We investigate the evolution of snow temperature, water content, density and stable water isotopes of δ18O at four Arctic snow-pit sites during early-season melt, in order to understand the effects of melt on snowpack stratigraphies and seasonal isotopic signals. We relate isotopic changes observed at these sites to temperature reconstructions derived from a 33 year firn-core record drilled on the same icefield. Decreases in seasonal isotopic amplitudes observed at all but one snow-pit site coincide with the percolation of more enriched meltwater into the snowpack, suggesting that meltwater percolation is the dominant process causing isotopic redistribution in Arctic snowpacks during the melt season. The decrease in isotopic range was accompanied by increases in mean δ18O values at all snow-pit sites. Positive degree-day (PDD) calculations are used to relate the amount of melt observed at the low-elevation snow-pit sites to the firn-core site. Results based on PDD values suggest an average overestimation of 1.1°C in average annual temperature reconstructions from the firn-core site from 1967 to 2006, with the possibility of errors in excess of 3°C during high-melt years.

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Abstract:A Lagrangian (Rayleigh) distillation model is used to track the evolution of stable isotopes in precipitation over mountainous terrain from the Pacific Coast of Canada to two alpine field sites in the Canadian Rocky Mountains. Precipitation υ 18 O at Vancouver constrains the model and air-mass back trajectories provide the water vapour pathway for 10 winter storm events. Isotopic values along storm pathways are modelled with a classical Rayleigh model that prescribes a linear decrease in temperature and pressure from initial to final conditions, and two models that account directly for orographic precipitation processes by: (i) applying an orographic rainfall model and (ii) using North American Regional Reanalysis data to calculate the change in vapour content along storm pathways. All models are significant predictors of snowpack υ 18 O, but the orographic model provides the best fit to precipitation-weighted υ 18 O for each storm. The improvement in modelled υ 18 O by accounting for terrain along storm trajectories illustrates the need to account for orographically driven moisture loss when modelling vapour transport to ice core sites with mountainous upwind terrain. This finding is also applicable to isotopic studies of paleoaltimetry and source areas of groundwater recharge.

Harvesting floodwaters to recharge depleted groundwater aquifers can simultaneously reduce flood and drought risks and enhance groundwater sustainability. However, deployment of this multibeneficial adaptation option is fundamentally constrained by how much water is available for recharge (WAFR) at present and under future climate change. Here, we develop a climate-informed and policy-relevant framework to quantify WAFR, its uncertainty, and associated policy actions. Despite robust and widespread increases in future projected WAFR in our case study of California (for 56/80% of subbasins in 2070–2099 under RCP4.5/RCP8.5), strong nonlinear interactions between diversion infrastructure and policy uncertainties constrain how much WAFR can be captured. To tap future elevated recharge potential through infrastructure expansion under deep uncertainties, we outline a novel robustness-based policy typology to identify priority areas of investment needs. Our WAFR analysis can inform effective investment decisions to adapt to future climate-fueled drought and flood risk over depleted aquifers, in California and beyond.

roundwater is a critical resource for California's agricultural sector, accounting for almost 40% of agricultural water use, and far more in drought years (DWR 2015). Many groundwater basins, particularly in the Central Valley, have experienced significant declines in groundwater levels over the past several decades, and the recent drought heightened concerns over these declines and associated impacts. In 2014, the California Legislature passed the Sustainable Groundwater Management Act (SGMA), introducing for the first time a requirement that local agencies manage groundwater sustainably or face state intervention. SGMA grants broad authority for groundwater management to locally formed groundwater sustainability agencies (GSAs). Local agencies were given until