The influence of below-cloud secondary effects on the stable isotope composition of hydrogen and oxygen in precipitation at Calgary, Alberta, Canada

Stable isotope compositions of hydrogen (δ²H) and oxygen (δ¹⁸O) for short-term precipitation samples (n = 436) collected at Calgary, Alberta, Canada, between January 1997 and December 2001 were determined. Linear regression between δ2H and δ¹⁸O values of snow and large amount rain samples (≥4 mm) yielded correlation equations δ²H = 7.72 × δ¹⁸O + 5.02 and δ²H = 7.50 δ¹⁸O + 0.27, respectively. In contrast, correlation equations between δ²H and δ¹⁸O values for small amount rain samples (≤4 mm) resulted in progressively lower slope and intercept values with decreasing precipitation. Correlations of isotope data with parameters such as local temperature, relative humidity, and precipitation amount provided evidence that small amount rain samples undergo secondary evaporation accompanied by mass dependent isotope fractionation during their descent from the cloud base to the ground. Hence, the isotope compositions of precipitation at Calgary, and likely also at other locations in the North American Great Plains region, are influenced by below-cloud secondary effects. Since about one-third of the precipitation events in the 5-yr observation period were rain samples accumulating less than 4 mm, below-cloud secondary effects resulted in a slight decrease of slope and intercept values of the local meteoricwater line (δ²H=7.43 × δ¹⁸O-2.79) calculated using amount-weighted monthly average δ²H and δ¹⁸O values compared to equations based on isotope data for snow and large amount rain events only. The correlation equation (δ2H = 7.11 δ¹⁸O-11.60) calculated using δ²H and δ¹⁸O values of individual samples (non-amount weighted) yielded the lowest slope and intercept values caused by the significant influence of small amount rain samples.