The Euphrates–Tigris characteristics.
Growing population, increasing basin development, and progressively declining water supplies are typical water resources issues in the Middle East. Drought is one of the most damaging climate‐related hazards that affect more people than any other. For identifying drought‐prone areas in the Euphrates–Tigris Basin, multifold aspects of drought and its features such as the frequency of drought occurrence and its spatial distribution were assessed. The long‐term precipitation data were collected from different meteorological stations of Turkey and Iran, and standard precipitation index (SPI) was calculated. Due to the lack of raw data, the literature works on drought were used in Syria and Iraq to obtain a drought perception in these countries. Moreover, the policy of water resources management and the hydraulic works in these regions were considered. The results show significant changes in the precipitation in these regions over the past decades. The projects undertaken in the basin are not in line with the principles of integrated water resources management and intensify the drought and caused marshland demise in the downstream of the basin. The results of a comprehensive analysis of precipitation variation and water management in this research can alter the policy of water resources management in order to avoid drought in the basin.
- Water Resources
Water has played a vital role in the Euphrates–Tigris Basin (ETB), especially in Mesopotamia, which is the fertile land between the Euphrates and Tigris (ET) Rivers. Climate change together with increasing population makes the water resources an important issue in the region , which affects the social and economic conditions of this region. Very low precipitation caused a steep decline in agricultural productivity in the rain‐fed ET drainage basins and displaced hundreds of thousands of people . Climate change could seriously affect the water resources and lead to serious disputes among the countries that have territories in the ETB . Bozkurt and Sen  investigated future climate change in the ETB and found that Turkey and Syria are most vulnerable to climate change, and downstream countries, especially Iraq, suffer more because they rely on the water released by the upstream countries. Middle East Region significantly suffers from decreasing water resources due to climate change . The Middle East and North Africa (MENA) region represents 5% of the total world population, whereas contains only 0.9% of global water resources. The number of water‐scarce countries in the Middle East and North Africa has risen from 3 in 1955 to 11 by 1990. Another seven countries, including Iran, are expected to join the list by 2025 . ETB is in the midst of a water crisis and the worst drought in decades. At the current decline rate of water, the water supply in the basin will not be enough to avert such a widespread humanitarian crisis . The recent drought that began from 2003 has further strained the limited water resources in the region . Voss et al.  used Gravity Recovery and Climate Experiment (GRACE) satellite mission and  concluded an alarming rate of decrease of approximately –2.7 cm/yr water height, equal to 143.6 km3, in the total water storage from 2003 to 2009. SPI - is an index for extract drought periods that considers precipitation only and the negative values of SPI indicate droughts. Karavitis et al.  used different scales of SPI for droughts in Greece and they declared that SPI has had high ability to announce drought. Paulo et al.  used SPI for several sites of Alentejo, Portugal, to find drought classes. A reported SPI in stations inside Syria, as a part of ETB, is presented by Skaf and Mathbout . Their findings and results from literatures showed that the duration of most extreme drought in terms of intensity and areal extent in Syria corresponds to 1998–1999, 1972–1973, 2007–2008, and 1999–2000, respectively. Syria is one of the most economically affected countries by drought. Erian  showed that in Eastern Syria rainfall dropped to 30% of the annual average in 2008 and a main tributary of the River Euphrates dried up. Timimi and Jiboori  investigated the frequency of drought for the period of 1980–2010 in Iraq, based on SPI. Their results reveal that the country, in the past 30 years, faced frequent nonuniform drought periods in an irregular repetitive manner.
As severity of drought may have different effects in different regions and systems due to the underlying vulnerabilities, so the assessment of drought is a complex process . The objectives of this study are to identify drought vulnerability at multiple time steps, so the effects of rainfall deficiency on water resources in the region and water resources management in ETB can be accessed. The drought vulnerability and water management information presented in this study can be applied in other sectors and used by water managers to ensure that they will act timely and effectively to tackle the drought‐related losses in the regions.
