Adaptive evaluation and comparison of SMDI, SPI and RDI drought indices inZarghan region, Fars province

Soil moisture is one of the most important factors that are affected by drought. Soil Moisture Deficit Index (SMDI) is one of the most important indicators that monitors drought based on soil moisture. But due to lack of direct measurement of soil moisture in weather stations and lack of information, it has been used to a limited extent. In this research, the meteorological data of Zarghan synoptic station was used during the statistical period of 30 years (1992 to 2022) and with the help of ET0 and AquaCrop models, the amount of soil moisture was estimated at the depths of 5, 15, 45 and 95 cm. Then the percentage of soil moisture deficiency and SMDI index were calculated to monitor drought. Also, the results of this index were compared and analyzed with two common indices SPI and RDI which are based on precipitation and evapotranspiration data.The highest value of this index occurred in 1996 after a period of heavy rain in 1994 and 1995, and the lowest value occurred in 2011 after the dry years of 2008 to 2010. Considering that the highest and lowest amount of rainfall is in 2004 and 2021, SPI and RDI drought indices have shown the most severe drought in the mentioned years. Severe droughts occurred in 2010, 2011, 2016 and 2021, and all three indicators had similar results. In examining the relationship between meteorological parameters and indices, the results showed that the SMDI index has the lowest coefficient of explanation with evapotranspiration (0.25), RDI index (0.4) and then SPI and precipitation index (0.45). Also, the results showed that there is a strong correlation between SPI and RDI. Regarding moderate and very severe droughts, where the amount of precipitation is greatly reduced and evapotranspiration increases, the RDI index is more accurate and shows the severity of the drought with higher accuracy, and its sensitivity to weather conditions is higher than the SPI index. Therefore, in arid and semi-arid regions where the severity of drought reaches the severe and very severe threshold, it is recommended to use indicators that take into account other factors in addition to precipitation.