Climate Variability Impact on the Spatiotemporal Characteristics of Drought and Aridityin Arid and Semi-Arid Regions
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Investigating the spatiotemporal distribution of climate data and their impact on the allocation of the regional aridity and meteorological drought, particularly in semi-arid and arid climate, it is critical to evaluate the climate variability effect and propose sufficient adaptation strategies. The coefficient of variation, precipitation concentration index and anomaly index were used to evaluate the climate variability, while the Mann-Kendall and Sen’s slope were applied for trend analysis, together with homogeneity tests. The aridity was evaluated using the alpha form of the reconnaissance drought index (Mohammed & Scholz, Water Resour Manag 31(1):531–538, 2017c), whereas drought episodes were predicted by applying three of the commonly used meteorological drought indices, which are the standardised reconnaissance drought index, standardized precipitation index and standardized precipitation evapotranspiration index. The Upper Zab River Basin (UZRB), which is located in the northern part of Iraq and covers a high range of climate variability, has been considered as an illustrative basin for arid and semi-arid climatic conditions. There were general increasing trends in average temperature and potential evapotranspiration and decreasing trends in precipitation from the upstream to the downstream of the UZRB. The long-term analysis of climate data indicates that the number of dry years has temporally risen and the basin has experienced succeeding years of drought, particularly after 1994/1995. There was a potential link between drought, aridity and climate variability. Pettitt’s, SNHT, Buishand’s and von Neumann’s homogeneity test results demonstrated that there is an evident alteration in the mean of the drought and aridity between the pre- and post-alteration point (1994).
KeywordsAridity index Climate data variability Climatic drought Multi-scalar drought index Trend analysis Homogeneity analysis
Research on long-term variations in meteorological data is important to identify climate change and variability as well as human-induced water resources management impacts (Mohammed and Scholz 2017b, 2018; Yue et al. 2018). Due to climate change impact and anthropogenic intervention, temporal and areal meteorological parameters would mostly vary in the long-term and cause alterations in the local and global hydrological cycle. Climate change and climate variability are anticipated to influence land use, land cover, water resources and ecological sustainability. Extreme hydro-climatic events such as floods and droughts can be considered the most important impacts of such changes (Michel and Pandya 2009; Mittal et al. 2016). Consequently, it would be useful to evaluate the hydrological process responses to such alteration to enhance decision makers understanding for the hydrological processes and to improve sustainable water resources management strategies.
Precipitation and temperature are key parameters to describe the climate over all scales (from local to global). Analysing the long-term trend of these parameters at a basin scale is useful for the assessment of the regional environment. The evapotranspiration variable is usually involved in water balance research. Accordingly, it can be considered as a more expressive variable for substituting temperature in water resources management (Mohammed and Scholz 2017a). Analysis of long time series concerning temperature, precipitation and potential evapotranspiration for an area may produce one of the following collections: + + +, + + 0, + + −, +0+, +00, + 0 −, + − +, + − 0, + − −, − + +, − + 0,− + −,− 0 +, − 0 0, − 0 −, − − +, − −0, − − −, where +, − and 0 represents a rise, a decline, and no change, respectively.
Many researchers have recently carried out extensive studies on trend analysis of climatic parameters such as precipitation (Koutroulis et al., 2011; Beguería et al. 2014; Khan et al. 2016; Ahmad et al. 2018; Asfaw et al. 2018), air temperature, meteorological drought (Banimahd and Khalili 2013; Trenberth et al. 2014; Moral et al. 2016; Deng et al., 2017; Cheng et al. 2018; Hazbavi et al. 2018; Yue et al. 2018) and regional aridity (Hrnjak et al. 2014; Djebou 2017; Mohammed and Scholz 2017a; Radaković et al. 2018). However, most research has focused on the spatiotemporal distribution of drought, examined potential drought patterns based on results from global and/or regional climate models (Koutroulis et al., 2011; Trenberth et al. 2014; Asfaw et al. 2018) and investigated the spatial and temporal variation of drought and aridity (Banimahd and Khalili 2013; Liu et al. 2015; Moral et al. 2016; Deng et al., 2017; Beguería et al. 2014) without evaluation of the potential impact of long-term variations and distributions of weather data on the drought and/or aridity at local scale For example, Tabari et al. (2012) investigated the rainfall and drought severity without linking it to the variation of the weather regional aridity.
Station addresses with corresponding aridity index (RDIα12) range and long-term average meteorological variables computed by the Thiessen network and the sub-area sizes
This research can be seen as a comperhensive study during which the relationship between climate variables, drought events and aridity are assessed at a local scale. This in turn can help to understand to what extent such relationship would affect basin hydrology in arid regions.
