Stable Isotope‐Inferred Hydrology of Ponds Created by the Mount St. Helens Eruption

Freshwater ponds are prevalent globally and provide critical ecosystem functions (e.g., water storage and groundwater recharge), yet little is known of their hydrological features immediately following formation. We analysed stable isotopes of water (δ 2 H, δ 18 O) to characterize spatio‐temporal hydrologic variation in ponds created by the Mount St. Helens eruption. We also examined how climate and landscape features interact to regulate local hydrology. Ponds were sampled for isotopic analysis in spring (2015, 2017, 2018) and summer (2018). Mass balance models characterized the water balance of ponds (evaporation:inflow; E:I), as well as water sources (rain, snow). Other variables were measured in situ (temperature, conductance), collected from data sources (meteorology) or quantified with remote sensing (vegetation). Bayesian estimates of standard ellipse areas (SEA B ) were used to compare isotopic values among years, whereas linear models were used to examine local and regional drivers of E:I, as well as intra‐annual isotopic shifts. We observed high interannual variability (as SEA B ), suggesting that snow was the main water source in wet years but that the proportion of rain and snow varied among sites in dry years. Spring E:I was negatively correlated with total precipitation, whereas the importance of evaporation in summer varied with pond morphology, with large shallow ponds exhibiting the greatest evaporation. Evaporation regulated the hydrology of ponds with higher dissolved organic carbon (DOC; as residuals of DOC and chlorophyll). We show that simple metrics of basin morphometry can predict seasonal variability in pond hydrology, allowing managers to better estimate pond sensitivity to future climate conditions.