2.1. Study area
The Euphrates and Tigris, with 3000 and 1850 km lengths, respectively, are the most important rivers in the Middle East, which have been supporting agriculture in the ETB region since many centuries. Both rivers are fed by numerous tributaries, and the entire river system drains a vast mountainous region. They start from mountain ranges of today's Turkey and Kurdistan, and flow southeast through Iraq to the Persian Gulf. In the southern Iraq, both the rivers merge to feed the Mesopotamian marshlands, the land between the Euphrates and Tigris rivers. These marshlands used to be the largest wetland ecosystem in the western Eurasia and Middle East , which once covered over 15,000 km2 of interconnected lakes, mudflats, and wetlands.
The headwater catchment generating the flow of ET is entirely located in the north and eastern parts of the ETB in the highlands of Turkey, Iraq, and Iran as a result of watershed's topography. The flow is regulated by the storage capacity of the limestone aquifers of the Taurus and Zagros mountains . The annual precipitation in the upstream of ETB typically exceeds 1000 mm whereas in the south of Iraq it was found to be less than 100 mm. Most of this precipitation occurs as snow in winter and the water resources are mostly available in the form of snowmelt water during spring and winter . As shown in Figure 1, the ETB spreads in the territories of Iraq (46%), Turkey (22%), Iran (19%), Syria (11%), Saudi Arabia (1.9%), and Kuwait (0.03%) . Details of ET river systems are presented in Table 1.
A challenge to conduct this research was the paucity of precipitation and water resources management data for the region. Inconsistent monitoring combined with a lack of data transparency and accessibility is a problem that plagues water managers in the ETB. Data shortage and inaccessibility result in an incomplete understanding of water availability and use in this area . In this research, daily precipitation data were collected from Iran Meteorological Organization and Regional Water Companies located in the west and northwest of Iran, and inside or close to the ETB. Similarly, in Turkey daily precipitation data were collected from the climate stations located in the East of Turkey, which are operated by the Turkish State Meteorological Service. For Syria and Iraq, where availability of raw data for precipitation and water management was very limited, the data analyses presented in previous researches were considered.
2.3. Drought analysis
In this study, drought vulnerability in the regions of ETB, which are the main origins to supply water to the whole basin, was investigated. To check the quality of dataset, each daily total is compared with the climatological daily total maximum for the corresponding site. For this, stations with consistent and complete precipitation records were selected. Cumulative distribution of daily precipitation for each month was applied to obtain the monthly values. Missing data of the stations were completed by using the linear stochastic model called the Thomas‐Fiering model. This model is based on the first‐order Markov model and represents a set of 12 regression equations. The well‐known Thomas‐Fiering model equation is described as 
where is the predicted rainfall for the jth month from the (j – 1)th month at time i, is the mean monthly rainfall during month j, Sj is the standard deviation of monthly rainfall during month j, aij is the independent standard normal variable at time i in the jth month, and rj is the serial correlation coefficient for rainfall from the (j – 1)th month to the jth month. Negative values obtained after applying Equation 1 were ignored. To identify the presence of drought or wet spells intensity for multiple time scales, several indices have been developed and adopted. The most well‐known index is the SPI developed by McKee et al. :
where is the rainfall, is the arithmetic mean, and is the standard deviation of the series. The SPI provides a quick and handy approach to assign a single numeric value to the rainfall which can be compared across regions with markedly different climates and to reflect the impact of rainfall deficiency on the availability of various water sources . The relative simplicity is a huge advantage of this index  and is among one of the most used indices by the researchers around the world. Therefore, its effectiveness in assessing the nature of the phenomenon has been tested in many climatic realities. Separate SPI value is calculated for a selection of time scales. McKee et al.  calculated the SPI for 3‐, 6‐, 12‐, 24‐, and 48 month time scales and defined the range for a “drought event” for any of the time steps and categorized the SPI to define various drought intensities (Table 2). In Table 2, negative and positive values of SPI represent rainfall less than and more than normal, respectively. For an equivalent normal distribution and adequate choice of fitted theoretical distribution of the actual data, the SPI can be considered as the value of standard deviations that the measured value would move away from the long‐term mean.
|−1||−1.00 to −1.49||Moderately dry|
|−2||−1.50 to −1.99||Severely dry|
To determine the area of influence of each individual station and utilized synthetic precipitation series in the study area, Thiessen polygons were employed. Since precipitation is not distributed normally, a transformation is first applied so that the transformed precipitation values follow a normal distribution. Different statistical distributions are announced to model the time‐series data. The gamma distribution to fit climatological data is the most well‐known distribution  which is defined using its probability density function:
where α > 0 is the shape parameter, β > 0 is the scale parameter, and x > 0 is the amount of precipitation. Here, the gamma function is defined as . To calculate the SPI value, the gamma stochastic distribution was transformed into the standard normal distribution. In this study, to compute the SPI for drought analysis, the Drought Indices Package (DIP) software presented by Morid et al.  was used. The package is capable of calculating and displaying SPI values at 3‐, 6‐, 12‐, and 24‐month time steps.
