Document Type : Research Article
Authors
1
Postdoctoral Researcher in Climatology, Faculty of Social Sciences, University of Mohaghegh Ardabili, Ardabil, Iran
2
Professor in Climatology, Department of Physical Geography, Faculty of Social Sciences, University of Mohaghegh Ardabili, Ardabil, Iran
Abstract
Climate change has led to alterations in the temporal and spatial distribution of precipitation across the globe. The present study aims to identify trends and project future precipitation in selected cities of northern Iran. To achieve this goal, the 30-year precipitation trend (1994–2023) for each city was first examined using the Mann-Kendall test and Sen’s slope estimator. In the second phase, precipitation was projected for the next 20 years (2040–2060) in the study area using the latest version of the LARS-WG8 weather generator under two scenarios—SSP2-4.5 and SSP5-8.5—with the ACCESS-ESM1-5 and CNRM-CM6-1 models. The outputs of both models were then evaluated and compared using validation indices (R², MAPE, and RMSE) to select the most suitable model. Trend analysis results indicated that over the past 30 years (1994–2023), the precipitation pattern in all stations in the region changed significantly, showing a decreasing trend at the 99% confidence level. This decrease was greater in the cities of Qazvin, Ardabil, and Zanjan, and less in Rasht and Gorgan. According to the projections from the two models, ACCESS-ESM1-5 and CNRM-CM6, over the next 20 years (2040–2060), the entire study area will face a decrease in precipitation in most months of the year. The decrease in precipitation in the northwest will be more than 60% in some months. Conversely, some cities will experience an increase in precipitation during the warm seasons, which will be more evident in low-precipitation cities such as Semnan. Finally, the comparison of the total annual precipitation of the observed and forecast periods confirmed a significant decrease in precipitation across the entire study area compared to the base period. The research findings indicate temporal and spatial anomalies in precipitation, confirm the effects of climate change on the region, and emphasize the need to adopt efficient management policies to optimize water resource consumption and develop strategies for adapting to climate change.
Introduction
Climate change has altered the temporal and spatial distribution of precipitation and its intensity throughout the world. Precipitation description, analysis, and modeling are among the most challenging issues in climate research, reflecting its complex and irregular temporal and spatial structure. In this regard, several modeling methods have been developed to deal with this complexity (Sánchez et al., 2011). A reduction in the number of rainy days and an increase in precipitation intensity over limited days can increase the probability of flooding in areas such as the coastal cities of the Caspian Sea, as well as cause soil erosion and reduce water infiltration into groundwater resources.
Material and Methods
In the present study, the trend and forecast of precipitation in selected cities in northern Iran (Ardabil, Tabriz, Bojnourd, Mashhad, Gorgan, Semnan, Zanjan, Tehran, Urmia, Gorgan, Rasht and, Sanandaj) were investigated. To analyzed the total annual precipitation, data from the last 30 years (1994-2023) were obtained from the Iranian Meteorological Organization, and Sen's slope estimator, a non-parametric method for estimating the slope of the regression line in trend analysis, was applied.
Results and Discussion
According to the results of the trend analysis, during the statistical period (1994–2023), the Q statistic (Sen’s slope estimator) and Z (Mann-Kendall test) were negative for all stations, indicating that the total annual precipitation in all 12 cities showed a decreasing trend. The largest Q values were observed in Qazvin (-81.8), Ardabil (-63.8), and Zanjan (-45.8). The highest negative Z values occurred in Sanandaj (-4.62), Zanjan (-4.57), and Qazvin (-4.42), while the lowest was in Rasht (-2.98). Since the Z scores in all cities were less than -2.58, a significant downward trend in precipitation during the past 30 years is confirmed at the 99% confidence level.
In the second part of the study, after validating the performance of LARS-WG8 in simulating base-period precipitation (1980–2010), the ACCESS-ESM1-5 and CNRM-CM6-1 models were used to predict future precipitation under the SSP2-4.5 and SSP5-8.5 scenarios. According to the results, Urmia is projected to experience decreased precipitation in all months under both models. Tehran, Tabriz, Ardabil, Gorgan, and Rasht will also face significant reductions in most months, with only slight increases (less than 20%) under some scenarios. Zanjan, Sanandaj, Qazvin, Mashhad, Bojnourd, and Semnan are expected to experience up to a 50% increase in precipitation in at least one late-spring or summer month. Overall, projections from both models suggest that all 12 cities will experience a significant reduction in precipitation compared to the long-term average during most months of the next 20 years.
Finally, model accuracy was assessed using R², RMSE, and MAPE. Although both models showed relatively high error rates, each city exhibited higher accuracy under one model. Consequently, the CNRM model was selected for Sanandaj, Ardabil, Tehran, and Gorgan, while the ACCESS model was selected for the remaining cities.
Conclusion
The aim of the present study was to forecast and analyze precipitation trends in selected cities of northern Iran using GCM models, in order to reveal the extent of climate change impacts on different climatic regions. Based on Sen’s slope estimator and Mann-Kendall tests, precipitation in all cities during the past 30 years (1994–2023) showed a significant decreasing trend at the 99% confidence level. The decrease was most pronounced in mountainous cities in the northwest (Qazvin, Ardabil, and Zanjan) and least in humid coastal cities (Rasht and Gorgan).
Projections from the ACCESS and CNRM models suggest that precipitation patterns in the study area will undergo significant changes. Over the next 20 years (2041–2060), most cities will face decreasing precipitation, with reductions exceeding 60% in some months in northwestern regions. At the same time, some low-precipitation cities, such as Semnan, may experience increases during warm months. The largest changes—both increases and decreases—are projected for the warm season (late spring to late summer).
A comparison of total annual precipitation during the observation and forecast periods confirmed a significant decline in all 12 cities, with the greatest decrease expected in Rasht and the smallest in Semnan. These results highlight temporal and spatial anomalies in precipitation, confirm the strong influence of climate change on the region, and underscore the need for effective water resource management and adaptation strategies.
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