During the dry months of the water year in Mediterranean climates, groundwater influx is essential to perennial streams for sustaining ecosystem health and regulating water temperature. Predicted earlier peak flow due to climate change may result in decreased baseflow and the transformation of perennial streams to intermittent streams. In this study, naturally occurring radon-222 (²²²Rn) was used as a tracer of groundwater influx to Martis Creek, a subalpine stream near Lake Tahoe, CA. Groundwater ²²²Rn is estimated based on measurements of ²²²Rn activity in nearby deep wells and springs. To determine the degassing constant (needed for quantification of water and gas flux), an extrinsic tracer, xenon (Xe), was introduced to the stream and monitored at eight downstream locations. The degassing constant for ²²²Rn is based on the degassing constant for Xe, and was determined to be 1.9–9.0 m/day. Applying a simple model in which stream ²²²Rn activity is a balance between the main ²²²Rn source (groundwater) and sink (volatilization), the influx in reaches of the upstream portion of Martis Creek was calculated to be <1 to 15 m³/day/m, which cumulatively constitutes a significant portion of the stream discharge. Experiments constraining ²²²Rn emanation from hyporheic zone sediments suggest that this should be considered a maximum rate of influx. Groundwater influx is typically difficult to identify and quantify, and the method employed here is useful for identifying locations for focused stream flow measurements, for formulating a water budget, and for quantifying streamwater–groundwater interaction.

Stable isotopes of water allow researchers to examine water pathways and better understand spatial and temporal variability in mixtures of municipal water sources. In regions such as Kyiv (Ukraine), with a water supply that is vulnerable to the effects of climate change, pollution, and geopolitical conflict, such understanding is critical for effective water management. Trends in stable isotope values and water sources can function as a confirmation of municipal data. Additionally, these data can provide an early signal for the effects of climate change on these sources, reducing uncertainty from physical measurements.For this study, tap water, surface water, and groundwater were collected over 14 months in Kyiv and nearby Boryspil, Brovary, and Boyarka and measured for hydrogen (δ²H) and oxygen (δ¹⁸O) stable isotopes. The stable isotope values from the tap water for each district show a general seasonal trend in water sources, with more groundwater used in the supply in the winter for most districts. Spatially, groundwater use increases from south to north in the left-bank districts in Kyiv city and groundwater use generally decreases from south to north in the right-bank districts. As precipitation patterns shift and temperatures increase, the reliance on particular water sources may need to shift as well. Overall, δ²H and δ¹⁸O data provide a baseline expectancy for current water use throughout the year and, from this, deviations can be assessed early.

North-central Ukraine is vulnerable to temperature increases and precipitation pattern changes associated with climate change. With water management becoming increasingly important, information on current water sources and moisture recycling is critically needed. Isotope ratios of oxygen (δ¹⁸O) and hydrogen (δ²H) in precipitation are sensitive to these variables and allow comparisons across the region. The δ²H and δ¹⁸O values from collected precipitation in Kyiv and Cherkasy in 2020 and published ³H data for Kyiv from the year 2000 show an influence of the North Atlantic Oscillation (NAO) and provide information about processes affecting precipitation along the storm trajectory. The δ¹⁸O values also show a correlation with temperature, indicating that precipitation patterns may be affected by the rising temperatures in the region, as predicted by recent regional studies using Representative Concentration Pathway scenarios and the global climate model GFDL-ESM2M. When compared to backtracked storm trajectory and NAO data, clear relationships emerged between water isotope ratios, storm paths, and likely moisture recycling. Overall, δ²H, δ¹⁸O, ³H, and backtracked storm trajectory data provide more regional and local information on water vapour processes, improving climate-change-driven precipitation forecasts in Ukraine.