Stochastic time-series models for drought assessment in the Gaza Strip (Palestine)

Climatic drought is a phenomenal evidential sign of climate change that affects the natural conditions of water resources (Dai et al. 2004; IPCC 2007; Sheffield & Wood 2011; Intergovernmental Panel on Climate Change (IPCC) 2014; Sheffield et al. 2014). Globally, drought is being reported as the most extensive natural hazard that occurs in arid and semi-arid regions, which can cause stark economic, social, and environmental damage (Nagarajan 2010; Mondol et al. 2017). Evidentially, droughts of greater severity are expected more frequently, for longer, and to cover a broader area due to global warming, which causes greater disruption (Han & Singh 2020). Agricultural production is among the most vulnerable sectors to drought where one instance could reduce global agricultural productivity by 0.8% and around three-quarters of globally cultivated areas experience periodic losses from drought estimated at approximately 6–7 billion USD per year (Kim et al. 2019). The losses caused by drought are projected to increase dramatically due to potential increases in the incidence of drought in the future. A commonly reported fact is that a 1 °C increase in temperature is strongly linked to a 1.4% decrease in annual income (Dell et al. 2012). In 1975, the drought-related crop losses in the Great Plains of the United States were estimated to be worth 700 million USD (White & Haas 1975). However, annual losses from drought in the United States amount to an estimated 6–8 billion USD (FEMA 1995). The prolonged drought of California triggered crop losses worth nearly 5.5 billion USD in the years 2011–2017, and the forests lost more than 130 million types of plants (Kam et al. 2019). The Millennium drought reported between 1997 and 2009 was one of the worst droughts of below-average precipitation in Southeast Australia, which severely impacted the ecosystem and economy (CSIRO 2012; Leblanc et al. 2012; AghaKouchak et al. 2014; Xie et al. 2016). In response to the drought, integrated national and state institutional frameworks in parallel with water conservation programs like water recycling and seawater desalination were adopted to alleviate the consequences of future drought variabilities (Low et al. 2015). Banerjee et al. (2013) estimated that the economic costs of handling the disruption in the hydrological environment in South Australia during the Millennium drought to be approximately 810 million USD. The projections of the local economy reveal that the consequences of drought lead to a decrease of around 1–2% in Australia's national gross domestic product (GDP) per year (Haque et al. 2017). In this context, the African Sahel region demonstrates an extreme example of multi-decadal climate variability that has occurred since the 1960s due to the sharp decrease in the rainfall patterns. From the 1950s until the present day, the temperature has risen to 3.5°C in the Sahel region, and this increases the evaporation process from the soil and water surfaces and negatively affects the water security and food production for about 100 inhabitants in this area (van der Geest et al. 2019). Principally, the classification of droughts relates to the decreases in the climatological factor of precipitation to a level less than normal over a geographical area that subsequently impacts the natural sustainability of the hydrological systems (Shukla & Wood 2008; Mishra & Singh 2010; Efstathiou & Varotsos 2012; Keka et al. 2012; Almedeij 2014; Shah et al. 2015). The drought is identified as a slowly evolving phenomenon that has an elliptical profile in the temporal and spatial propagation (Shahid & Behrawan 2008; Al-Qinna et al. 2010; Patterson et al. 2013; Shah et al. 2015). Therefore, the assessment of the drought severity, spatial and temporal dispersion, and the frequency of occurrence is a prerequisite for an integrated water resource, economic and agricultural management, and planning system (Mishra & Singh 2010; Kampragou et al. 2011; Moreira et al. 2015; Nam et al. 2015; Kwak et al. 2016; Alqaysi et al. 2017; Awchi & Kalyana 2017; Mehr et al. 2019; Baruga et al. 2020). Many drought investigations in the regions of the Middle East and North Africa (MENA) indicate that these regions face chronic consequences from drought, strains on the integrity of water resources, and increases in the impact of the water crisis (Sönmez et al. 2005; Abbaspour et al. 2009; Al-Qinna et al. 2010; Keramat et al. 2011; Sen et al. 2012; Kelley et al. 2015; Modarres et al. 2016; Awchi & Kalyana 2017; Habibi et al. 2018; Aladaileh et al. 2019). Decision-making requires a rigorous preventive drought mitigation strategy, dependent on accurate drought monitoring and forecasting that is based on defining drought contingencies in the context of oceanic atmospheric-land dynamics and hydrological conditions (Hao et al. 2018). Accordingly, multiple statistical, dynamic, and hybrid model