Preamble

ARID LAND GEOGRAPHY
Vol. 48 No. 8 Aug. 2025

Spatial Synergistic Evolution of Oasis and Oasis Cities in Arid Zones During Urbanization and Pattern Analysis

HAN Yuchen¹, SUN Qinke¹,²,³, ZHOU Liang¹,²,³, LI Yu'ang¹

¹Faculty of Geomatics, Lanzhou Jiaotong University, Lanzhou 730070, Gansu, China
²National Local Joint Engineering Research Center of Technologies and Applications for National Geographic State Monitoring, Lanzhou 730070, Gansu, China
³Key Laboratory of Science and Technology in Surveying & Mapping, Gansu Province (Lanzhou Jiaotong University), Lanzhou 730070, Gansu, China

Abstract: Oasis cities serve as core spatial units within oasis systems, concentrating population, economic activities, and public service functions. Their expansion is constrained by ecological patterns while simultaneously driving spatial evolution of oases. Systematically identifying the evolution characteristics of oases and oasis cities is fundamental to coordinating ecological protection and spatial development in arid zones. This study first delineates the spatial distribution of oases and oasis cities in northwest China's arid region, analyzing their spatial evolution trends. It then constructs an Oasis City Synergy Index (OCSI) to quantitatively evaluate synergistic evolution characteristics and further explores urban expansion patterns. Results show: (1) From 2000 to 2023, both oasis and city areas exhibited continuous growth, though oasis patch numbers decreased, revealing a trend toward "large-scale and concentrated" oases. (2) Synergy between oases and cities gradually strengthened, with 8 regions showing similar trends and 23 showing opposite trends. (3) Based on comprehensive evaluation, cities with infill and edge expansion patterns generally demonstrated high synergy, while those with leapfrog or river-dependent expansion showed lower synergy due to dispersed growth or linear spread along water bodies. These findings identify and reveal the spatial evolution relationship between oases and oasis cities in northwest China's arid region, providing theoretical foundations and decision-making support for alleviating human-land conflicts and promoting sustainable development.

Keywords: oasis; oasis city; synergy index; expansion model; northwest China arid zone

Introduction

Drylands cover approximately 41% of global land area and represent sensitive zones for climate change, characterized by scarce precipitation and ecological fragility. Oases—occupying merely 5% of dryland area yet serving as the sole stable water-driven ecosystems with highest productivity—support over 95% of dryland populations and provide critical habitat for flora and fauna, agricultural production, and industrial development. However, global climate change, intensifying aridity, and human activities pose severe challenges to oasis ecosystems and their dependent oasis cities. Climate warming has significantly increased drought frequency and intensity, particularly in arid and semi-arid regions where oases face water shortages, land degradation, poverty, social vulnerability, biodiversity loss, and human-land conflicts. Meanwhile, population growth and economic expansion further intensify oasis resource exploitation, unprecedentedly testing oasis sustainability. Sustainable Development Goal 15 emphasizes protecting, restoring, and promoting sustainable use of terrestrial ecosystems, particularly forests, deserts, and arid regions—directly relevant to oasis conservation. Thus, oasis research is theoretically and practically significant for dryland sustainability.

Oases are not single ecosystem units but typical "human-natural-economic" composite systems. They exist within arid regions, depend on limited water resources, feature concentrated vegetation and land distribution, and maintain relatively high biodiversity. Oasis ecosystems are highly sensitive and vulnerable to environmental changes, with distinct boundaries from surrounding deserts. As the most concentrated human activity areas in drylands, oases require combined natural and artificial interventions, with human activities playing crucial roles. Previous research has focused on oasis spatio-temporal changes, driving mechanisms, climate change relationships, and environmental assessments, revealing oasisification processes and providing scientific bases for evaluating dryland environmental trends. However, systematic studies on complex interactions between urban economic activities and oasis ecosystems remain lacking.

Promoting balanced oasis-city development is crucial for dryland sustainability. Existing studies generally acknowledge mutual support between oases and urbanization, yet lack empirical support and show inconsistent understanding. For instance, studies on Xinjiang's oasis ecological security patterns or landscape ecological risk assessments have not deeply explored spatial synergy effects between oasis expansion and urban evolution. Some research treats oasis towns as complex adaptive systems but fails to examine spatial relationships between oasis and urban patterns. Large-scale, long-term studies on oasis-city synergistic evolution remain scarce, particularly in northwest China, where quantitative identification and classification of coordinated evolution processes are urgently needed to support precise management and regulation of regional ecological-social systems.

This study selects northwest China's arid region as the research area, investigating 31 cities. Based on evapotranspiration and normalized difference vegetation index data from 2000 to 2023, we analyze synergistic evolution trends and propose the Oasis City Synergy Index (OCSI) to quantify their relationship. By identifying typical urban expansion patterns, this research aims to provide theoretical foundations and decision-making support for alleviating human-land conflicts and promoting sustainable development in arid regions.

