The increase in precipitable water vapor in the atmosphere due to rises in temperature has changed the seasonal and geographical distribution of precipitation in many regions of the globe [1
]. The climatic water availability (CWA), which is the difference between the evaporative demand of a reference crop and the total precipitation, has changed due to the rise in evapotranspiration and the alteration in precipitation. CWA provides an estimation of the surplus or deficit of precipitation to meet the evaporative demand of a reference crop and thus, it is a key factor that defines crop irrigation needs. The modification in CWA due to variations in climate can have severe implications in agriculture [4
One of the limiting factors of agricultural development in the Middle East and Mediterranean regions (MENA) is water as there is a general deficiency in the overall amount of water and irregularity in rainfall [6
]. The recent climate changes due to global warming have seriously aggravated the issue of water scarcity. The CWA and crop water demand (CWD) have also changed due to the changes in seasonal rainfall pattern and rises in temperature. The alteration in CWA and CWD is particularly important in Iraq where a gradual decrease in rainfall and a rise in temperature have been noticed in recent years [7
The temperature in Iraq is increasing at a higher rate compared to the average global rate [8
]. Such a rate of temperature increase has been confirmed to be continued in the coming years by climate models [9
]. The increase in temperature is believed to contribute to an increase in evapotranspiration which can result in a large increase in CWD and a decrease in CWA [10
]. It can have severe implications for water stress in the country and thus, food scarcity. Understanding the changes in water demand and water balance is vital for preparedness and planning adaptation to climate change effects on water resources and to achieve sustainability in agriculture.
Atmospheric water balance has been assessed in different regions of the earth to provide clues to climate change implications in agricultural water use in terms of variations in net irrigation needs [3
]. However, there is no study so far on detecting/assessing the spatiotemporal changes in CWD and CWA employing long-term precipitation and temperature data and trustworthy methods.
Trends in climate variables are usually assessed for the quantification of the changes in climate variables. The non-parametric Mann–Kendall (MK) test [21
] suggested by the World Meteorological Organization (WMO, 1998) is generally used for the evaluation of the trends in atmospheric variables. The MK test is independent of data distribution and less sensitive to missing values and therefore most popularly used for the assessment of trends in different climate variables all over the world. However, the major drawback of the MK test is that it is sensitive to autocorrelated data [9
]. Several revisions of the MK test have been made to make it insensitive to autocorrelation [24
]. Recent studies also revealed that long-term persistence (LTP) or the slow decay of the autocorrelation function that causes a longstanding cycle in data series also affects the significance in trend [27
]. The LTP in climatic time series occurs due to the influence of slow climatic processes that change over time. The LTP is a part of climate and therefore, it is required to distinguish the natural climate variations from the unidirectional trend of climate in order to estimate the trend caused by global warming [8
]. The recent modification of the MK test (MMK) by Hamed (2008) can be used for this purpose [32
The present study aims to evaluate the spatiotemporal pattern in the trends in CWD and CWA of Iraq using MMK tests. The monthly rainfall data of the global precipitation climatology center (GPCC) and temperature data of the climate research unit (CRU), having a spatial resolution of 0.5° for the period 1961–2013, were used for this purpose. The spatial variability in the trend in both annual and seasonal CWA was assessed. The changes in CWD for five common crops of Iraq namely, wheat, barley, millet, sorghum and potato were analyzed. Obtained results were compared with MK test results to distinguish the trends due to global warming and the natural variability of climate.
The robust trend tests adopted in this study for the evaluation of the spatiotemporal changes in CWD and CWA are expected to provide the secular trends in CWA and CWD due to global warming. CWA and CWD are important indicators of water availability and irrigation needs. Therefore, the changes in CWA and CWD would provide valuable knowledge required for long-term management of water resources in Iraq.
The spatial distribution and trends in CWA for different climatic and crop growing seasons for the period 1961–2013 were assessed in this study. The changes in CWD for the major crops in the irrigated regions were also assessed to provide an understanding of how the irrigation requirements for different crops have changed over time.
The results showed declination in annual and seasonal CWA in most parts of Iraq. The obtained results collaborate with the results of earlier studies on the impact of climate change on agriculture water demand in an arid environment. Shen et al. [56
] assessed the spatiotemporal variability of irrigation need for five major crops in the arid Northwest part of China. They reported an increase in irrigation need over the past two decades with the highest increase in July and August. The comparison of irrigation need with water availability showed April and May as the most crucial period for ensuring water supply. Ragab and Prudhomme reported an increase in CWD due to a decrease in rainfall by 20–25% in the arid region of the Middle East [57
Chowdhury et al. [58
] investigated climate change impacts on crop water needs in north Saudi Arabia, bordering southwest of Iraq and reported that 1 °C temperature rise may cause an increase in crop water requirement by 2.9%. They also indicated the lesser impact of rainfall compared to temperature on CWD. This also concurs with the results of the present study. The temperature of Iraq is increasing at a higher rate compared to other parts of the globe [43
], while the rainfall is reported to change very less [9
]. Therefore, a large decrease in CWA and an increase in CWD in Iraq are due to a large increase in temperature.
