Predicting Spatial Patterns in Precipitation Isotope (δ²H and δ¹⁸O) Seasonality Using Sinusoidal Isoscapes

Abstract

Understanding how precipitation isotopes vary spatially and temporally is important for tracer applications. We tested how well month‐to‐month variations in precipitation δ18O and δ2H were captured by sinusoidal cycles, and how well spatial variations in these seasonal cycles could be predicted, across Switzerland. Sine functions representing seasonal cycles in precipitation isotopes explained between 47% and 94% of the variance in monthly δ18O and δ2H values at each monitoring site. A significant sinusoidal cycle was also observed in line‐conditioned excess. We interpolated the amplitudes, phases, and offsets of these sine functions across the landscape, using multiple linear regression models based on site characteristics. These interpolated maps, here referred to as a sinusoidal isoscape, reproduced monthly observations with prediction errors that were smaller than or similar to those of other isoscapes. Sinusoidal isoscapes are likely broadly useful because they concisely describe seasonal isotopic behavior and can be estimated efficiently from sparse or irregular data.

Sensitivity of young water fractions to hydro-climatic forcing and landscape properties across 22 Swiss catchments

“…Instead, δ 18 O values from monthly cumulative precipitation samples were interpolated from long-term observations at nearby monitoring stations (the Swiss network for Observations of Isotopes in the Water Cycle -NAQUA-ISOT, the Global Network of Isotopes in Precipitation -GNIP, and the Austrian Network of Isotopes in Precipitation -ANIP). We used two different interpolation approaches that we summarize below: method 1 after Seeger and Weiler (2014) and method 2 similar to that of Allen et al (2018). More detailed descriptions of both interpolation methods 1 and 2 can be found in Seeger and Weiler (2014) and in the Supplement, respectively.…”

“…These interpolated coefficients were volume-weighted by the spatial pattern of precipitation over each catchment. An alternative approach (Allen et al, 2018; see Supplement) builds on the Jasechko et al (2016) method with an additional step that accounts for the residuals of the observations from the fitted seasonal cycles. To generate a high-resolution precipitation isotope map for Switzerland, Seeger and Weiler (2014) interpolated δ 18 O in monthly precipitation from long-term monitoring stations in central Europe, using an elevation-gradient approach.…”

Water source segregation along successional stages in a degraded karst region of subtropical China

“…This value is equivalent to mobile soil water and/or groundwater that normally undergo slight evaporative enrichment (Hasselquist, Benegas, Roupsard, Malmer, & Iistedt, ; Tiemuerbieke et al., ). Precipitation itself can also have either positive or negative LC‐excess (Allen et al., ); for example, the most negative LC‐excess of rainfall received during the study period was around −20‰ (Supporting information ). In these circumstances, except for the sixth sampling time (on October 25), the relatively more negative averaged stem water LC‐excess of the remaining sampling times does not necessarily mean rainwater has undergone strong evaporative enrichment before being absorbed by plant roots.…”

“…where a and b are the slope and y ‐intercept, respectively, of the LMWL. Precipitation itself can also deviate from the LMWL, and thus have either positive or negative LC‐excess (Allen, Kirchner, & Goldsmith, ). Soil water and plant stem water samples normally have negative LC‐excess values, a more negative LC‐excess value of stem water sample suggests that source water has experienced stronger evaporative enrichment before it was absorbed by plant roots (Landwehr & Coplen, ; Sprenger, Leistert, Gimbel, & Weiler, ).…”

The Seasonal Origins of Streamwater in Switzerland

“…The estimated seasonal isotope cycles were used to calculate δ Pw and δ Ps as sine‐curve offsets (i.e., the value around which the curve oscillates) ± amplitudes. Thus, the “summer” and “winter” seasons are defined by the fitted sine curves themselves, which peak in mid‐July and are lowest in mid‐January in Switzerland (Allen et al, ). Summer versus winter seasons may roughly correspond to liquid (rain) versus solid (snow) precipitation in the higher‐elevation catchments but not at lower elevations where rainfall occurs regularly in winter.…”