2 Data and Methodology
2.1 Illustrative Case Study Region
The Upper Zab River (UZR) is one the largest tributaries of the Tigris River in terms of water yield. The river has its spring in Turkey, runs through the northern part of Iraq, and subsequently joins the Tigris River covering a distance of about 372 km (Fig. 1). The UZR and its tributaries are located between latitudes 36°N and 38° N, and longitudes 43.3°E and 44.3°E. The UZRB covers an area of approximately 42,032 km2 with an elevation varying from 180 m above sea level (masl) to 4000 masl. Due to water erosion, the basin is filled with sandstone, gravel and conglomerate.
The UZR passes different ecological and climatic areas. The mean and the peak discharge of the river are 419 and 1320 m3/s, respectively. Annual precipitation ranges between 350 and 1000 mm (UN-ESCWA 2013). In general, most of the UZRB precipitation occurs in winter and spring. The distribution of annual precipitation is approximately as follows: 48.9, 37.5, 12.9 and 0.7% in winter, spring, autumn and summer, respectively. The UZRB flow regime shows considerable seasonal flow variations with a maximum discharge happening in May and low seasonal flow between July and December (UN-ESCWA 2013). The basin comprises many springs that are the main sources for irrigation proposes.
2.2 Data Availability, Collection and Analysis Techniques
The following climate data were gathered for this research purpose; daily precipitation amount and maximum and minimum air temperature from thirteen meteorological stations for the period from 1979 to 2013. The stations are spread within and outside of the UZRB with altitudes varying from 223 to 1980 masl. (Table 1 and Fig. 1). The data have been obtained from the Ministry of Agriculture and Water Resources (Kurdistan province, Iraq). The shape files of the Iraqi borders and UZRB have been obtained from the Global Administrative Areas (GADM 2012) and the Global and Land Cover Facility (GLCF 2015) databases, respectively.
For the projections of weather stations, shaping Thiessen network, and the delineations of the river and the basin, ArcGIS 10.4.1 has been used. XLSTAT, which isa user-friendly statistical software for data analysis add-in for Microsoft Excel, has been used for data analyses.
The standardised reconnaissance drought index (RDIst), standardized precipitation index (SPI), standardized precipitation evapotranspiration index (SPEI) and the alpha form of the standardised reconnaissance drought index (RDIα) were used for analyzing drought severity and regional aridity using precipitation and potential evapotranspiration. To estimate the potential evapotranspiration, RDI and SPI, the Drought Indices Calculator (DrinC1.5.73) (http://drinc.ewra.net/index_d.html) software has been applied (DrinC, 2018).
The Hargreaves method has been applied for the potential evapotranspiration estimation. Mohammed and Scholz (2016) stated that the Hargreaves method can be considered as the main tool to estimate potential evapotranspiration for many climatic conditions including arid and semi-arid climate as a result of its suitability for climate change research, which is supported by many research in water resources studies such as Vangelis et al. (2013) and Tigkas et al. (2012). Moreover, Mohammed and Scholz (2016) proved that the Hargreaves method was linked to the best findings that were similar to the full equation of the Food and Agriculture Organization Penman-Monteith Method. Additionally, Mohammed and Scholz (2016) proved that no significant impact on RDIst was detected by applying many potential evapotranspiration methodologies including the Hargreaves method at different elevations for a range of climate conditions. The SPEI has been estimated using the SPEI-package, which includes a set of functions for computing potential evapotranspiration and several widely used drought indices including SPEI.
To test meteorological data, many methods have been suggested (Duhan and Pandey 2013; Asfaw et al. 2018), which are generally classified into variability and trend analysis. The former set of tests applies the coefficient of variation (CV), anomalies (proportional departure from the average), precipitation concentration index (PCI) and the moving mean. However, the latter is normally performed by non-parametric and parametric analysis for regular climatic data (Duhan and Pandey 2013). The parametric analysis is a simple method, but requires climatic parameters to be normally distributed. Nevertheless, the non-parametric analysis does not assume any specific data distribution (Tabari and Taalaee, 2011).
The variability of precipitation and air temperature has been estimated using CV, the standardized precipitation anomaly and PCI. The Cv has been considered to assess the inconsistency of precipitation. A large value of CV indicates large variability and vice versa. The coefficient is calculated by using Eq. (1).
The PCI can be classified into low (uniform monthly distribution), moderate and very high precipitation concentrations. The respective ranges of PCI are as follows: PCI < 10, 11 < PCI < 15, 16 < PCI < 20, and PCI > 21, respectively. Furthermore, standardized anomalies of precipitation have been computed to study trend characteristics, enable the definition of dry and wet years in the measurements and to evaluate drought severity and occurrence (Asfaw et al. 2018) as represented by Eq. (3).
