ارزیابی تغییرات زمانی و مکانی شاخص‌های حدی اقلیمی بارندگی در بخش مرکزی استان اردبیل

Abstract
Objective: Climate change and its consequences have become a threat to the planet, the natural and man-made environment, and it leads to an increase in greenhouse gas emissions, global warming. In recent decades, the amount of carbon dioxide in the atmosphere has increased, and this factor of climate change, such as temperature, has decreased. One of the effective factors in the management of agricultural and industrial projects is that changing the pattern of distribution and its amount, especially its decreasing trend, can play important role in determining the time of crop cultivation and water resources planning. Temperature as one of the important climatic factors, causes changes in the temporal and mechanical pattern of climatic phenomena and droughts, floods, storms and heat waves. Destructive climate change is due to a great increase or intensity of extreme weather phenomena and climatic events. In fact, extreme events rarely occur, but they have a direct impact on communities and vulnerable areas. Studies have proved that climate change is driven by simultaneous increases in evapotranspiration and changes, contributing to water cycle processes. Social and environmental impacts of extreme events are locally significant as a result and can severely affect specific sectors and locations. From a statistical point of view, climatic events, changes in the average limit and deviation of climatic indicators alters the occurrence probability of number of frost days, cold and hot days and nights, and the length of the growing season. Climate change intensify the hydrological cycle and change the amount of evaporation and transpiration and the pattern of precipitation. Climate change and global warming increase droughts and their continuation, and this change also causes uneven distribution and affects water resources. The purpose of this study was to evaluate the temporal and spatial variations of climatic rainfall indices in the central part of Ardabil province.
Method: In this study, 11 climatic extreme indices were selected and evaluated in a 40-year period (1973-2014) to evaluate changes. At first, the required data were prepared and after processing, the values of the indices were calculated using Excel software, then the temporal trend of the indices was analyzed by ProUCL software. Evaluating the existence of a trend in the time series data is one of the important aspects of time series. In this regard, non-parametric methods are usually used due to the lack of statistical assumptions. In this research, the Man-Kendal method was used to investigate the changing process of climatic extreme indices. The non-parametric Mann-Kendall test has the ability to adopt with non-normal time series that does not follow a specific distribution. Also, this test has a small influence of the outlier values in the time series. Regarding the spatial interpolations, there are several methods to estimate the spatial changes of variables. The difference between these methods is the way of calculating the weight that is given to the observed points around the unknown point. In this research, the inverse distance weighting method was used, which estimates the values of unknown points through the weighted average of observational data with identical points.
Results: In general, the changes in rainfall extreme indices have a decreasing trend in all stations, but it is significant in Hir station. Based on the results of interpolation maps, the indicators of maximum 1-day monthly rainfall, maximum 5-day monthly rainfall and total annual rainfall on rainy days in the central and western parts show the least and most values, respectively. Raw rainfall intensity indicators, total annual rainfall in 95% of rainy days in the northern, southern and western parts of Ardabil province show the most values and in the eastern part of the region the least values. The indicators of the number of days with rainfall of 10 mm or more, the number of days with rainfall of 20 mm or more and the total annual rainfall in 99 percent of rainy days showed the least and most changes in the east and north-west of the region, respectively. The indicators of wet period length and Rnnmm indices show the most values in the western and eastern parts and the least values in the central part of the province. The high values of number of rainy days index with more than 20 mm precipitation are related to Sablan slopes. It should be noted that the maximum length of the dry season is longer in the middle parts than in the areas of Musharraf and the north, as well as in the slopes of Sablan. The spatial distribution pattern of the maximum wet period index is also such that the maximum length of the wet period is in the stations located on the path of rainfall caused by the incoming moisture from the Caspian Sea and Gilan province, and this effect of the increase of the wet period is also clearly visible in the stations of Sablan slopes.
Conclusions: According to the findings of the study, the least changes in precipitation extreme indices occurred in central stations and the highest values in precipitation extreme indices occurred in stations located in western part of the study area. Overall, it can be said that assessing changes in extreme indices can be a guide in assessing climate change and its negative effects on soil and water conditions. It should be noted that the changes in extreme indicators in different regions will be different based on climatic conditions, but in most areas, the changes in extreme indicators are such that extreme events are intensified. One of the important effects of climate change is changing the rainfall regime and the pattern in the future. In many cases, changes in precipitation characteristics and extreme events may not follow the general trend of precipitation reduction. Spatial evaluations of changes in the occurrence and trend of extreme climatic indicators can help in identifying the signs of climate change. The dominant trend of increasing changes in rainfall extremes can have a significant contribution in intensifying the flood occurrence. The extreme rainfall events have undergone many changes in recent years under the influence of many factors. Considering the occurrence of severe floods under the effect of extreme rainfall events, examining the changes in the trend of these events in the past years and predicting them in the future will be effective in planning and managing water resources and natural disasters caused by these events. Forecasting changes in climatic extreme events is necessary to take effective considerations to deal with the harmful effects of climate change. In general, it can be said that the analysis of indicators related to extreme events can complement the results of evaluating the annual average values of climatic variables.