CONTROLS ON STABLE ISOTOPE VARIABILITY IN PRECIPITATION DURING 2015-2018 IN SAN ANTONIO, CENTRAL TEXAS

Oxygen and hydrogen stable isotopes (δ¹⁸O and δD) in precipitation collected in rain events from 2015 to 2018 in San Antonio (SA), Central Texas, are presented in this research. The total correlation between δ¹⁸O and δD obtained here is similar to the Global Meteoric Water Line. However, variations of the regression line from 2015 to 2018 are likely caused by changes in moisture sources for the precipitation. Neither “amount effect” nor “temperature effect” occurred in the time series of δ¹⁸O values of daily precipitation in SA. However, moisture source is the controlling factor on daily precipitation δ¹⁸O and D-excess (d), which generally agrees with the air trajectory modeled analysis using HYSLPIT for specific rain events. Rain events with moisture from Gulf of Mexico (GoM) have high δ¹⁸O and low d signals; rain events with moisture from east Pacific have low δ¹⁸O signals; and rain events with continental moisture have high d signal. Results of this study imply that some climate proxies such as speleothem δ¹⁸O may indicate moisture sources instead of changes in temperature or the amount of precipitation.

Two severe convective storm events are investigated in this study. We collected and photographed 20 hailstones for the hailstorm of April 12th, 2016, northern SA. The isotopic analysis of the hailstone subsamples, together with their crystal structures, are examined here to provide information on the vertical profile of the moisture content, cloud droplet sizes, and temperature of the clouds in which the hailstones formed. Hourly precipitation samples of the Hurricane Harvey were collected during a 45-hour span between August 25 and 27, 2017 in SA. The stable isotope composition of precipitation changed remarkably before, during, and after Hurricane Harvey, which was due to changes of moisture sources from terrestrial to oceanic sources of water vapor.