The classes of drought severity are extreme, severe, moderate and no drought with the corresponding ranges Z > −0.84, Z < −1.65, −1.28 > Z > −1.65, and − 0.84 > Z > −1.28, respectively. The non-parametric Mann-Kendall (M-K) test was used to identify, if there is a monotonic descending or increasing trend in the climatic time series. A monotonic increasing (descending) trend shows that the variable constantly raises (declines) during the time-period, though the trend may or may not be linear. Tabari and Taalaee (2011) and Robaa and AL-Barazanji (2013) published more details about the M-K test. Pettitt’s, Standard Normal Homogeneity (SNHT), Buishand’s test and von Neumann’s test were applied to check the homogeneity of the climatic indices (Zahumenský, 2004). RDI, SPI and SPEI were applied to study the temporal variation of meteorological drought and aridity.
2.3 Meteorological Drought
2.3.1 Reconnaissance Drought Index
The RDI may be expressed in terms of the standardised (RDIst), normalised (RDIn) and initial (RDIαk). In general, the standardised form is used to evaluate the severity of drought and the initial form is used as an aridity index. The aridity index is mainly based on the accumulated values of precipitation and potential evapotranspiration (Vangelis et al. 2013). Online Resource 1.1 involved the theoretical background of the RDI index.
A positive RDIst number represents a wet period. In contrast, a negative one is symptomatic of a dry period compared to the normal environment of the corresponding research area. The drought severity rises when the RDIst magnitude becomes minimal. Drought severity may be classified as mild (−0.5 < RDIst < −1.0), moderate (−1.0 < RDIst < −1.5), severe (−1.5 < RDIst < −2.0) and extreme (RDIst < −2.0) classes (Tigkas et al. 2012; Vangelis et al. 2013).
2.3.2 Standardised Precipitation Index
The SPI can identify and monitor droughs. The evaluation of SPI at a certain location is based on a series of accumulated precipitation for a different monthly time scale such as 1, 3, 6, 9 and 12 months. The precipitation series is fitted to a probability distribution that is subsequently transformed to a normal distribution. It follows that the average SPI for the target location and the chosen period is zero. Negative numbers of SPI specify less than median precipitation, whereas positive SPI values are indicative of greater than median precipitation. The gamma distribution fits climatological precipitation time series well (Vangelis et al. 2013).
2.3.3 Standardized Precipitation Evapotranspiration Index
The standardised precipitation evapotranspiration index (SPEI) is a simple multi-scalar meteorological drought index that links values of precipitation and temperature with each other. SPEI is depended on the climatic water balance (precipitation-potential evapotranspiration) for a monthly time scale. The values are aggregated at several time scales and changed to standard deviations with respect to average values. For more details regarding SPI, see Online Resources 1.2.
3 Results and Discussion
3.1 Analysis of Climate Data
Statistical properties of the meteorological variables after applying a non-parametric test for the decadal change
Mean air temperature (°C)
Potential evapotranspiration (mm)
Descriptive statistics and Mann-Kendall (M-K) trend analysis of the Upper Zab River Basin precipitation during the time-period between 1979 and 2014
A significant (p < 0.05) rising trend for the potential evapotranspiration concerning the whole UZRB during the last half-century has been noticed (Fig. 3c and Table 2). The decadal increase in potential evapotranspiration rate was 27.10 mm. With a mean amount of approximately 1348.48 mm, the estimated potential evapotranspiration for varied from 1222.10 mm in 1982/1983 to 1429.132 mm in 1998/1999 (Fig. 3c). The research outputs show that the semi-arid environment, as illustrated through the example basin, is becoming hotter and drier as a result of climate variability during the previous three decades. For example, the annual precipitation declined and the annual average temperature rose (Table 2).
Figures 3d–f display the spatial distribution of the long-term average values of the meteorological parameters. Each box–whisker plot represents a meteorological variable for a certain station over UZRB, which ranged from the upstream to the downstream part of the basin. Despite that there are no coherent change trends among various stations, there are general increasing and decreasing trends in both mean temperature and potential evapotranspiration and precipitation, respectively, for the upstream to the downstream areas. The mean temperature varied between 12.55 and 17.23 °C.
