Global sinusoidal seasonality in precipitation isotopes

Abstract

Abstract. Quantifying seasonal variations in precipitation δ2H and δ18O is important for many stable isotope applications, including inferring plant water sources and streamflow ages. Our objective is to develop a data product that concisely quantifies the seasonality of stable isotope ratios in precipitation. We fit sine curves defined by amplitude, phase, and offset parameters to quantify annual precipitation isotope cycles at 653 meteorological stations on all seven continents. At most of these stations, including in tropical and subtropical regions, sine curves can represent the seasonal cycles in precipitation isotopes. Additionally, the amplitude, phase, and offset parameters of these sine curves correlate with site climatic and geographic characteristics. Multiple linear regression models based on these site characteristics capture most of the global variation in precipitation isotope amplitudes and offsets; while phase values were not well predicted by regression models globally, they were captured by zonal (0–30∘ and 30–90∘) regressions, which were then used to produce global maps. These global maps of sinusoidal seasonality in precipitation isotopes based on regression models were adjusted for the residual spatial variations that were not captured by the regression models. The resulting mean prediction errors were 0.49 ‰ for δ18O amplitude, 0.73 ‰ for δ18O offset (and 4.0 ‰ and 7.4 ‰ for δ2H amplitude and offset), 8 d for phase values at latitudes outside of 30∘, and 20 d for phase values at latitudes inside of 30∘. We make the gridded global maps of precipitation δ2H and δ18O seasonality publicly available. We also make tabulated site data and fitted sine curve parameters available to support the development of regionally calibrated models, which will often be more accurate than our global model for regionally specific studies.

Waters flowing out of systems are younger than the waters stored in those same systems

“…Although we suggest that these storage‐outflow age contrasts are expected to occur in any heterogeneous flow system, they may not always manifest in isotopic differences. Soil and xylem water isotopes can be used to infer their source because in most places, there are temporal variations in the isotopic composition of precipitation (Gat, ; Ingraham, ; Allen et al, ). As a consequence, soil water is a mixture of previous events with different isotope compositions and different ages.…”

Investigating young water fractions in a small Mediterranean mountain catchment: Both precipitation forcing and sampling frequency matter

“…In a seasonal climate, precipitation (or throughfall) is more enriched in heavy water isotopes during summer than during winter (Allen et al . 2019a), while evapotranspirative demand is higher in summer than in winter.…”

The Seasonal Origins of Streamwater in Switzerland

“…Given the widespread availability of streamflow isotope data (Halder et al, ) and maps describing precipitation isotope cycles (Allen, Jasechko, et al, ; Bowen, ), could be calculated for many landscapes and ecosystems. While strong seasonal cycles and high‐quality streamflow and precipitation data are required for calculating robust SOI values, such analyses could aid in advancing our basic understanding of how the partitioning of water into ET versus Q depends on its seasonal origin as precipitation.…”

“…Differences between the isotopic signatures of mean streamflow and precipitation can reflect the relative amounts of summer versus winter precipitation in streamflow. Isotope ratios in precipitation commonly exhibit strong seasonal cycles, especially outside of the tropics, with isotopically heavier precipitation in summer and lighter in winter (Allen, Jasechko, et al, ; Bowen, ). Past studies have used seasonal cycles in precipitation isotope ratios to distinguish between the relative contributions of summer versus winter precipitation in aquifers (e.g., as synthesized by Jasechko, ; Jasechko et al, ) or plant xylem (Allen, Kirchner, et al, ; Martin et al, ).…”