Drought analysis with different indices for the Asi Basin (Turkey)

Effective drought risk management is crucial for water resource planning and management, especially with the intensifying impacts of climate change on water bodies. Climate change increases pressure on water resources through floods, droughts, ocean acidification, and rising sea levels. While the scientific community investigates the impacts of global warming on oceans and seas, including changes in biodiversity and species habitats, complete estimations remain challenging due to the complex, multi-component nature of natural systems. The Mediterranean region is particularly vulnerable to climate change, experiencing extreme weather events like heatwaves and increased frequency of natural disasters. Drought, defined as a significant precipitation deficiency, impacts various sectors including meteorology, hydrology, and agriculture. Drought indices are vital for quantitatively monitoring drought duration and severity. This study focuses on the Asi Basin in Turkey, a region experiencing increases in natural disasters due to extreme weather events, to analyze drought using a variety of indices and to contribute to improved drought management strategies.

Numerous studies have examined drought conditions in relation to weather and climate patterns, including the frequency of severe weather events. The Mediterranean is identified as a climate change hotspot, rapidly responding to atmospheric events and impacting various systems, including water management, human health, biodiversity, agriculture, and socio-economic efficiency. The Euro-Mediterranean region frequently experiences extreme climatic and weather events. Existing research employs various drought indices, including SPI, SPEI, and SRI, often focusing on regional drought analysis, examining long-term precipitation changes and the influence of ocean-atmosphere forcings. Previous work on the Asi Basin investigated drought trends using meteorological and hydrological data; this study expands on this using additional indices.

This research employed meteorological, hydrological, and hydrogeological data from 1970 to 2016 for the Asi Basin. Missing data were completed through regional analyses. Four drought indices were used: * **Deciles Index (DI):** A simple index based solely on precipitation data, categorized into deciles to represent drought severity. * **Standardized Precipitation Index (SPI):** A widely used index based on precipitation for various time scales (1, 3, 6, 9, 12, 18, 24, and 48 months). Data are fitted to a Gamma distribution and then transformed to a standard normal distribution. * **Standardized Precipitation Evapotranspiration Index (SPEI):** An index incorporating both precipitation and potential evapotranspiration (calculated using the Thornthwaite method), reflecting the impact of temperature on drought. Data are fitted to a log-logistic distribution and then transformed to a standard normal distribution. * **Standardized Runoff Index (SRI):** Similar to SPI, but using runoff data instead of precipitation. It reflects hydrological drought. For each index, threshold values were established to classify drought severity (severe, moderate, mild, and normal/humid). Areal weighted averages of index values for the entire basin were calculated. Drought incidence rates were determined for each index and drought severity (S), magnitude (M), and duration (D) were calculated using a method where the threshold value is zero. Drought risk maps and recurrence-severity maps were then created using the calculated indices. The analyses included trend analysis of precipitation, temperature and runoff.

The analysis of the Asi Basin using DI, SPI, SPEI, and SRI indices revealed several key findings: * **Drought Severity Analyses:** Time series for each index (DI, SPI, SPEI, and SRI) illustrated periods of drought severity, with the severity and duration varying across indices. Figure 1 shows an example of the monthly DI time series. * **Drought Risk Analyses:** Maps generated from 12-month index values depicted spatial distribution of drought risk (Figure 2 provides a sample of the SPEI 12-month severe drought percentage). * **Drought Incidence Rates:** Analysis showed varying incidence rates of mild, moderate, and severe droughts across different stations within the Asi Basin for each index (Examples shown in Figure 3). The averages of the stations within the basin show severe drought incidence rate in the basin to be 9.7% according to DI, 7.3% according to SPI, and 6.2% according to SPEI. The moderate drought incidence rate was 9.6% according to DI, 8.8% according to SPI, and 9.3% according to SPEI. The mild drought incidence rate was 9.7% according to DI, 13.9% according to SPI, and 13.6% according to SPEI. * **Arid Periods:** Common arid periods were identified across the indices, specifically in the years 1973–1974, 1989–1991, 1993–1994, 2000–2001, 2004–2005, 2014, and 2016. These periods were consistent in drought indication across different indices (Figure 6). * **Drought Severity, Magnitude, and Duration:** Calculations of drought severity (S), magnitude (M), and duration (D) based on SPI-12 and SPEI-12 indices showed varying levels across stations. Some stations experienced more severe, prolonged droughts than others (e.g., Iskenderun showed the highest drought severity using SPEI-12). Figures 4 and 5 display parameters used to define drought and a map of the SPEI-12 drought magnitude. * **Drought Risk Maps:** Maps illustrating the distribution of drought severity were produced for both SPEI-12 and SPI-12. * **Drought Recurrence-Severity Maps:** Maps showing drought recurrence at 50-year return periods for SPEI-12 and SPI-12 were generated. * **Recurrence Intervals:** Table 1 presents index values expected to recur at 5, 10, and 50-year return periods for 50% of the basin. Results indicated the likelihood of various drought severities at different return periods.

The high correlations observed between the indices suggest a strong relationship between precipitation, evapotranspiration, and runoff in determining drought conditions within the Asi Basin. The upward trend in annual precipitation, though not statistically significant, coupled with the downward trend in runoff, may indicate changes in water resource availability. Although the temperature increase was more apparent. The use of multiple indices provided a more comprehensive understanding of drought characteristics, capturing both meteorological and hydrological aspects. The study's findings highlight the vulnerability of the Asi Basin to drought, suggesting the need for effective drought management strategies. The selection of SPEI as a potentially suitable index for other basins with similar characteristics is justified by its inclusion of evapotranspiration in addition to precipitation. This study's results are valuable to drought management strategies for the Asi Basin and similar regions, emphasizing the importance of considering multiple indices for a comprehensive risk assessment.

This research provides a comprehensive drought risk assessment for the Asi Basin using four drought indices. The findings highlight the importance of using multiple indices to capture the complex nature of drought and to account for the impact of climate change. The identification of common arid periods and the spatial distribution of drought risk provide critical information for developing effective drought management plans for the region. Further research could explore the impacts of climate change on drought patterns in the region and improve the accuracy of drought predictions, refining the used methodologies.

The study's limitations include the data availability which affected the analysis of evaporation. The study used a limited number of weather stations and might not reflect small-scale variations in drought conditions. The accuracy of the drought indices relies on the accuracy of the input data; any errors in these data will propagate through the analysis. The study's focus is mainly on meteorological and hydrological drought, so it lacks information about the socioeconomic impact of droughts.