simulations have been developed for drought forecasting. The dynamic drought forecasting models are future climate projections of the ocean–atmosphere–land processes based on coupling the general circulation models of climate. However, statistical forecasting approaches simulate the future climate by utilizing the previously observed values (Yuan & Wood 2013; Saha et al. 2014). The statistical models exhibit greater accuracy in addressing local drought behaviors than dynamic models that are more appropriate for large-scale studies. However, the limitations of statistical models in detecting the nonstationary nature of climate systems, and the weakness of dynamical models in long-term predictions due to the random nature of the ocean–atmosphere circulation, make the hybrid statistical–dynamic simulation approaches a preferable option to forecast the climatic phenomena (Mishra & Singh 2010; Madadgar et al. 2016; Hao et al. 2018; Xu et al. 2018; Strazzo et al. 2019; Han & Singh 2020). Nevertheless, the variety and the simplicity of the statistical forecasting models conceptual structures of machine learning (Kuswanto & Naufal 2019; Zhang et al. 2019; Khan et al. 2020), time-series models (Moghimi et al. 2019; Yeh & Hsu 2019), probability models (Paredes & Guevara 2013; Behrangi et al. 2015; Avilés et al. 2016; Hao et al. 2016; Jouybari-Moghaddam et al. 2017), regression models (Stagge et al. 2015; Mortensen et al. 2018), and hybridized models (Rezaeianzadeh et al. 2016; Prasad et al. 2018; Fung et al. 2019; Malik et al. 2020) reinforce the trend to rely on these approaches for establishing regional and local analytical drought studies (Anshuka et al. 2019). As aforementioned, the science and the historical data provide the basis for developing descriptive drought models. Hence, the aggregation of the prior distribution of observations conveys the predictive information about the immediate future (Nagarajan 2010). Therefore, the stochastic approaches utilize the power of stochastic models such as Autoregressive Integrated Moving Average (ARIMA) or Markov Chain to simulate the temporal propagation of drought (Cancelliere & Salas 2004; Mishra & Desai 2005; Modarres 2007; Serfozo 2009; Han et al. 2010). Similarly, the statistical approach employs various statistical techniques to estimate drought characteristics, and these techniques can also mainly be divided into statistical modeling and probabilistic distribution (Cancelliere et al. 2007; Kwak et al. 2016). Specifically, drought analytical studies are fundamentally based on the stochastic statistics of drought indices (Munger 1916; Cancelliere et al. 2007; Kwak et al. 2016). The index approaches are based on computing a unique value of drought in terms of different meteorological inputs, such as precipitation, evaporation, and transpiration, to classify the severity and the frequency of drought (Palmer 1965; Gibbs & Maher 1967; Palmer 1968; Shafer & Dezman 1982; Petrasovits 1990; McKee et al. 1993; Sastri 1993; Kogan 1995; Guttman 1998; Byun & Wilhite 1999; Szalai & Szinell 2000; Samkhtin and Hughes 2004; Morid et al. 2006; Tsakiris et al. 2007; Vicente-Serrano et al. 2010; Hao & AghaKouchak 2013; Wichitarapongsakun et al. 2016; WMO & GWP 2016). The Standardized Precipitation Index (SPI) is the most popular meteorological drought index used to assess the expected drought due to the change in the precipitation behavior over different periods (McKee et al. 1985, 1993, 1995). The SPI introduces a comprehensive multi time-scaled assessment methodology for the effect of precipitation deficit on water resources in comparison with the other indices (Manatsa et al. 2010; Du et al. 2013; Alqaysi et al. 2017; Halwatura et al. 2017). However, taking the influence of other climatic parameters, the Standardized Precipitation Evapotranspiration Index (SPEI) is considered a more reliable index to assess the impact of drought (Homdee et al. 2016; Tefera et al. 2019; Zhang et al. 2020). The spatial mapping of drought is necessary for policymakers to understand the regional characteristics of drought to manage it effectively, to reduce agricultural production losses, and to protect the environment (Alamgir et al. 2015; Karavitis et al. 2015; Mondol et al. 2017; Uddin et al. 2020). In general, this study aims to assess the fluctuation in the temporal and spatial distribution of drought in the MENA. Moreover, a microstudy of the Gaza Strip in Palestine was implemented by developing forecasting stochastic time-series models and detecting the statistical features of drought occurrence throughout the period and the area of the study. The study sought to overcome the dearth in available studies on drought in the Eastern Mediterranean countries of MENA as well as the challenge of a lack of and missing meteorological data for drought investigations in the Gaza Strip by expanding stochastic models for simulating missing data and forecasting the meteorological state in the Gaza Strip.