The Spearman correlations between meteorological drought indices and the annual standardised reconnaissance drought index (RDIst), annual standardized precipitation index (SPI), annual standardised precipitation evapotranspiration index (SPEI), and annual aridity index (RDIα12) for the thirteen meteorological stations within the Upper Zab River Basin (UZRB) and Precipitation concentration index (PCI) of UZRB for the time period between 1979 and 2014Station name
Meteorological drought indices
Meteorological drought indices and aridity index
RDIast vis SPIb
RDIst vis SPEIc
SPI vis SPEI
RDIst vis RDIα12
SPI vis RDIα12
SPEI vis RDIα12
Number of years
Low precipitation concentration (almost uniform)
Very high concentration
3.2 Drought and Aridity Identification, Classification and Correlation
To assess the occurrence of drought, the study applied the SPI, RDIst and SPEI, which are frequently applied drought indicators. However, for aridity identification, the alpha form of the RDI (RDIα12) has been considered.
The drought severity for UZRB has worsened considerably during the past 12 years. The drought amounts calculated from 1998 to 2011 illustrate that considerable droughts took place as the number of months with total periods of precipitation lack increased. The precipitation tendency and the long-term investigation show that the drought events and regional aridity were linked with the precipitation reduction and an increase in the potential evapotranspiration (Fig. 5a and b). Additionally, from the beginning of the year 2000, the precipitation trend shows that the area has experiencing a precipitation reduction as well as an increase in the potential evapotranspiration and the drought periods. In general, as a result of precipitation decrease coupled with the potential evapotranspiration increase, particularly during the years from 2006/2007 to 2007/2008, drought and aridity have worsened (Fig. 5a and b).
3.3 Change Point Identification
Station addresses with corresponding homogeneity test for the basin annual standardised reconnaissancedrought index (RDIst); annual standardized precipitation index (SPI), annual standardised precipitation evapotranspiration index (SPEI) and annual aridity index (RDIα12)
The homogeneity test for the basin annual standardised reconnaissance drought index (RDIst); annual standardized precipitation index (SPI), annual standardised precipitation evapotranspiration index (SPEI) and annual aridity index (RDIα12)
Figures 7b–h explain the change point years for the drought and aridity time series using the Pettitt method. The figures confirm that there is an evident change in the average of the indicator time series pre- and post-change point (1994). Accordingly, 1994/1995 is seen as a change point for the assessed time series, which reflects the impact of climate variability and anthropogenic interventions on the basin climate.
There was sufficient evidence for defining climate data trend alterations in the assessed region. The findings indicate that there are declining and rising trends in yearly mean temperature. However, most of them were not statistically significant (p > 0.05). A significant (p < 0.05) decreasing trend in precipitation was noted. Increasing trends in the potential evapotranspiration were computed. However, most of these trends were not significant (p > 0.05). An assessment of meteorological drought trends showed that droughts have surged.
Despite that there are no coherent change trends in the spatial distribution of the climate data, there are general increasing trends in average temperature and potential evapotranspiration as well as decreasing trends in precipitation from the upstream to the downstream areas of the basin. The average precipitation concentration indicates high precipitation concentrations. The precipitation anomaly witnessed for the occurrence of the trend being lower than the long-term mean becomes evident mainly after 1994/1995.
The long-term analysis of climate data reveals that the number of dry years has temporally risen and the basin has encountered succeeding years of drought, particularly after 1994/1995. Humid and dry sub-humid sub-basins are likely to become arid and hyper-arid due to climate variability. There is a strong relationship between drought, aridity and climate variability.
The potential differences and similarities among RDIst, SPI and SPEI indices were investigated by a comprehensive comparability analysis. Observations indicated that there is a better correlation with RDIst compared to the one with SPI and SPEI.
The drought and regional aridity variations and the role of climate variability were investigated applying linear regression and homogeneity tests. The annual RDIst, SPI, SPEI and RDIα12 values increased significantly (p < 0.05) at the annual rate of −0.0401, −0.035, −0.0085 and − 0.0085, respectively, and a remarkable alteration occurred in 1994. The increase in drought and the aridity indicated that during the last three decades UZRB became drier, which is affecting the regional water resources availability. Pettitt’s, SNHT, Buishand’s and von Neumann’s test results proved that there is an evident variation in the mean of the drought and aridity between the pre- and post-change point (1994). Consequently, 1994/1995 can be considered as a reflection for the potential impact of climate variability.
Finally, the results indicated that using only trend analysis, whether it is parametric or non-parametric, cannot be considered sufficient enough for climate variability evaluation. Adding a homogeneity test to the analysis would provide a clear picture concerning the long-term variations of the climatic variables, particularly drought and aridity.
The authors acknowledge the support of their respective institutions. This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.
Open access funding provided by Lund University.
Compliance with Ethical Standards
Conflict of Interest
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