Controlling Factors of the Stable Isotope Composition in the Precipitation of Islamabad, Pakistan

Significant temporal variations in delta(18) O and deuterium isotopes were found in the rainfall water of Islamabad, Pakistan, over a 15-year period (1992-2006). The data were obtained from the International Atomic Energy Agency/Global Network of Isotopes in Precipitation (IAEA/GNIP) database, and statistical correlations were investigated. In particular, this study provides the first detailed analysis of GNIP data for Islamabad. Both dry (1999-2000) and wet years (1994, 1997, and 2000) were chosen to investigate the correlations between precipitation amount, vapor flux, and temperature. We observed obvious differences between the dry and wet years and among seasons as well. Long-term features in the isotope composition agreed with the global meteorological water line, whereas short-term values followed rainfall amounts; that is, a total of 72% of the precipitation's isotopic signature was dependent on the rainfall amount, and temperature controlled 73% of the isotopic features during October to May. The lower d-excess values were attributed to conditions during the spring season and a secondary evaporation boost during dry years; precipitation originating from the Mediterranean Sea showed high d-excess values. Overall, the results of this study contribute to the understanding of precipitation variations and their association with water vapor transport over Islamabad, Pakistan.