The stochastic component is the dominant term in representing models while the existence of the deterministic component is not dominant (Machiwal & Jha 2012). The deterministic component is a combination of long-term mean, trend, periodicity, and jump (Kottegoda 1980). Generally, the time-series model is built by capturing the deterministic component and by simulating the behavior of the stochastic random component (Chatfield 2003).

Generally, to ensure purely stochastic random time series, the autocorrelation values for all lags except zero-lag autocorrelation should be statistically insignificant. The degree of dependency of any value on another value in the time series should also be addressed by partial ACF (PACF) analysis. The hypothesis that the time series follows the normal distribution is used to examine whether the sample time series comes from a purely stochastic series or not (Box & Jenkins 1970; Kashyap & Rao 1976).

Principally, a necessary condition in the stochastic models is that the series being modeled must be free from any significant periodicities. Thus, for periodicity to be removed from the time series, the original time series is simply transformed into another pure one by differencing or by standardization method (Box & Jenkins 1970; Kashyap & Rao 1976).

The stochastic ARIMA models are widely used in water resource management applications, especially for modeling hydrological stream flows, groundwater level fluctuations, and drought patterns (Bazrafshan et al. 2015; Mirzavand & Ghazavi 2015; Djerbouai & Souag-Gamane 2016; Khorasani et al. 2016; Myronidis et al. 2018; Takafuji et al. 2018; Sakizadeh et al. 2019).

Accordingly, as demonstrated in Table 1, the computed SPI and SPEI values are used to classify the severity of drought that has occurred, based on four designated drought categories by McKee et al. (1993).

The advantages of SPI and SPEI are due to its considerable ability to quantify the precipitation deficit for several time scales to address the consequential impacts of agricultural drought, hydrological drought, and groundwater drought (Bazrafshan 2007; Elkollaly et al. 2017; Javanmard et al. 2017; Caloiero et al. 2018; Abeysingha & Rajapaksha 2020). Hence, the recharge of the groundwater is evaluated every 12 months. The SPI period of 12 months is the commonly recommended time scale to address the impact of annual drought behavior on the groundwater balance.

Throughout this study, the MENA countries that lie on the eastern coast of the Mediterranean Sea, shown in Figure 1, were highlighted to describe the nature of the regional drought. The study area covers about 1.3 million km², where around 180 million people reside. The drought noticeably increases in this area, especially in Egypt, which has a negative effect on agricultural production and food security (Hameed et al. 2020). Prolonged droughts are inherently part of the natural climate of the area where there is a substantial decrease in precipitation in tandem with temperature rises in the area (Elasha 2010; IPCC 2014). The Middle East area is known to be the driest area in the world, and historical measurements reveal extreme and alternating periods of drought in the region (Kelley et al. 2015; Sharifi et al. 2015; Huang et al. 2016). The Mediterranean region of the Middle East shows more decline in precipitation trends than other areas, which leads to a more prolonged drought in the coastal region of MENA (Karami 2016). The coastal strip of North Africa faces continual drought warnings due to the effect of the Sahel Desert region (Elasha 2010). Additionally, the Eastern Mediterranean region is under intense and increasing drought conditions that have been recorded since 1998 as observations and model simulations have shown how the number of warm events have doubled since the 1970s (Cook et al. 2016; Lelieveld et al. 2016). Moreover, the drought is more likely to occur in the arid and semi-arid areas of MENA where the climate experiences losses in the effective moisture over the region, which is increasing in aridity (El Kenawy et al. 2016). In general, the prolonged drought in the MENA is attributed to anthropogenic and nonanthropogenic components; however, the nonanthropogenic components can be considered a more reliable explanation for the drought in the MENA (IPCC 2014). Principally, the interaction between precipitation and evaporation processes is the main driver for drought pattern formation and this is basically related to many climatic processes in the region such as El Niño-Southern Oscillation, Southern Annular Mode, Indian Ocean Dipole, and North Atlantic Oscillation (Golmohammadian & Pishvaei 2014). Noticeably, in the last 20 years, the region of MENA faces tangible losses due to drought, where considerable large agricultural areas were lost – around 50% of the irrigated land and 80% of the rain-fed cultivated land were absolutely lost (Huttner 2014).

To characterize the drought evolution in the Eastern Mediterranean region of MENA, the SPEI that is given by the Global SPEI database (https://spei.csic.es) was gathered, screened, and presented regionally. The Global SPEI database offers long-term, robust information about drought conditions on the global scale, with a 0.25 × 0.25° spatial resolution cell and a monthly time resolution. It has a multiscale character, providing SPEI time scales between 1 and 48 months. The average SPEI for the area of study was presented based on 3-, 6-, 9-, and 12-month time scales, shown in Figure 2, for the period from 1970 until 2019. According to the data presented, the 1970–1990 period was in a state of no drought. However, after 1990, climatic conditions become drier. Remarkably, the drought's origins could be geographically pinpointed to the region of North Eastern Egypt and specifically, the Sinai Peninsula. According to categories from McKee et al. (1993), the drought in the area can be classified as being between normal and severe. The southern part of the Eastern Mediterranean region of MENA is currently under severe drought conditions, which negatively affects the integrity of water resources and other environmental components.

