Tools for Drought Identification and Assessment: A Review

D rought is a natural phenomenon in many arid, semi-arid, or wet regions. This showed that no region worldwide is excluded from the occurrence of drought. Extreme droughts were caused by global weather warming and climate change. Therefore, it is essential to review the studies conducted on drought to use the recommendations made by the researchers on drought. The drought was classified into meteorological, agricultural, hydrological, and economic-social. In addition, researchers described the severity of the drought by using various indices which required different input data. The indices used by various researchers were the Joint Deficit Index (JDI), Effective Drought Index (EDI), Streamflow Drought Index (SDI), Standard Precipitation Index (SPI), Standard Evapotranspiration Index (SPEI), and Palmer Index (PI). According to the researchers in hydrology and for the most accurate description of the drought, more than one indicator for drought should be used. Most reviewed studies recommended using the Standard Precipitation Index (SPI) as the best indicator to describe the drought.


INTRODUCTION
Recently, it has been noticed that arid and semi-arid lands respond to climatic fluctuations very quickly.Global warming and climate change caused extreme events such as floods and droughts.Drought as a natural phenomenon may occur anywhere, and no region is excepted from the drought occurrence.(Beran and Rodier, 1985) stated that the chief characteristic of a drought is a decrease in water availability in a particular period and over a particular area.The consequences of drought on society and the economy are severe.Based on Benson and (Benson and Clay, 1994), drought shocks have the most significant economy-wide impact in countries at the intermediate rather than early stages of development.The above two statements highlighted that the definition of drought is continuously updated by considering its impacts and complex interaction on the environment and society.For different periods, (Guttman, 1998) determined the Palmer drought severity index and the standard precipitation index and compared the results from the two indicators.In that study, the standard precipitation index performed better than the Palmer index because the former was easy to interpret and calculate.At the same time, the latter has a complex structure and is difficult to calculate.(Keyantash and Dracup, 2002) described the drought as a deficit in precipitation, leading to a lack of water flow in streams that may extend for months or years.Three drought indices were generated and evaluated in Greece (Loukas et al., 2003) using information gathered from 28 locations over 40 years .Their study used the standardized precipitation index (SPI), rainfall abnormality index (RAI), and Z index.They initially determined the values of the indices for intervals of 3, 6, 12, and 24 months and then compared them.The findings showed that all three indices were equally effective at predicting the intensity and duration of the drought.A model was created by (Hong et al., 2004) to assess the risk of drought for two crops in Nebraska, the US: soy and maize.This study aims to present a straightforward definition of agricultural drought based on yield performance and suggest a new index to measure the severity of agricultural drought, taking into account all relevant factors as the final effects of the drought phenomenon in agriculture are on crops and fruits.(Yazdani et al., 2006) examined the dryness of the Zayanderud watershed, Iran, and surrounding areas using more than one index, such as the percentage of the normal index, the precipitation index, and the standard precipitation index.Through these indicators, they identified the drought based on the calculated value.The probability of a drought persisting in the center sections of Sistan and Balouchestan province, Iran, is over 70%, while it is less than 50% in the eastern areas, according to findings from (Raziei et al., 2007) collected using SPI.Aside from that, this province's center region experiences drought for about 30% of the time each year.As a result, this province is more vulnerable compared to the other analyzed provinces in this article.where  is the soil water deficit The SRI had the same SPI equations but was given a ( Tian et al., 2018) presented the results of using a few drought indicators, including Palmer's Drought Severity Index (PDSI), Palmer's Z-index, precipitation percentiles, Standardized Precipitation Index (SPI), and Standardized Precipitation Evapotranspiration Index (SPEI) and Z-index to assess agricultural drought in the central south of China.They found that using SPEI and Z-index was beneficial for monitoring agricultural drought.For spatial, temporal, and directional analysis of drought occurrence in the upper Tana River, Kenya, Wambua and his colleagues applied the standard and effective precipitation indexes (Wambua et al., 2018).They found that both indicators confirmed the negative effects and risks of the drought at the upper Tana River.(Polong et al., 2019) used SPEI at timescales of 6 and 12 months and monthly rainfall and temperature data gathered between 1960 and 2013 to investigate the spatial and temporal variability of dryness and wetness conditions and their extent, intensity, and severity in the Tana River Basin in Kenya.According to the index, dryness episodes predominated in the 1960s through the 1980s, whereas wetness incidents predominated from 1990 to 2000.The SPEI was chosen as the best index since it can consider future climate change effects in the area.(Asaad and Abed, 2020 ) conducted a study using three groups of monitoring data for the Tigris River in the Baghdad Governorate to characterize the low flow in the River Tigris decline.Based on the drought class, they recommend using inflatable dams as an effective solution.It is challenging to pinpoint the primary reason for a drought.It is well-recognized that droughts happen when there is no precipitation or below-average precipitation.Human activities, including land usage, mining, deforestation, and poor management of water resources, can readily increase both the frequency and severity of droughts (Abara and Budiastuti, 2020; Ayugi et al., 2020).A daily SPEI dataset was created by (Wang et al., 2021) using data collected over 58 years, from 1961 to 2018, at 427 meteorological stations spread out across the Chinese mainland.In addition, it was noted that the daily SPEI dataset might be used to explore droughts on various timesteps, including meteorological, agricultural, and hydrological ones.The study revealed that the SPEI value with a three-month (roughly 90 days) timestep illustrated no significant trends toward intensification in frequency, severity, and duration of drought incidents.Both academics and scientists have recently expressed serious concern about the use of drought indices, and a greater emphasis has been placed on understanding the phenomena of drought and delving further into its characteristics After reviewing the literature, no studies focused on various tools used to quantify drought in various regions worldwide.In addition, a recommendation for the most accurate drought identification numerical tool is required.The present study demonstrates the types of drought, describes the drought using various drought identification indices, assesses the application of various drought identification methods, and recommends the most accurate indices for drought identification.