1.1 Study Area

The study area spans 74°–115°E and 36°–49°N, covering Xinjiang, Gansu, Qinghai, and Inner Mongolia provinces. Oases are concentrated in the Hetao Plain, Hexi Corridor, Qaidam Basin, northern Tianshan foothills, and northern Kunlun foothills, covering approximately 1.77×10⁶ km². The region exhibits typical arid and semi-arid climate characteristics, with average annual precipitation generally below 200 mm and evaporation exceeding 2000 mm. Oasis formation and maintenance primarily depend on meltwater from the Qilian, Altai, Tianshan, and Kunlun mountains and Yellow River water. Irrigation systems in the Hexi Corridor and Hetao Plain ensure agricultural stability and support regional economic development. As China's important grain, cotton, and fruit production base, the region also carries significant ecological functions through complex irrigation networks supporting sustainable oasis agriculture. Given that cities are primary carriers of urbanization and oasis development, this study selected 31 cities (including prefecture-level cities, county-level cities, and central cities of some prefectures and autonomous regions) as urban study objects.

1.2 Data Sources

Research data include: (1) Land use data from the Annual China Land Cover Dataset (CLCD) using spatio-temporal feature construction and random forest classification algorithms at 30 m spatial resolution. (2) Normalized Difference Vegetation Index (NDVI) data from the National Earth System Science Data Center, calculating annual maximum values via maximum value composition at 1 km spatial resolution and annual temporal resolution. (3) Evapotranspiration data from the National Earth System Science Data Center Loess Plateau Science Data Center, interpolated to 1 km spatial resolution. (4) Fundamental geographic information data from the National Geomatics Center of China. (5) Statistical yearbook data from the National Bureau of Statistics. (6) OpenStreetMap (OSM) database.

1.3.1 Oasis Identification and Accuracy Assessment

This study delineates oasis spatial extent using the Aridity Index (AI) threshold method, selecting areas with AI between 0.05–0.65 to limit potential oasis distribution. Regional and annual optimal NDVI thresholds were determined using the double-peak segmentation method to optimize extraction stability. Combined with real geographic data and terrain interpretation, oasis patches were extracted for 2000–2023. According to UN aridity index standards, this threshold selection is reasonable. Accuracy verification using confusion matrix methods (Table 1) against the oasis spatio-temporal distribution dataset from the National Cryosphere Desert Data Center shows producer and user accuracies above 85% and 90% respectively, with overall classification accuracy exceeding 92% and Kappa coefficients above 0.84, demonstrating high validity and robustness.

1.3.2 Oasis-City Synergistic Evolution Trend Classification System

This study employs Oasis Expansion Rate (OER) to measure interannual oasis dynamics, representing expansion degree in given years:

$$OER_{n+1} = \frac{O_{n+1} - O_n}{O_n} \times 100\%$$

where $O_n$ and $O_{n+1}$ are oasis areas in years $n$ and $n+1$; $O_{n+1} \cap O_n$ represents overlapping area between consecutive years; $O_{n+1} - O_{n+1} \cap O_n$ is newly expanded oasis area.

Urban Expansion Rate (UER) measures urban dynamics similarly:

$$UER_{n+1} = \frac{U_{n+1} - U_n}{U_n} \times 100\%$$

where $U_n$ and $U_{n+1}$ are urban areas in years $n$ and $n+1$.

A synergistic evolution trend classification system was constructed based on 5-stage expansion rate sequences. Trend-similar types show synchronized growth/contraction across multiple periods, subdivided into "increase-increase" (Type A) and "decrease-decrease" (Type B). Trend-opposite types show directional divergence in at least one stage, classified as one-period opposite (Type C) and two-period opposite (Type D).

1.3.3 Oasis City Synergy Index

Based on the "Oasis Settlement Breeding Index" concept, this study proposes the "Oasis City Index" (OCI) to analyze synergy between city and oasis scales:

$$OCI = \frac{OCA}{OCSI}$$

where $OCA$ is total city area within oasis, and $OCSI$ is total oasis area within administrative boundaries. To characterize synergy strength and stability, a "dual-dimension synergy evaluation system" was constructed. The mean OCSI ($\mu$) measures overall synergy strength—lower values indicate stronger synergy, classified as: high synergy (OCSI ≤ 1.10), medium synergy (1.10 < OCSI ≤ 1.35), and low synergy (OCSI > 1.35). The range ($\Delta OCSI$) measures stability—lower values indicate higher stability, classified as: high stability ($\Delta OCSI \leq 0.60$), medium stability (0.60 < $\Delta OCSI \leq 1.00$), and low stability ($\Delta OCSI > 1.00$). Comprehensive synergy levels are determined by combining both dimensions.

