Characteristics of Dry and Wet Changes and Future Trends in the Tarim River Basin Based on the Standardized Precipitation Evapotranspiration Index

By inergency On Mar 20, 2024

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1. Introduction

Drought is among the world’s most significant climatic phenomena, due to the extensive harm and losses it causes [1,2]. Moreover, droughts are becoming more frequent and complex [3]. According to relevant organizations, drought is a meteorological phenomenon marked by an extended period of dry weather within the natural climate cycle, leading to water shortages [4,5]. It is important to underscore that unlike other natural disasters such as floods and wildfires, it is a slow-onset threat that becomes apparent as societies and ecosystems begin to feel its impacts. Additionally, drought has non-structural and widespread geographical ramifications [6,7]. Projections for the next 70 years show that drought-prone areas around the world may continue to expand, leading to an escalating negative impact of drought [8]. In China, the repercussions of drought are already severe, making it one of the nation’s most critical natural disasters. Drought affects not only the ecological environment and agricultural production but also wide-ranging economic and social aspects [9]. Statistics reveal that annual agricultural economic losses due to drought in China may reach as high as CNY 27.2 billion [10].

The Tarim River Basin (TRB) exemplifies the challenges faced by arid regions worldwide, making it an important case study for understanding drought dynamics. Against the backdrop of global climate warming, drought events in arid zones are steadily increasing in frequency and intensity [11,12,13]. The TRB, which is situated in the heart of Central Asia’s arid region [14], exemplifies a characteristic continental arid climate. It is an environmentally fragile area whose ecosystems are extremely vulnerable to worldwide climate change. Climate variations within the basin can result in the onset of numerous extreme weather events. The TRB experiences a temperate continental climate characterized by low precipitation, high temperatures, and intense evaporation. Drought is among the most frequent and predominant natural disasters in the basin, and extreme drought events have had significant impacts on both the basin’s ecological environment and human life [15]. Hence, monitoring drought and researching related mechanisms have become pressing priorities for development and disaster mitigation efforts in the TRB.

In its simplest form, drought manifests as a relative deficiency of water over a specific period compared to normal conditions. Water deficiency could refer to a severe imbalance in the surface water budget caused by significantly reduced precipitation or other abnormally dry climate conditions. This type of drought is often associated with meteorological conditions, such as changes in high-pressure systems or rainfall patterns, and is characterized by a wide-reaching impact and prolonged duration [16]. Meteorological drought typically manifests first and serves as a precursor to other drought types. It usually initiates with insufficient precipitation and accompanies the entire drought event [17].

The extent and intensity of meteorological drought largely determine the duration and intensity of other forms of drought events [18]. Since meteorological drought (like other forms) can be challenging to define, various indices are commonly used in meteorological drought-related research for quantitative analysis and monitoring of drought severity [19]. Currently, there is no optimal indicator to describe drought characteristics, as the complex relationships between evaporation, infiltration, groundwater, and surface water mean that different indices assess drought characteristics differently [20]. For instance, the PDSI considers temperature, precipitation, and groundwater factors, rendering it suitable for agricultural drought, but it has limitations in terms of its ability to capture multi-scale variability and cannot distinguish between different types of drought [21]. The SPI is simple to calculate and flexible in terms of time scale, but it only focuses on precipitation, overlooking the other important factors [22,23]. In comparison, the SPEI represents an improvement over both the PSDI and SPI, as it considers factors that are relevant to drought, such as temperature and precipitation. Furthermore, due to its temporal flexibility and spatial continuity, the SPEI can detect different types of drought occurrences across various temporal scales, making it well suited for reflecting regional drought characteristics [24].

Accordingly, the SPEI has become a crucial tool for assessing dry and wet conditions and is now widely applied in various fields of drought research [25,26,27]. In previous studies, this tool was extensively used in climate change investigations for analyzing drought variability and identifying drought impacts on agriculture and ecosystems [24,28]. Recent research has shown that temporal variations in Standardized Precipitation Evapotranspiration Indices (SPEIs) demonstrate differing reactions across various time scales. Specifically, it has been observed that shorter time scales, such as SPEI-3 and SPI-6, are characterized by a heightened frequency of drought occurrences coupled with shorter durations. At the end of the 1980s, some scholars suggested that there was a trend of “warm-drying” to “warm-wetting” climate change in the arid areas of Northwest China, and the amount of glacier ablation and precipitation showed a continuous increase, while the runoff volume also increased [29]. However, some scholars have questioned this statement [30,31], and since the 21st century, Xinjiang has witnessed a steady rise in temperature alongside a slight decline in precipitation, which is expected to influence the wet and dry climate patterns in the TRB [32]. Some scholars have used SPI to analyze droughts in Xinjiang, China, from 1957 to 2009, and found that the severity and duration of droughts have been increasing [33].

