Document Type : Research Paper
Precipitation is one of the most fundamental climatic elements, playing a crucial role in the hydrological cycle, the supply of surface and groundwater resources, and the sustainability of ecosystems. The spatial and temporal distribution of precipitation is highly heterogeneous across the Earth's surface, and this heterogeneity arises from the interaction of various factors including latitude, distance from moisture sources, atmospheric circulation patterns, and particularly elevation above sea level. In mountainous regions, precipitation variations with elevation are of particular importance, because topography significantly controls precipitation patterns through the forced ascent of air. The Zagros mountain range, as one of the principal mountain systems in Iran, plays a decisive role in the precipitation patterns of the western and southwestern regions of the country. The wide elevational range of the Zagros substantially modifies the climatic conditions of the region. Nevertheless, no comprehensive study has yet been conducted to investigate the distinct effects of different elevation classes on seasonal precipitation patterns using remote sensing data. The present study aimed to analyze the spatial distribution patterns of precipitation across different elevation classes in the Zagros region and to investigate the role of topographic factors in seasonal variability.
In this study, MSWEP satellite precipitation data with a spatial resolution of 0.1 degree (approximately 11 km) and a temporal period from 1990 to 2024 were employed. This product is generated by combining satellite data, reanalysis, and ground-based stations, and recent international studies have confirmed its suitability for mountainous regions. Additionally, the ALOS PALSAR digital elevation model with a resolution of 12.5 metres was used to extract topographical characteristics. The study area was classified into five elevation classes based on altitude above sea level (below 1000, 1000–1500, 1500–2000, 2000–2500, and above 2500 metres). Non-parametric Mann-Kendall tests and Sen's slope estimator were applied to analyse precipitation trends. Furthermore, due to the non-normal distribution of precipitation data, Spearman correlation was employed, and one-way analysis of variance (ANOVA) along with Scheffé's post-hoc test was used to examine precipitation differences among elevation classes. To avoid the pseudo-replication problem caused by spatial autocorrelation, a systematic grid sampling (0.1-degree resolution) was applied, reducing the sample size from 250,000 to approximately 2,850 independent pixels.
The results revealed a significant relationship between elevation and precipitation in the Zagros region; however, this relationship is neither linear nor direct. The highest annual precipitation occurred in the 1000–1500 metre elevation class, with a mean of 353.7 mm, while the lowest precipitation was recorded in the above 2500 metre class (256.6 mm). This pattern contradicts the common expectation that precipitation increases with increasing elevation. Autumn (ρ = −0.41) and winter (ρ = −0.34) precipitation exhibited significant inverse correlations with elevation, indicating the role of the rain shadow phenomenon in the Zagros. A different pattern was observed in spring, with the highest precipitation recorded in the 1500–2000 metre elevation class (114.1 mm), while the lowest spring precipitation was related to the below 1000 metre class (79.4 mm). ANOVA results indicated that the differences in mean precipitation among elevation classes were statistically significant across all seasons at the 95% confidence level (P < 0.001). The highest F-statistic value was observed in autumn (182) and the lowest in spring (47). Furthermore, temporal trend analysis using the Mann-Kendall test demonstrated that January (Z = −2.05) and December (Z = −1.99) precipitation exhibited significant decreasing trends. In contrast, other months, despite positive or negative fluctuations, lacked statistically significant trends. The greatest spatial variability of precipitation (standard deviation of 118.3 mm and coefficient of variation of 37.9%) was observed in the middle elevation classes (1500–2000 metres), a finding attributed to the complexity of topography and the diversity of slope aspects in these classes. In contrast, elevations above 2500 metres, despite having lower mean precipitation, exhibited greater statistical stability and uniformity (lowest standard deviation and coefficient of variation). It should be noted that summer precipitation (July–September) is negligible (near zero) due to the dominance of subtropical high pressure.
Elevation alone is not the sole determinant of precipitation in the Zagros region; rather, precipitation patterns represent a complex function of topographical factors including elevation, slope aspect, and position relative to moisture sources. The middle elevations (1000–1500 metres), which are located on the windward western and southwestern slopes, constitute the wettest elevation class during autumn and winter. The rain shadow phenomenon is active in the Zagros, with eastern leeward slopes receiving considerably less precipitation. In spring, with the activation of Sudanese and Red Sea systems and the increase in convective instabilities, the 1500–2000 metre elevations become the wettest class. The significant decreasing trends in January and December precipitation may serve as indicators of regional climate change and are consistent with the findings of other studies regarding reduced rainfall and increased dry periods in the Zagros. Based on the findings, it is recommended that middle elevations be prioritised for water storage in water resource management planning, and in agricultural planning, the high variability of spring rainfall in middle elevations should be taken into consideration. Furthermore, continuous monitoring of January and December precipitation trends is recommended as an indicator of climate change.
This research was supported by the Research Deputy of Lorestan University. The study is extracted from the PHD thesis of the first author. The funding body had no role in the design of the study, data collection,analysis, or interpretation.
Author1(phd student): Data curation, Formal analysis, software, Writing-original draft
Author2(Supervisor): Conceptualization, Methodology, supervision,Writing-review&editing
Author3(Advisor) Conceptualization, methodology, writing-review&editing
Author4(Advisor): Conceptualization, Methodology, Validation, Writing-review&editing
All authors havad and agreed to the published version of the manuscript.
During the preparation of this work, the authors did not use any generative Al or Al-assisted technologies. The authors take full responsibility for the content of the publication
The data that support the findings of this study are availabie from the corresponding author upon reasonable request.
The authors would lik to express their sincere gratitude to the Research Deputy of Loretan University for spiritual and administrative support. We also wish to htank Dr.Dariush Yarahmadi for his valuable guidance and structural comments, and Dr Sayyad Asghari and Dr. Hamid Mirhashemi for their scientific advice and review of the manuscript
The authors avoided data fabrication, falsification, and plagiarism, and any form of misconduct.
The authors confirm that they have avoided data fabrication, falsification, and plagiarism, and have adhered to all ethical principles in conducting this research. All authors take full responsibility for the integrity of data and the accuracy of the analysis.
The authors declare no conflict of interest.
