A 40-Year Analysis of the Hydrological Drought Index for the Tigris Basin, Turkey

Drought is a natural disaster that occurs globally when water availability is significantly below normal levels. Drought assessment is important for water resource planning, and therefore indexes can be used to characterize drought magnitudes. Using the monthly streamflow data at 47 stations from 1972 to 2011, the streamflow drought index (SDI) series with 3- (SDI-3), 6- (SDI-6), and 12-month (SDI-12) time scales were calculated, and the hydrological drought of the upper Tigris Basin in Turkey has been assessed. The results showed that almost all stations experienced at least one severe drought during the study period. The results revealed that since the early 1990s the study area has become drier. Using the data for a 12-month period and the area of the sub-basins for each flow monitoring station, area-weighted SDI-12 (WSDI) values were constructed. According to the WSDI results, the intensity and number of drought conditions increased during every 10-year period. This outcome has been verified using the maps of averaged SDI-12 values over the Tigris Basin. We noticed from observations of the maps that the time domain zones in lower latitudes experienced drought earlier as compared with zones in higher latitudes.