Currently, there has been minimal focus on the trends of wet and dry changes in small watersheds over extended time periods, particularly within the context of transitioning from warm–wet to warm–dry conditions in the arid regions of Northwest China [34], whether there is a wet and dry transition in the watersheds, and there is a deficiency in predictions of future spatial and temporal changes in wet and dry changes in the watersheds as a whole, which is crucial for drought risk assessment in the TRB. This study utilizes temperature, precipitation, and the SPEI from the TRB spanning the last 60 years (1962–2021). This research systematically analyzes the spatiotemporal characteristics of drought and wetness in the region, and the run theory is employed to identify the frequency and severity of drought events. This comprehensive analysis aims to offer insights into the frequency, severity, and spatial distribution of meteorological drought events. This study employs advanced statistical methods and climate model projections to enhance understanding and preparedness for meteorological drought risks in the TRB, serving as a valuable reference for drought warning research and strategies to mitigate meteorological drought risks in the drought-prone region.

4. Discussion

Global warming has contributed to significant changes in the TRB, shifting the region’s climate from “warm-wet” to “warm-dry”. Tang Qiuhong et al. discovered that the rise in temperature, decline in relative humidity, and increase in wind speed in Northwest China over the last 60 years offset the wetting trend brought about by the increase in precipitation, which is an important reason for the change from wet to dry [40]. Wan et al. suggested that the dominant factor for the drought characteristics of recent years in China is the increase in the PET [41], and some studies have shown that the Indian Ocean Basin-wide Modal (IOBM) and the Pacific Ocean Basin-wide Modal are the same. Additionally, they discovered that the IOBM and the Pacific Interdecadal Oscillation may serve as the primary drivers of climate change in Northwest China, whereas in Xinjiang, the Arctic Oscillation and North Atlantic Oscillation concurrently play crucial roles in drought evolution, particularly from January to March. In related work, Tao Hui et al. analyzed the wet and dry variations in the TRB and found that atmospheric circulation significantly influences changes in dry and wet conditions and that the effects of water vapor transport and atmospheric structure changes on the basin cannot be ignored [42]. Furthermore, under the scenario of continuously increasing temperature without a substantial rise in precipitation, the influence of a potential increase in evapotranspiration has gradually exceeded that of precipitation, and this region has been reversed from a humid trend to an arid one [43]. Despite the increasing precipitation in NW China, the primary cause of the escalating drought in Northwest China remains the increase in evapotranspiration due to rising temperatures [31].

Additionally, we found that the EOF decomposition of basin drought showed three main modes: the first mode exhibited consistency across the basin, while the second mode showed a north–south opposition, and the third mode displayed an east–west opposition, which was closer to the results of Zhao et al. [44]. The first mode shows the overall convergence of wet and dry conditions in the TRB, exhibiting obvious polycentricity, with the center of high values situated in the southwest part of the basin. The time coefficients corresponding to this mode indicate a transition from wet to dry, echoing the results of related studies on climate change in the northwestern region [29,45,46]. As the time scale increases, the time coefficients and spatial modes of wet and dry changes are inclined to show global rather than local trends, and generality rather than details [47]. Therefore, the SPEI benefits from multiple time scales and can be used as an important tool to analyze the characteristics of short-, medium- and long-term drought temporal changes as well as spatial distribution characteristics. Meanwhile, the EOF decomposition method was able to extract mutually orthogonal spatiotemporal modes from the complex drought variable field, accurately reflecting the spatiotemporal changes of drought, which is a key method for analyzing drought characteristics [48]. The dry and wet spatiotemporal change characteristics obtained from the analysis have strong regularity and identifiability.

Our analysis of future trends of wet and dry changes in the TRB found that drought levels will likely increase and that the area of drought intensification is concentrated in the central region of the basin. For the past several years, global warming and the increase in evapotranspiration have had a rising impact on the Northwestern Arid Zone. This factor is the primary driver behind the intensification of drought in this region [45]. Some scholars have also studied the drought characteristics of the TRB through four different scenarios of CMIP6 and the VIC distributed hydrological model. Their results show that under the different scenarios, the future drought trend intensifies in the basin’s central region but is somewhat weaker in the mountainous areas on the periphery of the basin, which aligns well with the findings of the current study [49]. Meanwhile, in the Taklamakan Desert in the central part of the TRB, the future drought trend increases sharply, demonstrating that the desert amplifies the impact of global warming within the context of a warmer climate in the future, making the whole area more sensitive to climate change. This sensitivity causes the arid and water-scarce land mass to become even more arid so that the risk of drought intensification within the basin is higher in the future [50].

