INTEGRATING STABLE ISOTOPES IN PRECIPITATION AND EARTH SYSTEM MODELS TO INTERPRET LOCAL WATER CYCLE DYNAMICS

The stable isotopes of water carry unique information on climate, hydrological, and ecological processes. Factors that affect stable isotope composition of precipitation (δ¹⁸O and δD) include the source of moisture, temperature, and local distillation and evaporation processes. At any given location, precipitation is a mixture of different moisture sources, including evaporated ocean water and local moisture recycled through evapotranspiration. A better understanding of the temporal variation in the contribution of different precipitation sources is important for understanding local water cycling process as well as predicting extreme weather events such as droughts and floods.

Here we report the stable isotope values of individual precipitation events in Carbondale, Illinois measured from November 2012 to December 2015. Carbondale is located in Midwestern United States and has an increase in topography in southern Illinois which is thought to increase annual precipitation from ten to fifteen percent than the counterpoints in the maximum low of the basin. The local precipitation patterns are affected by moisture transported from Gulf of Mexico as well as local moisture recycled through transpiration and evaporation of the biosphere.

Our results show that, the average local meteoric water line has a slope of 7.8 and an intercept of 12.4, values similar to those of GMWL. Precipitation isotopic and deuterium excess (d-excess) values show clear seasonality. D-excess parameter can be taken as a measure of recycled water to precipitation. The high-amount precipitation events had enriched isotopic signature and small or negative d-excess values, suggesting the input of moisture originated from Gulf of Mexico. However, the majority of precipitation events were low-amount, had a wide range of isotopic values and d-excess > 5, suggesting that a major component was moisture originating from local recycled water.

Since most of the surface water is stored in soil and vegetation, changes in the amount as well as transfer rates among these reservoirs can have a significant effect on the regional land-atmosphere water fluxes with direct implications on economical activities.