In terms of the drought occurrence, the projections significantly reveal an increasing trend in the incidence of drought as shown in Figure 3. Marginally, drought probability increases from less than 20% during the 1970s to more than 80% in the period 2010–2019.

In the Gaza Strip area, the drought became a recorded phenomenon that affected the recharge of the coastal aquifer and the sustainability of life. The drought conditions of 2007–2008, where the total rainfall amounted to 67% of the area's annual average, caused a direct loss in agricultural output and 114 million USD and around 200,000 types of livestock were affected (MoA 2008). As the previous figures show, the area of the Gaza Strip is experiencing drought that is increasing in severity and its probability of occurrence is approximately 60–80% every 3 months, and more than 80% every year. The aridity index in the Gaza Strip ranges between 0.16 and 0.3 and is expected to decrease due to the effect of climate change, decreases in precipitation, and increases in the temperature. From an economic perspective, 4.7% of the labor force is employed in the agricultural sector and around 10% in fishing. The unemployment rate is around 41% and as a result of drought intensity, it is expected that more of the labor force will lose their jobs, particularly in the agricultural sector (PCBS 2020).

The Gaza Strip (Figure 4(a) is a coastal stretch of land that extends on the southeastern coast of the Mediterranean Sea around 42 km long, between 6 and 12 km wide, and which covers an area of 365 km². The Gaza Strip is characterized as being one of the most densely populated areas in the world. Therefore, according to the estimations of the Palestinian Central Bureau of Statistics (PCBS) for the year of 2019, the population of the Gaza Strip was 1.99 million inhabitants and the statistical projections show that the population will reach approximately 2.11 million inhabitants by 2021 (PCBS 2020). Geographically, the climate of the Gaza Strip is generated by the interaction between the climates of the arid desert of the Sinai Peninsula and the semi-humid Mediterranean. In the Gaza Strip, the mean monthly temperature varies from 29 in summer to 10 in winter. The rainy months arrive in tandem with the winter season from October to March and as shown in Figure 4(b), the total annual rainfall in the area varies between around 470 mm in the north and 250 mm in the south of the Gaza Strip. In winter and summer, respectively, the actual monthly evaporation process is 95 and 185 mm, the monthly relative humidity fluctuates between 68 and 75%, and the mean duration of sunshine is between 190 and 305 h. The sea breeze blows in throughout the day and the land breeze blows during the night in the summer at an average regular velocity of about 8–9 mph, primarily from the northwest direction. At midday, the wind speed hits its peak intensity and declines until sunset. The storms are widely reported in the winter with a wind speed of up to 40–50 mph and, in normal conditions, the wind blows from the southwest at an average speed of 10 mph.

