scale infiltration models at Black Mesa and Yucca Mountain, respectively.INDEX TERMS: 1040 Geochemistry: Isotopic composition/chemistry; 1829 Hydrology: Groundwater hydrology; 1045 Geochemistry: Low-temperature geochemistry; 1836 Hydrology: Hydrologic budget (1655); KEYWORDS: recharge, Yucca mountain, chloride mass balance, chlorine-36, paleohydrology Citation: Zhu, C., J. R. Winterle, and E. I. Love, Late Pleistocene and Holocene groundwater recharge from the chloride mass balance method and chlorine-36 data, Water Resour.

ABSTRACT. Previous research h as d ocumented a close associatio n b etween highresoluti on snow-pit profiles of hydrogen and oxygen st a bl e-isotope ratios a nd multi-year Specia l Sensor Micr ow ave /Imager (SSM/I ) 37 GH z brightness temp e r a ture data in centra l Greenland. C omparison of the SSM/I data to profil es obtained during the 1989-91 field seasons indi cated that bD a nd 6 18 0 data from the near-surface snow at the Gree nla nd summit are a r eliabl e, high-resoluti on temperature proxy. To test this new technique further, additiona l sta ble-isotope d a ta were obtained fro m a 2 m snow pi t constructed during late-Jun e 1995 near the GISP2 site. This new profile, supported by pit stratigraphy and ch emistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. Th e sub-annua l variation of the 8D r ecord at the GISP2 site was determined using 15 m a tch points, from approxim ately D ecember 1991 throug hJune 1995 a nd was guided in p a rt by time-constr a ined hoar layers. The close associati o n of these tempe ra ture proxies supports the asserti o n that snow acc umulation occ urs frequ entl y throu g h th e year a nd that the iso top e r ecord initiall y co nta ins temperature information from m any times of th e yea r. Thi s is a lso independently confirmed by a na lysis of H 2 0 2 d ata . The slope of the multi-year T vs 6 co rrelati on was evalu ated along with the sub-annu al vari ation in the a m o unt, rate and timing of accumula tion. These new r es ults are consistent with those fr o m the previous study a nd they also d em onstrate tha t the snow in this a r ea initi all y conta in s temperature a nd ch emical record s with sub-a nnual r esolution. This enco urages confident interpretation of the paleoclimatic signal varia tions in the GISP 2 a nd GRIP deep cores.