Climate models projected a continuous rise in temperature in Iraq which would cause a further decrease in CWA and increase in CWD. Azad et al. [13
] used Soil and Water Assessment Tool (SWAT) to show the effect of climate change on winter wheat crop in the north of Iran, bordering northeast of Iraq and reported a rise in temperature and reference evapotranspiration in the middle of the last century which would cause an increase in water scarcity. Zamani et al. [12
] assessed climate change implications on CWD in the southwest of Iran and reported a large increase in CWD for all scenarios. Sun et al. [59
] estimated a decreasing trend in CWD for the main crops (apple, jujube, maize, potato, and soybean) in the arid Loess Plateau of China for different climate change scenarios. The results presented in this study can be utilized for the mitigation of climate change implications in Iraq.
There exists a clear nexus between water and energy in irrigated agriculture [60
]. An increase in irrigation demand causes an increase in energy needs for irrigation, which in turn can cause an increase in irrigation cost and reduction in farmer’s profit from agriculture. Therefore, increasing CWD can have a significant impact on the livelihood of the large population depending on agriculture in Iraq. Particularly, poor and marginalized farmers will be the most affected if proper adaptation measures are not taken. The higher use of energy for irrigation also increases the carbon emission from agriculture and contribute to global warming [60
The climate of Iraq is changing rapidly which is also projected to continue due to global warming. A fast rise in temperature and declination of precipitation in most of Iraq has been projected by the majority of climate models under all the climate change scenarios [62
]. This indicates that CWA will continue to decrease and CWD will increase over the present century. Continuous increases in CWD and decreases in CWA would cause a gradual increase in water scarcity in agriculture in Iraq. Crop productivity is directly related to the availability of water for irrigation. Therefore, increasing water stress may severely affect crop productivity and food security in Iraq, if appropriate adaptation measures are not taken. A vast population of Iraq depends on agriculture for livelihood. Reduction of crop productivity would certainly affect the economic benefits and livelihood of the population dependent on agriculture. The decreasing precipitation and increasing PET would also cause an increase in aridity in Iraq. Similar trends in Iran were found to cause a shift in arid lands [63
]. The shifting of aridity in the cultivated land of Iraq may cause land degradation and reduction of land fertility which in turn may also reduce crop productivity and food security of the country.
Iraq should consider improvements in irrigation practices, advantages of technological development, and changes in the cropping pattern to increase crop water productivity to mitigate the impacts of climate change in water and agriculture. The seasonal precipitation pattern in Iraq is changing due to climate change. The schedule of different crops can be adjusted according to the changes in seasonal precipitation pattern to take advantage of precipitation increase in some months. High water-consuming crops can be replaced with relatively less water-consuming varieties and the advanced irrigation technologies can be used to reduce irrigation needs. Moreover, awareness should be grown among the farmers of looming water scarcity and encourage them to adopt different traditional practices to conserve soil moisture for the reduction of irrigation demand.
Gauge-based gridded precipitation and temperature data of GPCC and CRU, respectively, were used in this study for the assessment of CWD. Though GPCC precipitation and CRU temperature were found to replicate observed precipitation and temperature in Iraq reliably [35
], significant uncertainties are still associated with gridded climate data [64
]. In future, other gridded precipitation and temperature data can be used to assess the uncertainties in CWA and CWD due to gridded climate data. The Thornthwaite method was used for the estimation of PET for the assessment of CWA due to the unavailability of data of different meteorological variables required for other PET methods. However, the Thornthwaite method overestimates PET in a predominantly semi-arid country like Iraq. Future studies can be conducted for the estimation of PET with reliable methods like Penman-Monteith and Hargreaves when other meteorological data are available. The crop calendar was approximated to a monthly scale for the assessment of the changes in CWD and CWA in Iraq. For example, cultivation of some crops like Millet and Sorghum starts in the middle of a month which was approximated to start at the beginning of the month for estimation of CWD using monthly climate data. In future, daily precipitation and temperature data can be used for a more reliable estimation of CWD.
The present study was conducted to assess the changes in climatic water availability and irrigation needs in Iraq for the period 1961 – 2013 using GPCC precipitation and CRU temperature data. Obtained result showed sufficient CWA for rain-fed agriculture in the mountainous region in the north of Iraq for all the seasons except the dry summer season. In contrast, a deficit in CWA was noticed in the southwest for all the seasons even in wet winter. Trends in CWA and CWD in Iraq revealed a significant decrease in annual and spring in the west and the south. In winter, it was found to decrease in a small region in the east. CWA revealed a significant decrease during summer crop growing seasons (millet and sorghum) over the entire of Iraq while a decrease in the west and the southeast during winter crop (wheat, barley and Potato) growing seasons was also revealed. The average CWD of the major crops of Iraq showed a significant increase in summer crops water requirement in April, May, June and July between 1961–1990 and 1984–2013. For winter crop, the CWD was found to increase in October and April, decrease in November and to show no significant change in other months. The spatial distribution of CWD in irrigation areas showed an increase in irrigation requirements in recent years compared to past years. The increase was found to be higher for summer crops than the winter crops. It was also observed that CWD in irrigation area along the Tigris River increased more compared to that along the Euphrates River. The highest increase in CWD was found for the summer crops in the south of the Euphrates River. The results revealed a significant negative impact of climate change on climatic water balance and agriculture water requirements in Iraq. This emphasizes the need for new strategies for sustainable water resources management for the mitigation of the implications of climate change in Iraq.