Comparison of Meteorological Drought Indices in Georgia (1931-2020)

Abstract

Recently, the severity of droughts has been increasing due to climate change. Due to the multifaceted nature of droughts (meteorological, hydrological, economic, ecological, etc.), it affects almost all aspects of community life directly or indirectly, both short and long term. Georgia is characterized by diverse terrain and, accordingly, climatic conditions. Most types of climates are present in Georgia except savanna and tropical forests (from humid subtropical to dry subtropical, and climate of eternal snows and glaciers). Therefore, droughts are expressed differently in this small area (67,900 km²). The complexity of different indices used in drought studies depends on the availability of the used data. The purpose of the study was to analyze the intensity of droughts in the short and long term in the territory of Georgia and their distribution for 1931-2020. In this study, we focused on the widespread Standard Precipitation Index (SPI) and Standard Precipitation Evapotranspiration Index (SPEI).  Both indices were calculated based on the data of more than 100 rain gauges located in the study area for several time-scales including 3, 6, 12 and 24 months covering the period from 1931 to 2020. As SPI uses only precipitation data, evapotranspiration is also taken into account in SPEI, which offers a more complete picture of the background of the diversity of Georgia's climate. Daily temperature (for calculation of ET) and precipitation data are used in the research. We calculated the Pearson correlation, R² and RMSE. The correlation of SPI and SPEI allowed us to determine climate type with decisive role of temperature in assessing droughts. The frequency of severe droughts has increased throughout the country, especially in recent decades. This trend is especially striking in the case of the eastern Georgian lowland. In the example of Eastern Georgia‘s precipitation data, another trend was revealed. Here the correlation of SPI and SPEI was relatively low and decreased as the period increases; for example, the correlation for 12- and 24-month periods was lower than for 3- and 6-month periods. This shows that when assessing droughts in East Georgia, it is crucial to take into account the change in temperature along with the change in precipitation. Therefore, in western Georgia, where there is a humid subtropical climate, it is possible to create an idea about the nature of droughts only by using SPI. In the lowland of Eastern Georgia, where it is unlikely to see the accurate picture with only one index, and it is better to use multivariable indices, where along with precipitation, temperature and other data will be taken into account.