Abstract

The effect of wind speed (WS) on rainfall measurement is well documented in hydrology. However, it is unknown how the latter translates into differences in the isotope composition of rainfall within and between events. Here we report a field study of precipitation from our site in Punta Arenas, Chile (average windspeed 12 ± 5.6 m·s-1). We hypothesised that gauge undercatch would increase with gauge height (as seen in other studies) and that isotopic effects would follow this trend whereby an evaporative fraction of rainfall and isotopic “amount-effects” would also follow trends in physical totals, both within and between events. We measured rainfall using three co-located pluviometers at heights of 0.05, at 0.5, and 1.2 m above ground level. A passive sequential water collector was attached to each pluviometer to collect samples for δ2H, δ17O and δ18O. Deuterium-excess and Δ’17O were calculated as proxies for precipitation evaporative enrichment at 1 to 3 mm volume increments. We measured WS at a nearby meteorological station. We examined 36 individual rainfall events with event rainfall totals ranging from 0.3 to 23 mm. Yearly total rainfall ranged from 446.1 – 515.8 mm. Individual storm totals were statistically different for 19 events. Within-storm WS for sampled events ranged from 0 to 10.5 m·s-1. Overall, our results showed more complex patterns than simple trends with height above ground. In general, gauge undercatch varied due to a combination of wind speed and direction. The consequences of these differences on an event basis were complex, reflected in the measured isotopic variability. Overall, the δ2H and δ18O data (the standard hydrological tracers) were in equilibrium and plotted along the Local Meteoric Water Line for Punta Arenas. More advanced analysis of δ’18O/Δ’17O suggests that wind may be enhancing kinetic fractionation of rainfall. We are exploring this now with air mass trajectory analysis.