Standardized soil moisture agricultural drought index modification based on probability distributions to using in different climates of Iran

According to drought type including meteorological, agricultural and hydrological, drought can start with a decrease in variables such as precipitation, soil moisture and runoff compared to the average values. Agricultural drought, which indicates a lack of soil moisture, will have important effects such as a decrease in crop yield. One of the most widely used drought monitoring methods is the use of standardized indices. The main aim of this study is to modify the standardized soil moisture index (SSI) by selecting the appropriate distribution fitted on soil moisture variable over various climate of Iran.

In this regard, daily measured data in 40 synoptic stations and monthly soil moisture data in the first (0-7 cm) and second (7-28 cm) layers from the ERA5 database during 1979-2020 have been used. In the second step, after collecting the required data, in order to evaluate the accuracy of the ERA5 database, the measured variables including precipitation, average temperature and potential evapotranspiration using the coefficient of determination (R2), mean bias error (MBE) and scatter index (SI) were compared with the ERA5 database. In the third step of the research, the best fitted distributions on the soil moisture variables of the first and second layer among 49 different distributions in EasyFit 5.5 software were identified by performing the Kolmogorov-Smirnov test at a significance level of 1 and 5 percent. Then, 12 SSI indices have been calculated for 40 studied stations in time scales of 1, 3, and 6 months. In the final step, the characteristics of drought including the number of drought events, frequency, duration and intensity of drought were calculated for all indices and compared with each other in different climates.

Generally, based on the SI, the highest accuracy of ERA5 data is related to temperature, potential evapotranspiration and precipitation, respectively. The results of fitting distributions show the significant fitting of gamma distribution as the superior distribution in only 11 and 5% of the first and second layer soil moisture in different stations, respectively. The three superior distributions fitted on the soil moisture of the first layer are Lognormal, Beta, and Logistic distributions, and for the soil moisture in second layer, Logistic, Beta, and Normal distributions, respectively. The minimum and maximum frequency of drought occurred in Abadeh, Anar, Minab and Kerman stations equal to 7, 3, 4 and 2% and in Iranshahr, Bam, Zabol and Iranshahr stations equal to 18.4, 17.8 and 21.0 and 25.4 percent, respectively. The minimum and maximum duration of agricultural drought occurred in Sanandaj and Anar stations, equal to 18 and 64 months, respectively. The intensity of droughts from the index based on the gamma distribution to the index based on the superior distribution decreases from 1.53 to 1.44 in the first soil layer (severe to moderate drought) and from 1.91 to 1.65 in the second soil layer.

The change in the characteristics of agricultural drought based on the change in the distribution used in calculating the index is greater in drier climates than in wet climates and in the soil moisture of the second layer compared to the soil moisture of the first layer. So that in some cases it changes according to the type of distribution used in calculating the drought class index. In general, the results of the research indicate the necessity of examining and selecting the superior distribution instead of the gamma distribution in the calculation of the standardized agricultural drought index.