عنوان مقاله [English]
Rainfall is one of the most important meteorological quantities, and a decrease in its mean natural value over a period of time causes drought. Therefore, understanding the annual and intra-decade changes is important to improve water resources planning and management. One of the main approaches for preventing climate crises is to analyze the causes and factors that cause such climate variations and be aware of and anticipate the occurrence of climate hazards. Climate patterns are defined as the simultaneous relationship between the oscillations of the climatic elements of a place with changes in pressure patterns and sea surface temperature at other geographical locations. The aim of this study is to investigate rainfall and drought identification of Urmia Lake basin and the effect of some teleconnections patterns (NAO, EA, SOI) on climatic conditions and climatic factors such as rainfall and SPEI drought using wavelet correlation. In this study, 8 synoptic stations of Urmia Lake basin are selected in a period of 29 years, then the trends of rainfall and drought are determined using non-parametric tests. Then the effects of teleconnection patterns (NAO, EA, SOI) on rainfall and drought in Urmia Lake basin are determined using wavelet correlation.
The results showed that in the studied stations, the highest rainfall is in spring, winter, autumn and summer, respectively. The annual drought study using the SPEI index in the studied stations showed that the most severe droughts occurred in 1990, 2017, 2008 and 2010 and the most severe droughts occurred in 1993, 1994 and 1995 at the basin level. The results of trend analysis showed that transpiration evaporation in all stations except Takab station has an upward and significant trend at the level of 95% and Mahabad and Tabriz stations have an increasing slope and significant compared to other stations. Also, the trend of rainfall showed that rainfall only in Tabriz, Urmia and Takab stations had an upward trend and in other stations there were downward trends that in none of the stations a significant upward and downward trend was observed. An examination of the SPEI annual drought trend showed that the drought in Urmia and Ahar stations has been on an upward trend and in other stations it has been declining. The correlation between rainfall and index (EA) in the studied stations showed that there was a negative correlation in all the studied stations on an annual scale. In the negative phases of EA, the monthly rainfall showed positive values and the EA index has changed the rainfall. Also, the results of the correlation between rainfall and NAO in the studied stations showed that there was a positive correlation in all stations. Also, for Urmia, Tabriz, Takab, Saqez, Sahand and Mahabad stations correlations were significant at 99%, and Ahar station is significant at 95%. The results showed that the NAO time series has affected the changes in the rainfall time series. The results of the correlation between rainfall and SOI showed that there was a negative correlation between the indicators, which was significant at Mahabad station at 95%. These indicate that SOI has affected the changes of rainfalls. The correlation results between SPEI drought index and teleconnection patterns showed that NAO index had positive and significant correlation and EA and SOI index had negative correlation with monthly drought values.
Due to the fact that the Urmia Lake basin is semi-arid in terms of climate, determining the relationship between rainfall and drought with the patterns of distant links plays an important role in planning water resources, especially droughts for preventing damage. In this study, the results of rain wavelet correlation with NAO, SOI, EA bonding patterns of stations studied on an annual scale indicate that NAO index has a positive and significant correlation in which rainfall decreases in its negative phase and with decreasing values of NAO index. Also in the negative phase of NAO, the high-pressure centers of Siberia and Azores and the low pressure of Iceland are weakened, which causes less pressure of the Siberian high-pressure extention to wards the basin, and consequently the amount of precipitation decreases significantly. However, the two indicators of SOI and EA have negative correlations, which have increased the negative phase of rainfall, and the changes of the two indicators are effective on the changes of rainfall. The results showed that the impact of NAO on rainfall and drought on the surface of the basin was higher than SOI and EA. Due to the fact that rainfall and drought in the Urmia Lake basin have been correlated with some of these teleconnections identifying the factors affecting rainfall and drought and predicting their total amount or trend has a significant role in planning and developing water resources.