Investigating drought in Iran using Keetch-Byram drought index (KBDI) and evaluating it with surface soil moisture (SSM) of active microwave sensors

Drought is a natural phenomenon that is mainly caused by less than normal precipitation over a long period that occurs in almost all climates. Since the late 1970s, the occurrence of drought has increased globally due to increased evaporation (caused by global warming). This phenomenon occurs slowly and can be short-term or terminated within a few months or continue for several years. drought has increased significantly in Iran in recent decades. The increase in droughts has many negative consequences in different areas of agriculture, water resources, and wildfire. The purpose of this study is to investigate the drought situation in the vegetation areas of Iran using the Keetch-Byram drought index (KBDI). For this purpose, the KBDI drought index derived from the data set of the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth edition (ERA5) with a horizontal resolution of 0.25o during the period of 1981-2020 has been used. Also, the combined product of surface soil moisture (SSM) from microwave sensors AMI-WS (ERS-1 and ERS-2 satellites) and ASCAT (MetOpA-B satellite) with a horizontal resolution of 0.25 o to evaluate the KBDI drought index results as well as the moisture condition. The results showed that the khalij-Omani and Irani-Turani vegetation areas have very low soil moisture and high drought in Iran. The amount of soil moisture in these areas is low and the area-averaged is equal to 12.65% and 18.42%, respectively, which has been somewhat constant in all months, in other words, dryness is the predominant climate feature of these areas. The monthly analysis of the KBDI index shows the spread of drought severity towards the west, i.e., the vegetation area of Zagros, in the hot months of the year. A decrease in soil moisture and dryness can be observed in a major part of Iran from early spring to mid-autumn. Also, in the Zagros region, from mid-summer to mid-autumn, increasing values of the drought index and decreasing soil moisture can be observed. The minimum drought index is in the vegetation areas of Hyrkani and Arsbaran. The correspondence between the results of the surface soil moisture (SSM) of the satellite product and the KBDI drought index obtained from ECMWF-ERA5 shows the appropriate efficiency of the KBDI drought index in identifying drought centers in Iran and investigating the spatio-temporal patterns of drought occurrence. frequent low-intensity drought may favor more drought-tolerant species and adapt forests and grasslands to future conditions without the need for management measures.