1.3.4 Oasis City Expansion Pattern Classification

Through visual interpretation and remote sensing analysis, four typical expansion patterns were identified: (1) River-dependent expansion: New urban patches locate on new oasis patches near rivers, showing linear spread. (2) Infill expansion: New urban patches develop within existing oasis areas, filling unurbanized oasis spaces. (3) Edge expansion: New urban patches concentrate at oasis edges, gradually expanding outward. (4) Leapfrog expansion: New urban patches appear on new oasis patches far from original oasis patches, showing discontinuous distribution.

2.1 Spatio-Temporal Evolution Characteristics of Oases and Cities

From 2000 to 2023, significant spatial changes occurred in northwest China's arid zone oases and cities. Oasis area increased from 2.44×10⁵ km² to 3.34×10⁵ km² (36.89% growth), while patch numbers decreased from 135 to 101, indicating "large-scale and concentrated" development. Urban area expanded dramatically from 1.20×10³ km² to 5.54×10³ km² (361.60% growth), far outpacing oasis expansion. Xinjiang showed the largest oasis area and most significant growth (1.94×10⁵ km² to 2.67×10⁵ km², 37.63% increase), followed by Gansu. Inner Mongolia had the smallest oasis area. Oases were classified by size: micro (<50 km²), small (50–100 km²), medium (100–300 km²), large (300–500 km²), and giant (>500 km²). The reduction in oasis numbers primarily resulted from decreases in micro, small, and medium oases (42.86%, 52.38%, and 41.86% respectively), while large and giant oases remained stable.

2.2.1 Classification Results of Oasis-City Synergistic Evolution Trends

Based on 5-stage expansion rate combinations, 74.19% of cities (23) showed trend-opposite patterns, while 25.81% (8) showed trend-similar patterns. Trend-similar cities include Hotan, Kashgar, Shuanghe, and Alxa League. Trend-opposite cities include Kizilsu Kirghiz Autonomous Prefecture, Zhangye, Jiuquan, Korla, Hami, and Wujiaqu. Most cities showed improved synergy over time, with Kunyu exhibiting the highest synergy. However, Haixi Prefecture and Jiayuguan showed declining synergy due to urban expansion far exceeding oasis growth.

2.2.2 Trend Analysis of Oasis City Synergy Index

OCSI analysis of 31 cities from 2000–2023 shows an overall declining trend, indicating increasingly coordinated expansion. After removing Haixi's extreme values (where urban expansion vastly outpaced oasis growth), the mean OCSI dropped to 1.25, showing achieved synergy. Most cities demonstrated enhanced coordination, though Wujiaqu showed the lowest synergy. Kunyu and Hotan maintained high synergy levels.

2.3 Analysis of Oasis-City Expansion Patterns and Synergy Relationships

Northwest China's oasis-city expansion patterns show significant regional variation. Infill expansion cities (Zhangye, Jiuquan, Turpan) develop within oasis boundaries, reducing landscape fragmentation and showing high synergy with low OCSI fluctuations. Edge expansion cities (Urumqi, Karamay) expand toward oasis peripheries, maintaining high spatial overlap and synergy. River-dependent expansion cities (Yili Prefecture, Jinchang, Aksu) show linear spread along water bodies but generally low synergy with high OCSI volatility. Leapfrog expansion cities (Alxa League) exhibit discontinuous, discrete urban patches with poor oasis connectivity and low synergy.

Discussion

Urban development relies on spatial foundations and resource carrying capacity provided by oasis ecosystems, while oasis patterns undergo spatial restructuring and boundary compression during urban expansion, forming a potential two-way feedback mechanism through land use evolution. Research demonstrates significant spatial and temporal inconsistencies in oasis-city synergistic relationships. River-dependent expansion shows strong spatial orientation with linear coupling along water bodies, but exhibits lower synergy than other patterns—possibly related to water source distribution, though mechanisms require deeper analysis.

Synergy evolution patterns are diverse and require differentiated management strategies. River-dependent expansion cities should strengthen riparian oasis protection and establish ecological buffer zones ensuring water priority for ecosystems. Infill expansion cities can promote urban renewal and improve land use efficiency while safeguarding ecology. Edge expansion cities need strengthened guidance for peripheral oasis land use to avoid interference and ensure sustainable buffer functions. Leapfrog expansion cities should limit discontinuous growth and enhance oasis spatial connectivity to improve resource utilization.

Conclusions

This study reveals that from 2000–2023, northwest China's arid zone oases and cities showed overall expansion with significant differences in speed and pattern. Oasis area grew substantially while patch numbers decreased, concentrating toward large-scale development through edge and leapfrog expansion. Urban expansion outpaced oasis growth significantly. Synergy trends showed stage-specific fluctuations with gradually strengthening coordination, though regional and structural differences were evident. Different expansion patterns corresponded to varying synergy levels—spatial layout directly influenced oasis-city synergy. Infill and edge expansion cities, relying on oasis space, reduced fragmentation and showed high synergy, while leapfrog and river-dependent expansion cities exhibited lower synergy.

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