Comparative study of drought meteorological (SPI) and hydrological (SSI) indices based on the best cumulative distribution function for Urmia Basin

Atmospheric conditions such as climate change, ocean temperatures, changes in the jet stream, and changes in the local landscape are all factors that contribute to drought. Drought not only threatens agriculture but also causes a series of ecological, social, and economic damages. Therefore, it is necessary to have valid and appropriate indicators that can evaluate and measure drought efficiently. Drought monitoring by meteorological and hydrological indicators has always been of interest to researchers. Decreasing or increasing precipitation has a direct effect on river flow rate and groundwater level.

Drought monitoring was performed for 10 meteorological stations and 10 hydrometric stations in the western basin area of Urmia with different distribution functions to identify the best distribution function for fitting data based on standardized precipitation index (SPI) as meteorological drought index and standardized streamflow index (SSI) as hydrological drought index. SPI and SSI values were measured with four cumulative functions, i.e., Gamma, Weibull, logarithmic normal, and Gaussian normal functions. Some performance criteria such as MAE were used to evaluate the efficiency of the functions. This research has been conducted on an annual scale for 31 years from 1989 to 2019. Drought continuity zoning was done through the Inverse distance weighting (IDW) method based on drought continuity.

The results of the SSI showed that the cumulative gamma function had the best performance for fitting data for nine stations. Only at the Merakand station, the Weibull cumulative distribution function provide a better fit for the data. Babarood station with 99% correlation and MSE of 0.017 had the best fit among other stations. In order to understand the drought conditions of the region, according to the selected indicators and based on positive values for wet conditions and negative values for drought conditions, the stations have been studied. Finally, a zoning map of drought continuity is presented.

The SSI and SPI indicators are directly related to each other. The SPI is better for predicting the onset of drought and the SSI is better for measuring the severity and persistence of drought. During the last three decades (1998-2000), Urmia basin has witnessed the highest severity of hydrological drought. Based on the zoning map of the northwest region, especially the upper Chihriq station, it has always enjoyed more favorable conditions and has experienced a milder drought than other regions. However, the extent of severe drought changed from the western part in 1998 to the eastern part in 2000. the severity of drought in 1998 was less than in 2000. In 2000, the drought intensity reached the highest value just in the Merakand station.