Using Google Earth Engine in Monitoring and Trend Analysis of Hydrological Drought Time Series Based on SWSI Index in Western Sub-Basins of Lake Urmia

Drought as a natural phenomenon is one of the most important challenges in water supply, especially in arid and semi-arid climates such as Iran. In this study, the hydrological drought has been investigated in the western sub-basins of Lake Urmia using the Surface Water Supply Index (SWSI) from Oct. 2009 to Sep. 2019. For this purpose, annual precipitation data, snow water equivalent, surface runoff, and reservoir storage (or changes in groundwater storage) have been used. The methodology of the study is based on the SWSI index, remote sensing using Google Earth Engine, determining the snow cover area using the NDSI index, and analyzing the drought trend using the Mann–Kendall trend test. The results showed that according to the weights of the components obtained by the Analytic Hierarchy Process (AHP), precipitation is the most important factor in the study area in hydrological droughts. Also, according to the SWSI index, during the 10 water year period, wet, extreme drought, and severe drought conditions have not occurred in the study area and generally, in 54% of the cases near-normal conditions, in 19% of the cases incipient drought conditions prevail and in the next ranks are abundant and moderate drought conditions with respectively 14% and 13% incidence. Also, the wettest water year was from Oct. 2018 to Sep. 2019 and the driest water years were from Oct. 2016 to Sep. 2017 and from Oct. 2011 to Sep. 2012. In addition, in all studied sub basins except Zolachay, the near-normal hydrological condition was the most common condition during the study period. Also, the results of Mann-Kendall non-parametric trend test showed that only in the Baranduzchay and Rashkan-Talkhab sub basins, the trend of hydrological drought time series has been increasing (increasing humidity).