جستجوی مقالات مرتبط با کلیدواژه "precipitation anomalies" در نشریات گروه "جغرافیا"

Defining and measuring drought is a difficult concept. Perhaps the most general definition of drought refers to "a specific period of abnormal rainfall in a specific area", in other words, a drought occurs when a significant water shortage is widespread in time and space. But there has long been disagreement over the details. Disputes about the concept of drought and the uncertainty of measuring its characteristics can be an obstacle to research and planning related to drought. This shows that the qualitative and quantitative analysis of drought is a challenging issue. Although several studies have been conducted in connection with the evaluation of drought indices around the world, the somewhat forgotten point in past studies is that drought indices sometimes do not provide the same results in estimating and quantifying the severity of drought. Using the estimates obtained from multiple indicators as input for other similar studies, evaluating and managing the vulnerability and risk caused by this hazard, which is of great interest, and using its results in planning and even allocating budget and insurance coverage, doubles the importance of the issue. Therefore, the present research, focusing on meteorological drought indicators as a basic drought monitoring indicator that requires limited variables (rainfall, and in some cases temperature) but is effective in other types of drought, tries to provide a general presentation (introduction, calculation details, required data) of the main meteorological drought indicators, to address the difference in estimation and quantification of the drought index.

To evaluate the severity of drought for the period of 1979-2021, precipitation and temperature were used as their main variables on a daily and monthly scale according to the input of the indicators. The studied indicators were carried out in the selected year 2016-2017 when different regions of Iran were affected by drought and wet. It should be noted that the beginning of the annual period is considered from September. After developing the database, the drought indicators studied in the research were used to calculate the severity of the drought on an annual scale. Meteorological drought indices were classified into two categories of drought indices. In the last step, the drought hazard index was calculated, which expresses the severity of the drought event and is used as the first step to calculate the vulnerability and drought hazard index.

The 12-month SPI drought intensity values are lower than the results of RAI, DI, and ZSI indices, which is a significant point that is considered one of the disadvantages of this index. The estimation results of the ZSI index are almost equal to the RAI index, and the results of the CZI index are almost the same as the 12-month SPI index. The results of the EDI index showed that although this index can show the severity to some extent, it underestimates the severity of the annual drought events. In addition, this index has not exactly corresponded to the actual conditions caused by the drought. But this index is used to identify the length of the period and the start and end of the drought. which is a great advantage for this index. The spatial distribution of the RDI index estimate shows that half of Iran’s area had normal conditions in the year under review. In terms of space pattern, it is similar to the CZI index. The estimation of the SPEI index, similar to SPI and EDI indices, has not estimated the end of extreme conditions (very severe drought and wets).Among the indicators, the coverage percentage of drought severity in the ZSI, CZI, and RAI indices are almost closer to each other. While the intensity of the EDI index is different from other indices. According to the amount of internal standard deviation, it seems that RAI, DI, and ZSI indices had a more appropriate estimation of drought conditions.In addition to the mentioned cases, the comparison between the intensities of the investigated indicators also showed that the differences in normal conditions, droughts, and wets weak are greater and as the intensity increases, this difference decreases. The results of the drought hazard index resulting from the drought indices have had significant differences from each other.

According to the obtained results, it can be said that although the results of the three indices RAI, DI, and ZSI are closer to each other, the drought indices used (RAI, DI, SPI, PNPI, ZSI, CZI, EDI, RDI, SPEI) in the selected year, the drought intensities have been estimated differently. The difference in drought intensities was more in normal conditions, wet, and weak droughts. The standard deviation of the ranges of negative intensities of the index (drought) is more than that of drought conditions. Calculating the severity of drought hazard (DHI) using the intensity results obtained from the indicators showed a significant and sometimes extreme difference in this estimate. Estimation of drought intensity is used in other important studies such as calculating vulnerability, loss, and risk caused by drought. These differences and as a result the difference in results can affect environmental planning and management and economic considerations such as budget allocation and insurance coverage. Therefore, it is very important to check the accuracy of drought intensities estimation using indicators. Although there have been studies aimed at achieving a suitable index, an index that is approved and accepted in the diverse geographical-climatic area of the Country has not been introduced, which shows the need for a more comprehensive study in this field.

Increasing greenhouse gases could increase the frequency of changes in precipitation in different regions of the world. So, identify the effects of global warming on precipitation in a country like Iran which is located in the dry area of the earth is of great importance in planning related to water resources are. According to the study, statistical analysis method used. for analysis, total precipitation data of 34 stations from 1984 to 2012 was received from the National Weather Service. Also data of methane Gase (ppb) as one of the most important factors of global warming, the site of NOAA received. Outset this elements by method The Pearson correlation coefficient determined, afterwards long-term changes in precipitation than was calculated. The results of this study showed that, Bharhdr rainfall in the north and northwest parts of the country grew 13 mm; and Precipitation stations southern half of the country have faced long-term reduction of 46 percent. Due to the increasing amount of greenhouse gases, analysis of the data suggests a reduction of 30 mm of precipitation in summer than long-term stations in southern Iran, but in contrast rainfall in western and northern stations, an increasing of 14 mm. The results showed that the autumn precipitation is most relevant to global warming, in this season, about 24 mm of rainfall western stations to long-term decline, but the Southwest and Southeast stations surged 17 mm have experienced. Finally, the results of the Winters precipitation showed which, the West and Northwest stations were reduced 18/6 mm.