Key Takeaways & Executive Findings
- •• • Annual global solar radiation increased at +7.03 MJ/m² per decade over 1961–2023, with a mean of 5946.57 MJ/m² and a maximum of 7333.62 MJ/m² in 2017; this sustained upward trend underpins long-term bankability for utility-scale solar deployment in the region. • • Mann-Kendall change-point analysis reveals a significant decline of −364.01 MJ/m² per decade during 1961–1986 (α = 0.01), reversing to a significant increase of +147.55 MJ/m² per decade during 1987–2023 (α = 0.05); this regime shift invalidates stationarity assumptions in historical resource assessments and requires time-dependent yield modeling. • • Seasonal radiation partitioning is summer 33%, spring 29%, autumn 23%, and winter 15%, with spring exhibiting the strongest upward trend at +23.66 MJ/m² per decade (α = 0.05) while autumn and winter decline at −10.33 and −9.37 MJ/m² per decade, respectively (not significant); this asymmetry affects seasonal storage sizing and maintenance scheduling. • • Precipitation, overcast, and cloudy conditions cause the most severe attenuation of global solar radiation, whereas clear and partly cloudy skies dominate the annual energy budget; accurate cloud-cover classification is therefore critical for intra-day power forecasting and grid integration.
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Abstract
This study quantitatively analyzes the temporal variability of global solar radiation and its attenuation under different sky conditions in Hotan City, an oasis on the southern margin of the Taklimakan Desert, using monthly and hourly global radiation, temperature, cloud cover, precipitation, and weather phenomenon records from the Hotan National Reference Climatological Station for 1961–2023. The results show that Hotan possesses abundant and stable solar energy resources. Annual global solar radiation exhibits a fluctuating upward trend with a climatic tendency rate of +7.03 MJ/m² per decade, driven primarily by sustained warming and a reduction in dust weather. The annual mean global radiation is 5946.57 MJ/m², with an annual amplitude of 2473.28 MJ/m² and a maximum of 7333.62 MJ/m² in 2017. Mann-Kendall tests identify a significant decline from 1961 to 1986 at −364.01 MJ/m² per decade (α = 0.01), followed by a significant increase from 1987 to 2023 at +147.55 MJ/m² per decade (α = 0.05). Seasonal contributions follow summer (33%) > spring (29%) > autumn (23%) > winter (15%). Monthly radiation is unimodal, peaking in June at 697.21 MJ/m² and reaching a minimum in December. Diurnal radiation is low in the morning and evening and high at midday, with the maximum generally occurring at 13:00 local time. Precipitation, overcast, and cloudy days produce the most pronounced attenuation of global solar radiation. These findings confirm that Hotan’s solar resource is highly abundant and stable, supporting large-scale, sustained development.
1. Introduction
Solar radiation is the primary energy input governing oasis stability and agricultural productivity on the southern margin of the Taklimakan Desert, where precipitation is scarce, evaporation is extreme, and dust activity is frequent. Despite the region’s recognized solar resource richness, existing assessments remain inconsistent: some studies report declining annual radiation, others increasing trends, and seasonal signals vary by station and period. These discrepancies stem from short records, differing data sources, and the failure to resolve regime shifts in long-term series. Consequently, project developers and grid operators lack a reliable, station-grounded baseline for resource planning in this climatically sensitive zone.
This study addresses the bottleneck by applying Mann-Kendall change-point detection and climatic tendency analysis to a 63-year (1961–2023) quality-controlled observational record from the Hotan National Reference Climatological Station. By quantifying annual, seasonal, monthly, and diurnal radiation variability alongside sky-condition attenuation, the analysis isolates the timing and magnitude of trend reversals and identifies the weather types that most degrade solar yield. The resulting empirical parameters provide a defensible foundation for solar resource assessment, system sizing, and long-term performance forecasting in the southern Taklimakan oasis belt.
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LIU Hongxia, GAO Jiacheng, MAIMAITIAILI Maimaitiyiming, HUANG Ling, ZHANG Guanfeng, GONG Qing (2026). Long-Term Variability of Solar Radiation over an Oasis on the Southern Margin of the Taklimakan Desert, 1961–2023. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9686
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Frequently Asked Questions
What is the statistical significance and magnitude of the long-term radiation trend, and how does it affect project yield projections?
The annual global solar radiation increased at +7.03 MJ/m² per decade over 1961–2023. Mann-Kendall tests identify a significant decline of −364.01 MJ/m² per decade during 1961–1986 (α = 0.01) and a significant increase of +147.55 MJ/m² per decade during 1987–2023 (α = 0.05). This regime shift means that yield models calibrated on pre-1987 data will underestimate modern resource availability by a wide margin; project finance should use post-1987 trends or, preferably, a piecewise regression that accounts for the 1986 change point.
Which weather conditions cause the greatest attenuation of global solar radiation, and what are the operational implications for PV plant design?
Precipitation, overcast, and cloudy days produce the most pronounced attenuation. The study classifies sky conditions by total cloud cover: clear (<2 tenths), partly cloudy (2–5 tenths), cloudy (5–8 tenths), and overcast (≥8 tenths). Because overcast and cloudy conditions dominate the attenuation signal, PV plants in this region must be designed with enhanced inverter headroom and storage capacity to manage frequent, deep irradiance drops. Intra-day forecasting algorithms should prioritize cloud-cover thresholds rather than precipitation alone, as overcast skies without rainfall still cause severe losses.
How does the seasonal distribution of radiation affect storage sizing and maintenance windows?
Seasonal contributions are summer 33%, spring 29%, autumn 23%, and winter 15%. Spring shows the strongest upward trend at +23.66 MJ/m² per decade (α = 0.05), while autumn and winter decline at −10.33 and −9.37 MJ/m² per decade (not significant). The summer peak and winter trough create a 2.2:1 seasonal ratio, requiring storage or grid export capacity sized for summer surplus and winter deficit. Maintenance should be scheduled in winter when radiation is lowest and the opportunity cost of downtime is minimized.
What is the diurnal peak time and monthly maximum, and how do these inform tracking system design?
Diurnal radiation is low in the morning and evening and high at midday, with the maximum generally occurring at 13:00 local time. Monthly radiation is unimodal, peaking in June at 697.21 MJ/m² and reaching a minimum in December. The 13:00 peak indicates that single-axis tracking systems should be oriented to maximize collection during the early afternoon; fixed-tilt systems should be tilted to favor the southern sky with a slight westward bias to capture the afternoon maximum. The June peak aligns with the summer solstice, confirming that tracking gains are highest in late spring and early summer.
What are the primary drivers of the observed radiation increase, and how reliable is the 63-year record?
The upward trend is attributed primarily to sustained temperature increases and a reduction in dust weather. The dataset comprises monthly observations from the Hotan National Reference Climatological Station for 1961–2023, with hourly data for 2019–2023, all sourced from the Xinjiang Meteorological Information Center and subjected to strict quality control. The Mann-Kendall change-point detection provides a robust non-parametric assessment of trend reversals. The record length and quality control support high confidence in the identified regime shift, though local topography and episodic dust events introduce residual uncertainty that should be quantified in site-specific feasibility studies.
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