Evaluation of drought in the Gharasou basin using meteorological, hydrological, and remote sensing indices

The purpose of this study is to investigate drought using meteorological, hydrological, and remote sensing indicators in the Gharasou basin located in Ardabil province with an area of 495 square kilometers. Therefore, to calculate the SPI index was used monthly rainfall values related to 22 rainfall stations (1985-2015) and to calculate the SDI index, the monthly discharge values of 14 hydrometric stations (1985-2015). Then, SPI and SDI indices were compared with each other on a time scale of 1, 3, 6, 9, and 12 months, as well as seasonal and half a year. To study vegetation drought and calculate NDVI and VCI indices, were used MODIS images, with a spatial resolution of 500 meters for 15 years (2000-2015). The results of the meteorological drought time series in Gharasou showed that the drought occurred comprehensively in the years (2001-2000, 2006-2005, 2008-2007, and 2014-2013).  So that the most affected areas of drought were related to the northern and northeastern of the basin (Namin and Abarkooh). The results showed that meteorological drought has been more persistent in recent years and has been significantly correlated with hydrological drought with a delay of 1 to 3 months. NDVI and VCI indices showed that in identifying drought periods, these indices are consistent with meteorological drought, but are different correlation values. Finally, the comparison of NDVI and VCI indices with the SPI index showed that the VCI index with a value of r = 0.44 has the highest correlation with the SPI index. However, the correlation between NDVI and SPI indices is r = 0.38. The results of the VCI index showed that in drought conditions, only in the east of the basin and in the areas around Sabalan mountain, favorable vegetation conditions are observed, but in other areas of the basin, moderate and weak drought has occurred.