Stable isotopes of oxygen and hydrogen in precipitation and river water in Georgia

The article presents the first comprehensive evaluation of the isotopic composition of precipitation and runoff in Georgia. It is based on data from the monitoring of delta O-18 and delta H-2 in monthly precipitation at eleven precipitation stations and monthly runoff from five large, mostly nested catchments in Georgia, collected between the years 2013 and 2022. The data represent different climatic and altitudinal conditions ranging from the wet western part of Georgia affected by the Black Sea to the dry eastern part of the county presumably affected also by the air masses from the Caspian Sea. Specific conditions affecting the isotopic composition of precipitation and river water seem to occur close to the main ridge of the Greater Caucasus Mountains. We defined the National Meteoric Water Line (NMWL) and the National River Water Line (NRWL) for Georgia as well as the Local Meteoric Water Lines (LMWLs) and River Water Lines (RWLs) for individual precipitation stations and river gauges and investigated their slopes and intercepts. Altitude gradients of delta O-18 and delta H-2 in the rivers were similar to those in precipitation (-0.16% and -1.2% per 100 m of altitude for delta O-18 and delta H-2, respectively). The rivers were supplied from precipitation of the colder part of the year (November/December to March/April) while the isotopic composition of precipitation in the remaining warmer part of the year was clearly detached from that of the rivers. The isotopic signal in precipitation was dampened in the rivers by a factor of 5-6. The scaled line-conditioned excess (lc-excess*) did not indicate any significant deviations of river waters from meteorological conditions in the catchments.