Concerning issues regarding water, existing conditions in the Gaza Strip are described as undergoing a severe water crisis. The coastal aquifer is the only available water resource for water supplies in the Gaza Strip. Nonetheless, owing to low recharge levels and the excessive abstraction from water wells, the coastal aquifer is being continuously depleted. The Gaza coastal aquifer monitoring program indicates that more than 95% of groundwater is unacceptably polluted (Palestinian Water Authority (PWA) 2014). Driven by the effect of seawater intrusion, chloride concentration is found to a considerable degree in groundwater, which exceeds the World Health Organization (WHO) chloride level (Qahman & Larabi 2006; Zaineldeen et al. 2013; Dentoni et al. 2014; El Baba et al. 2020). The annual water obtained from the coastal aquifer via the metered water wells amounts to around 200 million cubic meters, which is approximately four times the volume that the aquifer can recover sustainably per year (PWA 2011, 2012, 2013, 2014, 2015; Abualtayef et al. 2017; Mushtaha & Walraevens 2018). Agricultural lands comprise approximately 33% of the total land of the Gaza Strip and require around 90 million cubic meters of water annually, which is largely obtained from groundwater wells (PWA 2013; Ministry of Agriculture (MoA) 2016). Like other Eastern Mediterranean countries, the region of the Gaza Strip is experiencing a rise in extreme drought events due to the influence of climate change on the rainy seasons, which significantly raises the vulnerability of food and water resources (Gampe et al. 2016; FAO 2018). Relatedly, Palestinian governmental institutions have adopted sustainable adaption strategies by implementing effective mitigation measures to cope with the adverse effects of extreme droughts. For instance, the MoA has adopted modern water harvesting strategies and has increased the knowledge and expertise among farmers about the need to move toward the most drought-resistant farming systems (MoA 2016). Elsewhere, the PWA has initiated many interventional initiatives such as the Strategy for the Water and Wastewater Sector, the Draft Water Resources Management Strategy, the National Water Policy, Water Sector Strategy Planning Study (WSSPS), the Coastal Aquifer Management Program (CAMP), the National Water Plan (NWP), the Stormwater Infiltration Plan, Gaza Emergency Technical Assistance Program (GETAP) on Water Supply to the Gaza Strip, and the Comparative Study of Water Supply Options for the Gaza Strip (CSO-G) to maintain the integrity of the coastal aquifer and to alleviate the impacts of extreme drought events and water scarcity. These studies suggest the use of nonconventional water supplies such as recycled wastewater, desalinated seawater, and collected stormwater as an effective potential water resource in the water cycle to satisfy water demands and to mitigate drought consequences. Furthermore, in response to the drought episode of 2010–2011, the Water Scarcity Task Force (WSTF) was established as a new institutional framework to improve coordination between government agencies and to coordinate and manage data accessibility under extreme conditions to facilitate the practice of multiple humanitarian and emergency measures (WSTF 2011). The Gaza Strip suffers from the lack of real data-based forecasting models that could be used for simulating future drought in the Gaza Strip. Specifically, Gampe et al. (2016) provided a predication for future drought in the Gaza Strip; however, the model's accuracy is contingent on getting image data from satellites and the correlation of the model does not exceed 55%. Therefore, the study's novelty is wrapped up in its ability to provide a reliable simulation model for the future based on the study of real acquired data from working meteorological stations in the Gaza Strip.

The methodology of this study is to describe fluctuating drought patterns in the Gaza Strip using stochastic time-series modeling to integrate a comprehensive spatial and temporal spread of droughts over previous periods and the commencing 20 years. This approach is illustrated in Figure 5.

The target period of 20 years, up to the year 2040, is based on the expectation that current and proposed water resource management plans would be comprehensively in operation within the next 10–20 years. Therefore, simulating drought behavior over the target period is anticipated to guide policymakers to evaluate response efficiency for coastal aquifer sustainability and drought resilience measures.

Daily rainfall data between 1974 and 2016 were collected, screened, and statistically analyzed through the nine meteorological stations of Beit Hanon, Beit Lahia, Shati, Remal, Mughraka, Nussirat, Deir Al-Balah, Khanyounis, and Rafah. The meteorological stations, as depicted in Figure 1, are well distributed across the entire geographical area to record the sensitivity of the distribution of rainfall from the south to the north of the Gaza Strip. The statistical analysis for the data, as shown in Table 2, shows that there is a prevalence of zero-rainfall in the summer, and the average monthly rainfall over the Gaza Strip is estimated at 21–39 mm. In the rainy season, the significant portion of rainfall drops, and it may reach a monthly average of around 135–330 mm. The findings indicate that in certain seasons, the Gaza Strip recorded massive quantities of rainwater. The statistical analysis demonstrates that the maximum rainfall values could reach a value of about 1.5–2.5 higher than the upper limit of outliers, which strongly confirms that during certain historical periods the Gaza Strip experienced floods. The average falling rainfall is distributed between 253 and 468 mm per annum, and the total annual volume of water that dropped in the Gaza Strip amounts to 92–171 million cubic meters.

Regionally, the northern Gaza Strip governorates contribute more to the recharge of the coastal aquifer than the southern governorates; therefore, all of the projects for infiltration and water harvesting and storage are planned for location in the northeastern regions of the Gaza Strip.

For the purposes of quality control and quality assurance, both R-statistical analysis language and the Statistical Package for Social Sciences (SPSS) were used in this research to evaluate the nature of the rainfall time series. The R-statistical analysis language is highly recommended for the purposes of climatic studies because of the vast availability of case studies. The SPSS is a familiar and established tool to confirm the consistency of results. The information inherent in the time-series data was derived through the analysis of the ACF, PACF, and the power spectrum. For the observed time series at the Beit Hanon station, the depicted ACF in Figure 6(a) shows a slow decay correlogram in a sinusoidal shape, which indicates that the data are in strong correlation with each other. The stationarity null hypostasis for the rainfall time series was tested using the Augmented Dickey–Fuller (ADF) test (Dickey & Fuller 1979). The t-statistic test gives a p-value of less than 5%; therefore, the null hypothesis is rejected, and this implies that the time series is stationary. The highlighted correlogram in Figure 6(a) illustrates that after a long time-lag of about 300 months, the correlation between the time series values becomes statistically insignificant at a significance level (α) of 95%, and this means that the time series should be converted into a more stationary form to extract the order of the MA components to structuring the time-series ARIMA model. The PACF expresses the degree of dependency of data on each other, and the PACF in Figure 6(b) reveals that 41% of the information in the time series is interpreted by the first observed value while the remaining 59% of the information is represented by the other observations.