3. Results and discussion
Drought indices were analyzed at a regional scale in reference to the rainfall regime in the countries of ETB. Figure 2 presents samples of calculated SPI values and the measured rainfalls for stations in Iran for the 24‐month time scales.
Apart from few stations such as the Khooshemehr station (Figure 2a), over the years of study, the calculated SPI indices indicated significant drying in many parts of ETB located inside Iran. There were both short‐duration and long‐duration droughts. Most of droughts were occurred during 1999–2014 in the stations (Figure 2b–d). Furthermore, the hyetographs shown in Figure 2 indicated the considerable spatial and temporal changes in the precipitation total series of Iran, during the study period. Comparing the negative value of SPI in Figure 2, it could be concluded that the severity of drought in the Sanandaj station is more than the other stations considered in this study.
Biox et al. and Chen et al. [15, 23] showed that dams and reservoirs intensify the effect of drought on downstream community composition and structure. Therefore, it can be concluded that the severity of drought for these regions is attributed to the construction of dams or much withdrawal of water for irrigation in Sirvan watershed as a sub‐basin of the Euphrates‐Tigris basin. Consistent with the findings of this research, the drought occurrence in Iran is particularly more in its western and eastern parts, as reported by many researches [24, 25].
Turkey is a part of highlands of the ETB that receives much of the precipitation in form of snow in winter season. The samples of results for drought analysis using SPI and recorded rainfalls in the upper part of ETB, particularly in Turkey, are presented in Figure 3.
In this research, the results of drought analysis for many stations in Turkey revealed that the regions of these stations experienced frequent moderate, severe, and extremely droughts for all years as shown in Figure 3a. However, in a few stations no drought was detected using SPI index (Figure 3b). Overall, a significant change in rainfall was determined in Turkey. These results are consistent with that reported by Bozkurt and Sen .
3.1. Water management
The ETB is associated with ancient civilization where irrigation schemes had been developed about the 5 millennium B.C. The headwater basin generating ET flows was entirely located in the north and eastern parts of the basin in the highlands of Turkey, Iraq, and Iran. Its hydrology and topography provide critical insight into understanding how hydraulic works affect the lower ETB, particularly the marshlands in south and middle of Iraq. Huge drainage and damming operations on the ET river systems in Iraq, Syria, Iran, and Turkey have diminished around 85% of the Mesopotamian marshlands, which originally covered an area of 20,000 km2 . The three rivers that feed the marshlands originate from riparian countries and all of these countries have extensive plans for control water resources to expand their irrigated agriculture. Construction and planning of mega structural water resources development projects began in the early twentieth century in the region . United Nation Environment Programme (UNEP) - in 2001 reported that the dams stores five times more than the Euphrates annual flow and twice that of the Tigris. Table 3 shows the large dams according to the International Commission on Large Dams, ICOLD .
|Country||Name||Nearest city||River||Year||Height (m)||Capacity (MCM)|
|Eilam||Eilam||Baraftab and Chaviz||–||65||71|
|Syria||Al Tabka||At Thawrah||Euphrates||–||–||11,200|
|Derbendi Khan||Ba‘qubah||Diyola river||1962||128||3000|
While the water resources development has been considered as one of the causes for damaging the marshland ecosystem  and intensifying the drought effects, it can be seen from the data in Table 3 that the basin is now deeply regulated with cumulative storage capacity of riparian countries. Apart from the dams listed in Table 3, more than 20 dams are planned or are currently under construction in the basin. Further investigation shows that increased agricultural demand driven by land use policy and cropping pattern intensified the pressures on water resources in the ETB. In addition, the use of well water, mostly for agricultural needs, has rapidly increased in the ETB, causing a drop in the water level of the aquifers. Groundwater loss is found to be the major source of water reduction in the region. Voss et al.  reported a reduction in groundwater within last 12 years, equal to 1.73 cm/yr height.
