Isotopic evidence for widespread cold‐season‐biased groundwater recharge and young streamflow across central Canada

Transformations of precipitation into groundwater and streamflow are fundamental hydrological processes, critical to irrigated agriculture, hydroelectric power generation, and ecosystem health. Our understanding of the timing of groundwater recharge and streamflow generation remains incomplete, limiting our ability to predict fresh water, nutrient, and contaminant fluxes, especially in large basins. Here, we analyze thousands of rain, snow, groundwater, and streamflow δ¹⁸O and δ²H values in the Nelson River basin, which covers 1.2 million km² of central Canada. We show that the fraction of precipitation that recharges aquifers is ~1.3–5 times higher for precipitation falling during cold months with subzero mean monthly temperatures than for precipitation falling during warmer months. The near‐ubiquity of cold‐season‐biased groundwater recharge implies that changes to winter water balances may have disproportionate impacts on annual groundwater recharge rates. We also show that young streamflow—defined as precipitation that enters a river in less than ~2.3 months—comprises ~27% of annual streamflow but varies widely among tributaries in the Nelson River basin (1–59%). Young streamflow fractions are lower in steep catchments and higher in flatter catchments such as the transboundary Red River basin. Our findings imply that flat, lower permeability, heavily tiled landscapes favor more rapid transmission of precipitation into rivers, possibly mobilizing excess soluble fertilizers and exacerbating eutrophication events in Lake Winnipeg.     

洞庭湖流域下落雨滴蒸发研究

通过在亚热带季风区内的洞庭湖流域对降水事件取样以及气象资料的收集,采用降水同位素对流域下落雨滴蒸发以及影响因素进行了研究。研究发现：流域不同季节大气水线的理论斜率均高于观测斜率,由此可判断形成流域降水的雨滴在下落过程中经历了不同程度的蒸发。进一步计算表明流域下落雨滴的蒸发比率介于0.21%~19.29%之间,蒸发比每增大1.0%,过量氘（d）将减小1.38‰。从主要影响下落雨滴蒸发的气温、湿度和雨滴直径来看,流域下落雨滴蒸发与气温呈显著正相关,与相对湿度、雨滴直径呈显著负相关。另外,流域下落雨滴蒸发随降水量增大呈指数函数递减,其并非指示降水量直接影响雨滴的蒸发,而是湿度、雨滴直径等综合作用的结果。     

Deuterium‐excess determination of evaporation to inflow ratios of an alpine lake: Implications for water balance and modeling

The numerous lakes on the Tibetan Plateau play an important role in the regional hydrological cycle and water resources, but systematic observations of the lake water balance are scarce on the Tibetan Plateau. Here, we present a detailed study on the water cycle of Cona Lake, at the headwaters of the Nujiang‐Salween River, based on 3 years (2011–2013) of observations of δ¹⁸O and δ²H, including samples from precipitation, lake water, and outlet surface water. Short‐term atmospheric water vapor was also sampled for isotope analyses. The δ²H–δ¹⁸O relationship in lake water (δ²H = 6.67δ¹⁸O ? 20.37) differed from that of local precipitation (δ²H = 8.29δ¹⁸O + 12.50), and the deuterium excess (d‐excess) in the lake water (?7.5‰) was significantly lower than in local precipitation (10.7‰), indicating an evaporative isotope enrichment in lake water. The ratio of evaporation to inflow (E /I ) of the lake water was calculated using both d‐excess and δ¹⁸O. The E /I ratios of Cona lake ranged from 0.24 to 0.27 during the 3 years. Observations of atmospheric water vapor isotopic composition (δ _A ) improved the accuracy in E /I ratio estimate over a simple precipitation equilibrium model, though a correction factor method provided nearly identical estimates of E /I ratio. The work demonstrates the feasibility of d‐excess in the study of the water cycle for lakes in other regions of the world and provides recommendations on sampling strategies for accurate calculations of E /I ratio.     

Vegetation water sources in California's Sierra Nevada (USA) are young and change over time,a multi-isotope (δ18O, δ2H, 3H) tracer approach.

Sierra Nevada forests transpire a significant amount of California's water resources, sparking interest in applying forest management to improve California's water supply. Determining the source water of evapotranspiration enables forest managers to make informed decisions. To this end, a significant interest in critical zone science is to develop new methods to work across time scales to predict subsurface water storage and use. In this study, forest vegetation accessed young water and switched sources depending on availability, suggesting that forest drought vulnerability may depend on the range of water sources available (rain, snowmelt and deeply stored water). This finding also suggests that changes in transpiration rates may have immediate effects on water sources in close proximity to vegetation, and delayed effects on storage and runoff. New δ¹⁸O, δ²H and ³H data were used to track precipitation, runoff, evapotranspiration and storage through the critical zone seasonally, including seasons where evapotranspiration and snowmelt were in phase (winter snowmelt) and out of phase (seasonally dry summer). The main source of this headwater catchment's runoff is derived from its meadow saturated zone water, which was dominated by snowmelt. Water that originated as snowmelt contributed to transpiration, unless other sources, such as recent rain, became available. In cases where xylem δ¹⁸O and δ²H signatures matched those of deeper saturated zone water, ³H data showed that xylem water was distinctly younger than the deep saturated zone water. During 2016, which experienced relatively normal snowpack in winter and seasonally dry summer conditions, mean summer saturated zone water and vegetation water were similar in δ¹⁸O, −12.4 ± 0.04 ‰ and − 12.5 ± 0.3 ‰, respectively, but were distinctly different in ³H, 5.5 ± 0.2 pCi/L and 13.7 ± 1.1 pCi/L, respectively. While δ¹⁸O shows that vegetation and meadow saturated zone water have similar origins, ³H shows they have dissimilar ages.     