The power spectrum, presented in Figure 6(c), implies that the time series explicitly repeats itself substantially every 12, 6 and 4 months. The statistical test at these spikes was significantly higher than the F-statistic test, and this reflects that the periodic pattern dominates the time series. To recapitulate, the original time series ought to be more stochastic in its behavioral process; therefore, the time series was processed and modified by inserting the series into a first-order seasonal differentiation to remove the substantial dependency of data on one another and the excessive periodicity. The modified time series for rainfall observations at the Beit Hanon meteorological station were reanalyzed through the ACF, PACF, and the spectrum as shown in Figure 4(a)′, (b)′, and (c)′, respectively. Generally, the identical analysis for all the rainfall observations at all meteorological stations implies that a seasonal ARIMA model (p, d, q) (P, D, Q)_s of order (4,0,1) (5,1,1)₁₂ is the suitable structure to represent and to forecast the rainfall time-series records at the Gaza Strip meteorological stations. In term of processing time, the model introduces the simulation results within a reasonable period of time. Moreover, a unique model was chosen to represent the simulation process at each station to mitigate the inaccuracies that could result from the redundancy of the structured models.

The algorithms of the R-statistical analysis language were utilized to investigate the structure of the stochastic models for each of the observed rainfall time series at each of the nine meteorological stations. The nonseasonal and seasonal components of AR and MA for the configured stochastic seasonal ARIMA (4,0,1) (5,1,1)₁₂ were specified for each of the nine rainfall time series as summarized in Table 3.

The seasonal ARIMA models were calibrated using 90% of the data, while the validation was performed on the remaining 10% of the data to test the performance of the models to forecast projections as shown in Figures 7 and 8. The degree of interpretation of simulated values to the observed values was investigated by the statistical measures of root mean squared error (RMSE) and r, which demonstrates the power of correlation between the simulated and observed data. The data fitted in the model calibration range implies that the model correlation was around 93–97% of the response data variability around its mean. In particular, the model exhibits good behavior in the forecasting framework where model testing reveals that the model can predict future conditions with a predictive correlation accuracy of about 92–99%. In other cases, the correlation between the observed and the simulated data is classified as a high correlation where in term of RMSE, the correlation strength in the calibration range is about 7–21, and a correlation of about 11–21 is provided in the validation test.

The Gaza Strip is a limited area of land so a unified structure of stochastic seasonal ARIMA model is deemed adequately appropriate for simulating the distribution of rainfall as well as simplifying data handling and reducing the inaccuracies that could emerge from using a myriad of models.

Devoting a clear framework for national-scale monitoring and evaluation of drought is a major challenge in articulating the impacts of climate change that directly threaten the coastal aquifer resilience and agricultural resources in the Gaza Strip. Intrinsically, the water budget appraisals in the Gaza Strip are prepared on an annual basis; therefore, it is more convenient to develop an annual national drought index to empower decision-makers to align the deficit and surplus in national water resources. Consequently, the SPI was evaluated on an annual basis every 12 months after compiling the historical data and simulating data over the following 20 years to 2040, using the previously mentioned stochastic seasonal ARIMA models. The annual SPI₁₂ was computed at each of the nine meteorological stations to demonstrate the change in drought models due to the change in the rainfall patterns over the specified period. The rainfall threshold value that aligns the zero SPI₁₂ and defines the separable limit between the state of dryness and wetness was specified by matching the rainfall and the SPI₁₂ graphs of the CDF as shown in Figure 9. The rainfall threshold level rises regionally from 250 mm in the south to around 474 mm in the north of the Gaza Strip. Therefore, a mean rainfall threshold level of 370 mm was considered as a credible local reference scale in this study to assess the temporal and spatial dispersion of drought in the area of the Gaza Strip.

The comparative review for the matching graphs of the cumulative probability distribution for the rainfall and SPI at each of the meteorological stations indicates that the representative local drought scale for the Gaza Strip could be established by fitting a probability rainfall distribution function with a mean and variance of 370 mm and 3,932, respectively, to match a probability SPI distribution function with a zero mean and a variance of 1, as shown in Figure 10.

