Investigation of different drought indices to find the best-fitted index (Case study: Damghan township)

Although Iran has 1.1% of the world's land area, it has only 0.34% of the available water resources. On the other hand, in most parts of Iran, precipitation is not evenly distributed. The average annual rainfall in Iran is about 250 mm and is one of the arid and semi-arid regions of the world and drought is one of its main climatic characteristics. Drought can be defined as a period of time with abnormally dry climatic conditions that cause serious hydrological imbalances due to prolonged lack of sufficient rainfall. Drought is a natural, temporary and reversible phenomenon in any climate, which is more pronounced in arid and semi-arid regions. According to different definitions, the drought phenomenon is divided into four different categories based on the damages that drought causes to different sources such as river flow, groundwater level, soil moisture, agricultural products and people's living conditions. These four categories are named meteorological, hydrological, agricultural and socio-economic drought, respectively. Recognizing the drought phenomenon and quantifying it by using drought indicators, suitable for the conditions of each region, is one of the main ways to adapt and control this phenomenon and is also a suitable tool for managers and policy makers in different sectors to be able to reduce drought damages as much as possible with coherent planning. One of the main sources of fresh water needed by human societies is groundwater resources, which are the largest reserves of fresh water on earth after glaciers. Groundwater resources are used for different purposes. For example, more than 90% of cities and about 70% of the needs of the agricultural sector worldwide are supplied from these sources. Droughts and precipitation occurrance are the most important factors that affect groundwater resources in the long run. In arid and semi-arid regions, groundwater depletion and quality play a key role. As a result of the drought phenomenon, the amount of groundwater consumption also increases. Severe droughts have occurred in different provinces of Iran in recent years. This shows that it is important to study the drought in this country to prevent the environmental crisis and the damages caused by it. Occurrence of drought in Damghan city, located in Semnan province, has caused the development of deserts, especially in its southern regions, and in the crisis of groundwater and surface water shortages, agricultural lands, gardens and livestock are in danger in this regard and the damages caused through this has economically reduced the quantity and quality of products. Investigation and monitoring of drought are important tools for drought management, which could determine its beginning and end, areal extent and severity by using climatic and hydrologic parameters. Knowledge of the occurrence of drought is essential to design environmental and executive planning to raise awareness of this issue and mitigate its negative and harmful effects as much as possible and make the right decisions for the study area.

The objectives of the present study are summarized as follows:A) Understanding the general situation of Damghan city in terms of long-term rainfall characteristicsB) Examining the trend and characteristics of drought and determining the various indicators of meteorological drought in the regionC) Investigating the correlation between drought indicesD) Finding the most suitable drought index in the regionE) Analysis of the relationship between drought and rainfall changes with groundwater level in the region.In the current research, meteorological drought of Damghan region is studied by using drought indices. For this purpose, monthly precipitation data of Damghan, Hossein Abad, Khourzan, Ghousheh, Mabad, and Astaneh stations were obtained for a common statistical period (1997-2019). After studying annual precipitation trend for each station, duration and maximum drought were determined using SPI, DI, PN, CZI, MCZI and ZSI indices. To investigate the performance of these indices and select the best-fitted index for Damghan region, the minimal theorem of precipitation was used.

Based on the results, the best-fitted index for Damghan township was DI, which showed proper performance in both prediction of severe drought and wet years. In the next step, using Spearman correlation between precipitation and drought indices, the PN index had acceptable performance in prediction of severe droughts, but could not predict wet years. To compare the indices for prediction of fluctuations of groundwater level, the 3, 6, 9, 12, 18 24 an 48-month moving averages were used. Finally, the SPI index was selected as the best-fitted index for predicting groundwater level in Damghan region.