واکاوی خشکسالی‌ ایستگاه های ایران در دوره سرد سال

Since the scientific study ofdrought provides a basis for reducing the effects of this climatic phenomenon,the study of drought in Iran,especially on the basis of multivariate indicators that use other climatic parameters to estimate drought in addition to rainfall is very important.In addition, this study is based on network data with high spatial and temporal resolution such as GPCC,CRU, TRMM.Due to climate change in recent decades and increasing water demand in different parts of the country and comparing it with the results of station data is doubly important and necessary.This research,in terms of purpose,is an applied research.In terms of the nature of data,this research is a quantitative research that presents results by collecting data and analyzing them with quantitative methods.To achieve the purpose of this study,network data along with statistical methods have been used. At first,CRU networking data with spatial resolution of 0.5 × 0.5 ° was obtained from NOVA site and was extracted for Iran using the capabilities of GIS and MATLAB software during the study period.Thus, first,the received CRU data was transferred to MATLAB software and the relevant area in Iran was separated from the rest of the world.Then, using the programming index in MATLAB environment,SPEI index as a new index that besides to precipitation considers the effects of temperature and evapotranspiration in estimating drought.Drought and wet periods are calculated in season time scales and drought characteristics were extracted in different parts of the country.Finally,the results of drought calculation using GIS and office software environments were shown as maps,graphs and tables.

Based on this, the whole country was divided into 7 clusters. In order to identify the spatial distribution of different drought classes in the country,the variables of each cluster were zoned in GIS software.For monthly drought assessment,the selected drought index is fitted to 50-year rainfall cultivars of 621 points of the CRU database nationwide,and considering the value (-0.5 and + 0.5) as the normal situation,the monthly conditions are checked from January to March.The drought situation of each of the cold months of the year is described below.

Drought is one of the natural disasters that compared to other natural disasters in terms of magnitude, severity, duration of the event, regional expansion, casualties,economic losses and long-term effects.It is one of the most important climatic phenomena that strongly affects all aspects of human activities.Studying the characteristics of drought and predicting it can be effective in reducing the damage caused by it.But as we know,one of the most important phenomena of climate and climatology is drought,which causes their intensity, continuity and expansion on human activities, transportation, energy, environmental issues and the activities of living things.Considering that the main source of fresh water supply for agriculture is domestic and industrial consumption and can lead from mild effects of personal life to major disasters at the national level.The general trend of rainfall in winter is decreasing.As in the 90s, most of the rainfall anomalies of this season in the country are positive; However,with the onset of the 2000s,the incidence of dry periods has increased, and this situation has continued almost to the end of the period under study.

On a monthly scale,the most stable rainfall regions of the country are the first (northwest) and third (north and northeast) clusters; Because these clusters have the highest frequency of normal periods in most months. Instead,the fifth (south) and seventh (southeast) clusters have the highest fluctuations and the lowest frequency of the normal period. For most months of the year,the frequency of dry periods is higher than wet periods, and it is only in December that in four clusters, the frequency of wet periods is higher than dry periods. In November, in three clusters (second, third and sixth), the frequency of wetlands is higher. The fifth and seventh clusters have the highest frequency of monthly droughts and the second and sixth clusters have the highest frequency of monthly wetlands. The duration of dry periods is more in most months of the year than wet periods. The most severe duration of the drought occurred in April and May, in the seventh and fifth clusters, respectively. In these two months, the continuation of drought in the two mentioned clusters has lasted more than 10 years, while the continuation of wet periods in most years and clusters of the country is 2 periods and the maximum duration of wet period in most months of the year is 4 periods. In the central regions (fourth cluster) and western (second cluster) the most continuous wet periods are observed. However, sometimes in the fifth dry cluster we see wet periods of 4 years. In terms of severity, a significant percentage of droughts and wetlands in the country in all clusters are mild and moderate, and severe events of low frequency and very severe events are usually accidental. In general, it can be said that the intensity of drought in the country has been more than wet season, because most wet season are mild, but in some droughts, especially in the fifth to seventh clusters, the frequency of the middle class is noticeable and even in some months is higher than mild droughts. In addition, intensity of droughts is more common than intensity of wet periods. The frequency of intensity of droughts in January, February, April and especially in December is much higher than intensity of wet periods. Spatially, intensity events of drought and wet season mainly belong to the arid regions of the country, especially the two clusters 5 and 7. In terms of time trend, the rainfall is decreasing in most months of the year and the frequency of droughts in recent years is more than wetlands. In October and November, of course, the precipitation trend has a slight upward slope and wet periods have been observed more in recent years. Therefore,the occurrence of drought has significant effects on economic,social and agricultural issues.And this requires studies to be done in this area and with proper management and careful planning to avoid potential risks and losses.