According to the generated local drought SPI scale, the SPI₁₂ for the rainfall time series at each of the nine meteorological stations was recomputed as the shown sample in Figure 10 for Beit Hanon and Rafah stations. The simulation of the rainfall over the coming 20 years shows stability in the SPI with some interruptions. Specifically, for the Gaza Strip, conditions are classified as extreme drought when annual rainfall is less than 230 mm. Meanwhile, for rainfall of 230–265, 265–300, and 300–370 mm, the drought is categorized as a severe, moderate, and normal drought, respectively. Likewise, the wet state may be defined by the amount of precipitation that exceeds the threshold value of 370 mm per year. The temporal distribution of the drought SPI₁₂ over the years 1974–2040 reveals that the Gaza Strip witnessed drought conditions in the past and that the drought may also be described as chronic at some of the southern sites of the Gaza Strip.

The consequences of climate change in MENA's Eastern Mediterranean countries are demonstrated dramatically by the rise in temperature compared with the decrease in precipitation. Analyzing the SPEI drought index shows that the drought status in the Eastern Mediterranean has shifted over the past 50 years from wet to severe drought. Moreover, the current probability of the incidence of drought is three times the probability than in the 1970s. At the regional level, the Gaza Strip area is facing severe drought with a probability of more than 80%. The lack of real field measurements due to weak possibilities and climate tracking programs is a major obstacle in the face of providing detailed prospects for the drought status in the Gaza Strip. In light of the scarcity of surveying studies about local drought conditions in the Gaza Strip, this study utilized the available data on precipitation to conduct a systematic drought assessment over the past 46 years, in tandem with an assessment of the next 20 years. Employing the ability of the stochastic time-series models, the rainfall time series were presented using the stochastic seasonal ARIMA model of (4,0,1) (5,1,1)₁₂ that shows a calibration accuracy of 93–97% and a future prediction accuracy of 92–99%. Based on the calibrated seasonal ARIMA models, the rainfall data were extended to the years of 2040. The trend analysis for the generated time series from historical and future rainfall implies that rainfall pattern is relatively consistent over the time frame in the northern governorates of Beit Hanon, Beit Lahia, Shati, and Remal. In contrast, the southern governorates of Mughraka, Nussirat, Deir Al-Balah, Khanyounis, and Rafah reveal a decreasing trend in the precipitation pattern, which confirms that the southern parts of the Gaza Strip are experiencing greater drought in the climatic conditions. More specifically, the annual SPI₁₂ that was devised at each of the nine meteorological stations indicates that the rainfall threshold value decreases distinctly from the north to the south of the Gaza Strip. The annual precipitation threshold levels, which are the precipitation values of the no drought and no wet conditions, i.e., SPI = 0, were 474, 440, 390, 403, 358, 366, 340, 315, and 250 mm at the meteorological stations of Beit Hanon, Beit Lahia, Shati, Ramal, Mughraka, Nussirat, Deir Al-Balah, Khanyounis, and Rafah, respectively. In this study, to unify the comparison of the drought assessment investigations, a unified local SPI scale was established to provide a baseline comparative study to represent local drought conditions in the Gaza Strip. The local SPI scale was established at an annual precipitation threshold level of 370 mm, which demonstrates the droughtless condition at an SPI₁₂ of zero. The temporal and spatial drought assessments that were mapped in Figure 11 confirm that the Gaza Strip faced serious evolving waves of drought over previous periods. The temporal assessment demonstrates that drought conditions distribute in an elliptic pattern over time, and the spatial distribution shows that the southern governorates of Khanyounis and Rafah are under extreme drought the majority of the time, as precipitation at these stations is less than average; the trend presenting in a decreasing pattern. The years of 1974, 1992, and 2016 were the most prosperous in terms of water. However, the years of 1980, 1998, and 2010 were the most barren. On average, the next 20 years’ drought conditions are to be severe in Rafah and Khanyounis with a rainfall amount of less than 230 mm per year. In central Gaza and Gaza, the dominant conditions are of moderate drought with an annual rainfall of 230–265 mm. The northern area of the Gaza Strip is forecasted to face moderate to normal drought with an annual rainfall of about 300–370 mm. The probabilistic analysis shown in Figure 12 signifies that the region of the Gaza Strip risks drought status with a likely incidence rate that varies from 8% in the north to 100% in the south. The northern parts of the Gaza Strip have mostly precipitation levels above the specified average precipitation of 370 mm, and this is reflected in the low occurrence of drought. However, the precipitation of the southern parts of the Gaza Strip is almost below the average precipitation, and this leads to permanent drought in this area. More specifically, the normal drought is the prevalent drought in the northern governorates, although the hazardous potential of extreme and severe drought is high in the southern governorates with an incidence risk of about 83%. Sequentially, southern governorates of Rafah and Khanyounis experience a chronic annual drought, while the period within which drought returns is reported to be every 9–12 years in the northern governorates of the Gaza Strip as depicted in Figure 13. The zoning of the Gaza Strip area reveals that 50% of the land experiences chronic drought events every year, while 28% of the Gaza Strip could face drought conditions every 2–3 years. Fifty percent of the land experiences chronic drought events every year, while 28% of the Gaza Strip could face drought conditions every 2–3 years. In this regard, the remaining 22% of the land endures drought with a return period of 3–12 years. In this context, the ongoing and future projections of drought reveal that urgent mitigation and intervention measures have to be taken to sustain the only available water resource of coastal aquifer in the Gaza Strip, which faces decline in the recharge process and is overexploited for domestic, agricultural, and other uses. In response, the PWA recommended water intervention strategies to maintain the quality of life in the Gaza Strip. The PWA suggests implementing a large-scale seawater desalination plant to be operated with the current short-term low volume seawater desalination plants in the Gaza Strip. The small-scale seawater desalination plants introduce about 7% of total water needs in the Gaza Strip. In addition, the large-scale central seawater desalination plant in Gaza is expected to provide 55 million cubic meters of the water in its first stage and about 110 million cubic meters in full operational capacity throughout the coming 20 years, which will improve the water situation by approximately 60% of total water needs. Moreover, the potential of reusing wastewater as a new water resource in the water cycle draws the attention of the PWA toward the willingness of implementing wastewater reclamation projects to exploit these resources in the replenishment of the coastal aquifer to remedy the impact of drought on groundwater sustainability. In the agricultural field, the PWA provided a plan to close all the illegal agricultural wells that exploit the aquifer arbitrarily and new irrigation systems were adopted to enable the farmers to use the reclaimed wastewater in the crops irrigating safely. Economically, the drought is expected to increase the price of water per unit; therefore, prospective water supplies are going to be mainly dependent on the nonconventional water resources that need national investments in constructing the new assets. Environmentally, the drought in the Gaza Strip will have real impacts on the environment, and this emphasizes the need to establish an environmental monitoring program to monitor the quality of the environment. The construction of seawater desalination projects and operating wastewater reclamation projects is a direct reflection of the increase of greenhouse gas emissions in the environment, which increases global warming and hikes up global temperatures.