新德里季风降水中过量氘与季风水汽来源

收集和分析了新德里降水中同位素资料(δ¹⁸O和δD),利用季风水线方程对个别年份缺测的δD资料进行估计,建立了新德里36 a夏季过量氘序列.基于降水中过量氘和水汽源区相对湿度关系考虑,利用NCEP/NCAR再分析资料,研究了新德里夏季过量氘序列和水汽源区相对湿度的关系.研究发现,西阿拉伯海相对湿度变化和新德里季风降水中过量氘变化较为一致.结合西阿拉伯海风速和印度西北地区季风降水量资料分析结果,认为西阿拉伯海是新德里季风水汽的主要来源.     

Vapour source and spatiotemporal variation of precipitation isotopes in Southwest Spain

The δ²H and δ¹⁸O composition of 77 precipitation samples collected between January 2014 and April 2019 from two sites across the Guadalquivir Basin, SW Spain, were analysed. The first site is located in an urban area of Seville at 100 km distance to the Atlantic coast and the second site is located in a dune area of the Doñana National Park a few kilometres from the coast. Sampling was performed within intervals of at least 14 days if rain occurred but frequently intervals were longer according to the rainfall incidence. Samples from both sites are available for the period February 2016 to June 2018 with six samples containing identical rain events at both locations. Precipitation weighted averages and local meteoric water lines produced by weighting and non-weighting regression methods are presented for its use in hydrological applications. Results show a remarkably high variability in δ²H and δ¹⁸O values and precipitation weighted average d-excess values of 11.8‰ and 13‰ at the sites Plaza de España and Doñana, respectively. Temperature and amount effects were found to be weak. A significant influence of secondary evaporation for single rainfall events during summer was identified by enriched isotopic signatures with reduced d-excess values plotting close or below the global meteoric water line. Backward trajectory analysis of 115 days with daily rainfall above 3 mm yield a predominant Atlantic Ocean vapour source with negligible Mediterranean influence and therefore, d-excess variability is attributed to the different ocean surface conditions of relative humidity and sea surface temperature. Parallel sampling indicate very similar isotopic signatures at both sites and point to the existence of thermal effects of the Plaza de España site in Seville city during the summer season.     

Geochemical and stable isotopic studies of Gulf of Suez’s hot springs, Egypt

The Gulf of Suez region is one of the most interesting geothermal areas in Egypt because of the high temperatures of its springs.The eastern and western shores of the Gulf of Suez are characterized by superficial thermal manifestations including a cluster of hot springs with varied temperatures.Variations of deuterium and oxygen-18 concentrations in thermal waters have been used to aid in describing the source of recharge in the Gulf of Suez hot springs.Isotope and geochemical data for the Gulf of Suez thermal waters suggest that recharge to the hot springs may not be entirely from the Gulf of Suez water,but possibly from the meteoric water that comes from areas of higher altitude surrounding the hot springs.     

Deuterium Hugoniot up to 120 Gpa (1.2 Mbar)

The shock compression curve (Hugoniot) of D₂ has been controversial because the two data sets measured previously with a laser (L) and with pulsed currents (PC) differ substantially. Recently, Hugoniot points of D₂ have been measured at shock pressures of 123, 109, 61, 54, and 28 GPa using hemispherically converging, explosively-driven systems (CS). The CS results are in good agreement with the PC data and the error bars of the CS-PC data are less than half those of the L data. The limiting compression obtained from the best fit to the CS-PC data is 4.30 ± 0.10 at 100 GPa. The CS-PC data are in good agreement with PIMC and DFT calculations, which is expected to be the case at higher shock temperatures and pressures, as well.     

Insights into groundwater salinization from hydrogeochemical and isotopic evidence in an arid inland basin

In the Manas River basin (MRB), groundwater salinization has become a major concern, impeding groundwater use considerably. Isotopic and hydrogeochemical characteristics of 73 groundwater and 11 surface water samples from the basin were analysed to determine the salinization process and potential sources of salinity. Groundwater salinity ranged from 0.2 to 11.91 g/L, and high salinities were generally located in the discharge area, arable land irrigated by groundwater, and depression cone area. The quantitative contributions of the evaporation effect were calculated, and the various groundwater contributions of transpiration, mineral dissolution, and agricultural irrigation were identified using hydrogeochemical diagrams and δD and δ¹⁸O compositions of the groundwater and surface water samples. The average evaporation contribution ratios to salinity were 5.87% and 32.7% in groundwater and surface water, respectively. From the piedmont plain to the desert plain, the average groundwater loss by evaporation increased from 7% to 29%. However, the increases in salinity by evaporation were small according to the deuterium excess signals. Mineral dissolution, transpiration, and agricultural irrigation activities were the major causes of groundwater salinization. Isotopic information revealed that river leakage quickly infiltrated into aquifers in the piedmont area with weak evaporation effects. The recharge water interacted with the sediments and dissolved minerals and subsequently increased the salinity along the flow path. In the irrigation land, shallow groundwater salinity and Cl^? concentrations increased but not δ¹⁸O, suggesting that both the leaching of soil salts due to irrigation and transpiration effect dominated in controlling the hydrogeochemistry. Depleted δ¹⁸O and high Cl^? concentrations in the middle and deep groundwater revealed the combined effects of mixing with paleo‐water and mineral dissolution with a long residence time. These results could contribute to the management of groundwater sources and future utilization programs in the MRB and similar areas.