The effect of manmade projects is greatest in the lower part of the basin in the marshlands in Iraq mainly in intensifying dust storm origins inside Iraq . The marshlands have been desiccated through the combined actions of upstream damming in riparian countries as well as the development of extensive downstream drainage projects, in the past 30 years . Without doubt Turkey is in the strongest position with regard to its potential control of a large part of the water resources of the ETB mainly due to the Southeast Anatolia Project (GAP). The project area neighbors with Syria in south and Iraq in southeast, which includes 22 dams and 19 hydropower plants and irrigation networks on the ETB.
It seems that there is a mismanagement of water resources in the region. As a few instances, the Tabqa dam in Syria planned to irrigate 640,000 ha of land. However, so far only 240,000 ha of land can be irrigated due to salinization and poor quality of land. Consistently, since 1990s large water management projects have been developed in Iran under a major policy to control surface water to serve other purposes such as irrigation and drinking. Thus, more than 20 dams have been constructed or are under construction in the Tigris Basin and an increasing amount of water is diverted from the rivers. The Karkheh is one of the largest dams in Iran with a reservoir capacity of 5.6 Billion Cubic Meter to irrigate 320,000 ha of land and produce 520 MW of hydroelectricity. Conversely, because of the water crisis and low water levels in Iran, the Karkheh dam irrigates one‐third of the anticipated area and cannot produce even 1 kWh of energy in its lifetime. In Sirvan watershed, as a part of ETB inside Iran, more than 10 dams were constructed in the last decades without accomplishing the irrigation and drainage system. While Iran is facing a critical water shortage currently, huge amount of water is lost through evaporation from unusable dams and reservoirs.
In Iraq, because of inadequate leveling, lack of know‐how, and poor water management practices, water is often poorly distributed. In the southern part of Iraq, diversion canals within the irrigation command area divert the ETB water from rivers to cultivated lands. Poor maintenance throughout the primary formal supply system caused water losses at all its stages of primary delivery. However, only about 30% of water supply available for irrigation annually actually reaches crops . Overall, the irrigation system in Iraq is inefficiently managed.
More than 90% of water resources are used for agriculture. The on‐farm water application rates in the region are high, and irrigation has a low efficiency. Keshavarz et al.  reported that overall irrigation efficiency in Iran ranges from 33 to 37%, which is lower than the average for both developing (45%) and developed (60%) countries. The most prominent causes of irrigation inefficiency in the study areas include improper design of irrigation facilities, poor maintenance, careless operation, negligible water prices, and inadequate training of farmers.
In this research, drought and water resources managements were investigated in ETB. Moreover, the calculated SPI for Iran and Turkey and those reported for Syrian and Iraq confirmed that there are various dry periods, which affect these countries, especially during the past 15 years. Among these four countries, Turkey has less severity and frequency of drought than the other three countries, and Syria has the most. This problem will affect the stream flow strongly. Thus, the annual flow of the Euphrates and Tigris entering Iraq and groundwater sources in the riparian countries declined drastically. Drought has negative impact on health, the agricultural production, and economic condition of most people who live in ETB; and food scarce makes migration from these dry areas and expected to increase further in the future. Apart from deficiency of rainfall, the rules to control the amount of water in riparian countries of ET lead to a decrease in the water flow in the two rivers. It should be noted that each riparian country has the right to use, in an equitable and reasonable manner, the water of the international watercourses in its respective territory. Emphasis on acquired rights without considering the principles of integrated water resources managements to achieve the optimal use of water by the riparian countries is the major cause of water decline in the ETB. The ETB has to be considered as forming one single transboundary stream system, and should be managed accordingly. It seems that the impact of hydraulic works needs to be reassessed and mitigated by ensuring a minimal water flow to sustain life in the ETB, particularly in Mesopotamian marshlands. A portfolio of water and land resources should be drawn up and jointly evaluated. Such realities remind us that we need to act now to restore ETB ecosystems on a global scale.
The authors are thankful of Ministry of Higher Education, Malaysia, and Universiti Teknologi Petronas for supporting this research under grants no. FRGS_ 0153AB_i61.
- Standardized Precipitation Index
- United Nation Environment Programme