Monitoring and forecasting the trend of climatic parameters reveal the impact of climate change on temporal and spatial distribution of droughts or flood events. The performance of the SPEI gives an accurate indication of drought interaction in terms of precipitation and temperature; however, the SPI reveals the conditions of drought depending on precipitation only. The robustness of stochastic models promotes these models as a simulator to mimic the pattern of observations and to forecast future conditions depending on the inherent trend of the time series. However, the stochastic models show significant deficiency in attaining outlier observations overcome by integrating other techniques like artificial neural networks. In this study, the stochastic models demonstrate high accuracy in modeling and forecasting the precipitation in the Gaza Strip due to low precipitation and low outlier measurements. The regional SPEI drought assessment for the Eastern Mediterranean region of MENA demonstrates that drought conditions in the Gaza Strip moved from the wet or nondrought conditions in the 1970s to moderate to severe drought within the last 10 years with an increase in drought occurrence probability of more than 40%. Locally, depending on the SPI scale and the adopted average precipitation amount by the PWA, the local temporal and spatial diagnostic assessment of drought is crucial to monitor the effectiveness of the anticipated national water supply projects that are planned to be implemented within the next 20 years to protect the sustainability of life in the Gaza Strip. Thus, the historical analysis demonstrates that the Gaza Strip faced serious evolving drought conditions where, in general, the potential occurrence of drought conditions in the Gaza Strip ranges between 8% in the north, which faces considerable precipitation amounts almost higher than 370 mm per year, and 100% in the south, which bears chronic annual drought events due to the low precipitation conditions of less than 370 mm per year. In conclusion, for the next 20 years, the drought pattern in the Gaza Strip is droughtless in the north with an average precipitation of about 433 mm per year, and normal to severe in the south with an average precipitation of about 242 mm per annum. The PWA adopted strategic water interventions to remedy the impact of hazardous drought over the forthcoming two decades. The principal part of these interventions is to operate a large-scale seawater desalination plant to decrease the deficit in water by about 60%. In addition, reclamation wastewater projects are to be completed to introduce water for groundwater recharge and agricultural irrigation.

All relevant data are included in the paper or its Supplementary Information.