Research indicates that climate warming amplifies the dynamic alterations of vegetation in the Northern Hemisphere [51]. Against the backdrop of climate warming, vegetation coverage in Xinjiang exhibits a declining trend [52]. Recent studies have demonstrated that extreme temperatures and intense precipitation in Xinjiang play a crucial role in influencing changes in vegetation coverage. Hence, the poor state of vegetation degradation may be caused by the combination of climate “wet-dry transition” and frequent recurrence of extreme climate events. Climate change will affect glacier melting. This, in turn, affects the generation of meltwater runoff and makes a significant contribution to the total volume of water resources. The runoff of TRB is heavily dependent on glacier melting [35]. Over the past few decades, many glaciers have experienced a complete retreat; the ongoing decrease in runoff since the 21st century is intimately associated with the diminishing glacier area, the thinning of glacier thickness, and the elevation of the equilibrium line in the basin [53]. The present study also found that the trends of the SPEI were not significant at short time scales but showed a significant downward trend at long time scales. Moreover, the frequency of changes in the SPEI gradually weakened, while the magnitude of changes gradually increased with the expansion of the time scale. This indicates that the index’s response to climate slows down for long time scales, reflecting the general trend of wet and dry changes [54].

In this paper, a limited amount of station data within the watershed is extended to the entire study area by inverse distance weight interpolation. The findings vary depending on the interpolation methods. The inverse distance weight method is simple and flexible, but it only considers the influence of distance and ignores the spatial variability between variables. The inverse distance weight interpolation method used in this paper has the advantage of simplicity, but the effect of terrain factors on drought may be underestimated, especially since some areas in southern TRB are located in the hinterland of the Taklamakan Desert and lack meteorological stations. Therefore, because the results of our analyses may be subject to certain limitations and uncertainties, the potential advantages of surface source data in the in-depth study of drought need to be further explored.

5. Conclusions

This study employed data from 37 meteorological stations and the CMIP6 dataset to compute the SPEI at various time scales in the TRB. Empirical orthogonal decomposition of the indices was carried out to make a detailed analysis of the climate dry and wet changes during a historical interval (1962–2021) and the forthcoming interval (2022–2100), along with their characteristics of spatiotemporal distributions. The main conclusions of this study include the following:

(1): From the latter part of the 1980s to the conclusion of the 1990s, the TRB showed a clear trend of warming and humidification, but from 1998 onwards, the basin as a whole began to change from wetness to dryness, and the proportion of mild drought, moderate drought, and extreme drought notably expanded at all measured sites. Since then, the proportion of mild drought, moderate drought, and extreme drought has increased significantly.
(2): The salient features of the spatial distribution of drought in the TRB are “more in the north and less in the south”, but drought severity characteristics are “less in the north and more in the south”. Overall, the drought severity and the spatial distribution of the number of droughts have little consistency, and droughts are frequent in the north, whereas they are severe in the south. In other words, the severity and frequency of drought events do not exhibit spatial consistency, with frequent but less severe droughts in the north and fewer but more severe droughts in the south.
(3): There were three main types of spatial modes in the TRB: regionally consistent, north–south opposite, and east–west opposite. About 75% of the cumulative variance contribution was attributed to the first three modes, with the first mode primarily characterizing the basin.
(4): Anticipated future climate change will elevate drought risk in the TRB, exacerbating the drought trend and concentrating the spatial distribution more in the basin’s center and less at its periphery.

The climate warming of TRB leads to the acceleration of glacier melting, which increases the water resources within the basin during a specific time interval. However, in the long run, as the temperature continues to rise, the glaciers will face depletion, which is very unfavorable to the development of the already arid TRB. Therefore, the government should take energy conservation, emission reduction, and consumption reduction as important starting points for economic development; develop and utilize new renewable energy sources; strengthen water resource management and optimal allocation; and coordinate ecological environment and economic and social development. These results are consistent with the drying trend in Central Asia that started in 2004. This paper uses EOF and integrates the CMIP6 dataset for future dry and wet prediction, providing a reference for promoting climate modeling and future climate prediction methods.

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