DROUGHT CLASSIFICATION AND CHARACTERISTICS
After defining drought and finding out the causes of drought occurrence, most of the researchers highlighted the linkage between precipitation and drought.Accordingly, (Wilhite and Glantz,1985) classified drought into three categories, as shown in Fig. 1.Besides the drought types, other drought characteristics such as severity, length, magnitude, frequency, and duration are important in identifying and analyzing the drought.Depending on a region's location, drought duration can range from a week to several years.The drought magnitude is the cumulative water deficit in precipitation, soil moisture, or runoff during a drought period that falls below a certain threshold.The drought intensity is the ratio of drought magnitude to its duration.Drought frequency (return period) is the average amount of time between drought events with a severity equal to or higher than a threshold, known as drought frequency or return period

Metrological Drought
This type of drought is specified to meteorology and results from decreased precipitation and the duration of this deficiency.(Shiru et al., 2019) evaluated the changes in the intensity and frequency of the meteorological drought during the agricultural seasons in Nigeria using the SPEI as an index.In addition, data on temperatures confirmed that the drought was greatly affected by the temperature increase due to global warming.The Joint Deficit Index (JDI) was used in northwestern Iran by (Mirabbasi et al., 2013) to determine and assess drought prediction since it offers important information about water resources.The JDI indicator only depends on monthly rainfall data.For Iran's Western Azerbaijan, (Nosrati and Zareiee, 2011) analyzed drought using SPI as an indicator and considering the intensity, duration, and frequency of droughts since the results showed that the drought gets worse as the yield period and scope grow.

Agricultural Drought
When agricultural drought occurs, it results in a decrease in production.This results from the soil moisture deficit, which decreases the plant's productivity.(Kamruzzaman et al., 2019) performed a study on Bangladesh to evaluate the characteristics of agricultural drought using an index called the Effective Drought Index (EDI).The findings of their study were similar to those obtained by (Rafiuddin et al., 2011), who carried out a study on the same area using the SPI index.Both studies revealed that the northern, southeastern, and central regions are the most affected by agricultural drought, as shown in Fig. 2. (Qiang et al., 2020) describe the drought in Inner Mongolia using annual SPEI.The SPEI-12 in December can be used to characterize the annual droughts.Fig. 3 shows the relationship between drought and plant growth (Hao et al., 2016).

Hydrological Drought
A hydrological drought occurs when a region's precipitation.The main impacts of the hydrological drought are lowering river and groundwater levels.When there is a long shortage of precipitation, the effect of a hydrological drought starts after the end of the meteorological drought.(Jahangir and Yarahmadi, 2020) conducted a study to detect Hydrological drought in Lorestan Province, Iran, using the Streamflow Drought Index (SDI) and knowing the drought trend for different periods.Their results were consistent with the discharge values for the period under study.Fig. 4 shows the spatial analysis of the drought in the studied area.The standard precipitation index and the streamflow index for various periods were employed by (Boudad et al., 2018) to study the two most significant types of drought in northern Morocco: meteorological and hydrological droughts.The findings revealed a link between these two parameters; mainly, such results are helpful to the Department of Water Resources taking necessary actions for the 12-month scale

Socioeconomic Drought
Socioeconomic drought tends to happen due to increased demand for natural products such as fish, grain, and hydropower.However, these products depend on the availability of water.
In addition, the availability of water depends on climatic fluctuations.For example, the Uruguayan drought that occurred in 1988-1989 reduced the flow of water and reduced hydroelectric power production.

