Analysis of Wet and Dry Spells Intensity and duration using Precipitation-Based and Evapotranspiration Influenced Indices

Spatio-temporal variability of wet and dry spells can be controlled by climate variability within a watershed and will affect availability of water resources and management plans. The application of the wet-dry spell analysis is presented for seven synoptic stations in the western part of Iran (Kurdistan Province). Numbers of consecutive months with standardized amount greater or less than the zero value were determined using Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI) as meteorological drought monitoring indices using R programming software. The characteristics of wet-dry spells which are parameters derived from 3-month SPI and SPEI indices were quantified and used to classify drought occurrence in the study area. Several drought parameters, such as number of wet-dry months/spells, maximum-minimum of drought intensity, max/min and average duration, were compared. The SPEI values calculated higher average intensity of wet and dry events in all stations in the study area. It is found that there is not a significant difference between SPI and SPEI indices in determining the long wet-dry spell durations. The SPEI value determines the highest number of change points between wet and dry spell occurrence which can be related to seasonal variation of temperature and calculated evapotranspiration. There is not a good correlation between SPI and SPEI in identification of the number of wet months, whereas, a significant positive correlation obtained between calculated number of dry spells using employed indices. Results reveal that the SPI and SPEI tools are not in agreement in detection of extreme wet and dry intensities according to insignificance of correlation coefficient. In this regard, the severe drought condition were identified to be 13 and 5 months in the Sanandaj and Baneh stations, respectively. Also the SPEI index were recognized at least 1 month as extreme drought at four stations. A strong correlation was found between the intensity of wet periods through applying SPI and SPEI. Comparison of the SPEI with other methods is necessary to understand the occurrence and variability of wet-dry events through different climatic regions.