DROUGHT INDICES
A symbol or a number that indicates the occurrence of a drought is referred to as a drought indicator.Scientists developed these indicators because they are important and useful for various purposes, but they did so while considering the available data for the area under study.(Friedman, 1957) prepared a list of the most important necessities for any specified index.For this purpose, steps outlined the requirements that any drought index must satisfy, as shown in  drought assessment  Depicting the idea of drought in a particular area, enabling the communication of drought conditions among multiple interested entities, and connecting with quantitative drought impacts over variable scales of geography and time.The researchers have strived to create various drought indicators, each of which has advantages over the others regarding data and the drought it can identify.A list of some of the most popular drought indicators is presented in Table 1.The most significant strength of this indicator is its reliance on rain data that may be available.It is one of the powerful indicators that can be used to monitor different types of droughts.The SPI-3 is used to monitor meteorological drought for three months or less.The SPI-9 is used to monitor the effects of agricultural drought for nine months.The SPI-6 and SPI-12 are used for six months to monitor hydrological drought, where drought begins at -1 and ends when its value becomes zero.This indicator is similar to the SPI indicator.Still, it includes temperatures in addition to precipitation since it provides a comprehensive image of drought for various periods and requires monthly data with an extended series for a long time, which can be challenging to obtain.

APPLICATION OF DROUGHT INDICES: PERFORMANCE ASSESSMENT
Several studies compared various drought indicators utilized worldwide and determined the effectiveness of each indicator (Askarimarnan et al., 2021).(Tirivarombo et al., 2018) conducted a study on northern Zamia to detect drought in the Kafu River basin.Two indicators were used: the standard evapotranspiration index (SPEI) and the standard precipitation index (SPI), as the first depends on both precipitation and temperature and the second on precipitation only.The correlation coefficient between these two indicators for different time scales (1, 3, 6, and 12) months was determined and shown in Fig. 6.The results of the drought index can be utilized for measuring the intensity of drought and forecasting the drought.This is very useful in the planning and management of water resources.Also, forecasting the drought is essential, and it helps to take the preventive measures that can alleviate the drought effects on the public, including using early warning and taking appropriate actions before the drought reaches a dangerous level.Most researchers recommended using more than one indicator in drought studies since determining indicators is a significant step in dealing with drought as a natural phenomenon.Most researchers recommended using more than one indicator in drought studies since determining indicators is a significant step in dealing with drought as a natural phenomenon, and using more than one index can help confirm the type of drought in the studied region.

(
Loukas et al., 2008) assessed how climate change has affected the severity of the drought throughout a large region of Tesla in Greece.They evaluated using information gathered over 30 years from 50 locations.They concluded that drought had occurred at different periods and varying degrees in all areas of this region using the SPI.An index called the Palmer drought index (PDI) and also called the Palmer drought severity index (PDSI) is a regional drought index commonly used for monitoring drought events and studying areal extent and severity of drought episodes (Mishra and Singh, 2010).Based on SPEI data (Yang et al., 2016) looked at spatiotemporal drought fluctuation patterns in the Haihe River Basin (HRB) between 1962 and 2010.The results showed that drought occurrence and severity have increased during the study period on both an annual and seasonal scale.All drought-affected areas exhibit a temporal pattern of increasing variability (Liu et al., 2018) used the drought indices to assess agricultural drought in northern China.The drought indices used in the assessment were the standard Precipitation Index (SPI), Standard Evapotranspiration Index (SPEI), and Palmer Drought Severity Index (PDSI).In Ethiopia, specifically at Upper Blue Nile, (Bayissa et al., 2018) used six indices to describe historical drought and compare the results.The indices used were SPI, SPEI, Evapotranspiration Drought Index (ETDI), Soil Moisture Drought Index (SMDI), Total Drought Index (TDI), and the Standard Runoff Index (SRI).The assessment obtained from all these indicators enabled them to determine the beginning of the drought.The above indices were described respectively using the following equations (Mckee et al., 1993), the indices SPI: according to(Mckee et al., 1993), the values of  0 ,  1 ,  2 ,  1 ,  2   3 are 2.521, 0.803, 0.010,1.432,0.189, and 0.001, respectively.

Fig. 5 .
The following are the operational benefits of employing an index for drought characterization:  Real-time monitoring and drought detection (Niemeyer, 2008)  announcing the start or conclusion of a drought (Tsakiris et al., 2007)  enabling drought managers to announce levels and initiate responses;

Figure 5 .
Figure 5. Flowchart for choosing the best index good dry results practically identical to SPI values has the drawback that the results change without flow, producing less-than-ideal results.Its values, representing dry and wet circumstances, range froms use of daily precipitation data makes it one of a kind and allows for the determination of the start and end of the drought period.It is also one of the most effective indicators for determining meteorological and agricultural drought.Still, the data itself presents a problem because it is possible for updates to be made to all data daily.

Figure 7 .
Figure 7. Time series for a) SPI and b) EDI with precipitation (Wambua et al., 2018)

(Nkunzimana et al., 2021; Myronidis and Theofanous, 2021; Stefanidis and Alexandridis, 2021; Ndayiragije and Liu, 2022).
By creating new or improved techniques and choosing indicators, much work has been put into creating statistical drought prediction models that are accurate and dependable.
The most key aspect of this indicator is that it provides a clear visualization of the long-term drought, unlike other indicators that do not provide a clear visualization for periods shorter than a year.The indicator is complicated, data is hard to come by, and it cannot be compared across regions like SPI.According to the maps of operational agencies like NOAA, the index values range between -4 to +4, and occasionally, it extends beyond that range because positive values imply wetness and liquid over drought.