جستجوی مقالات مرتبط با کلیدواژه "آزمون روند من کندال" در نشریات گروه "آب و خاک"

In this study, the decade variability of frequency and severity of drought in Iran has been investigated. The one-month scale data from the standardized precipitation-evapotranspiration index (SPEI 01) in the period 1956 - 2015 have been used. Based on the common numerical thresholds, the characteristics of the frequency and severity of drought for each pixel have been calculated and they are the basis for the analysis of the drought situation. Then, the frequency of drought severity classes was calculated and its trend was investigated using the non-parametric Mann-Kendall test. The findings indicated the spatio-temporal variability of drought frequency and intensity patterns in Iran. The frequency of mild droughts has decreased from south to north and from east to west; while the frequency of more severe droughts has increased from north to south and from west to east. The frequency of mild droughts in the southeast, northwest, and northeast has increased by 5 to 40 percent. While the frequency of more severe droughts in most parts of Iran has increased between 10 and 20 percent. Variability in the frequency of more severe droughts is more pronounced in the Central Plateau catchment area as well as in the Persian Gulf-Oman Sea. The trend of drought intensity is decreasing (drought intensification) at the same time as the prevailing rainfall regime in Iran. A significant increase in drought intensity (wet season intensification) is observed only in southeastern Iran at the same time as the monsoon regime. However, extra-arid and arid regions of southeastern Iran are affected by the frequency and severity of drought and have a high degree of vulnerability.

Aired areas are often affected by rapid soil erosion, land degradation, and desertification. Therefore, continuous monitoring of land cover changes is required. Remote sensing data with spatial and temporal information are suitable for this purpose. In the present study, the time series of TGSI and Albedo remotely sensed indexes were used to monitor desertification in the center of Khuzestan province. After constructing the above-mentioned indexes for the period of 1999-2019 using ETM+ sensor images, the values of 411 randomly selected samples on the images were used to construct the Albedo-TGSI feature space model for each year and the correlation between the variables was calculated 0/48-0/83 in different years. The DDI (Desertification Degree Index) was then obtained based on the slope of the fitted line, and the value of DDI was calculated for each sample in each year. By applying a natural break classification on DDI, different levels of desertification and the break values were obtained and considered as the representative of the class in each year. Therefore, five time series of five desertification degrees were formed. Finally, a Man Kendall trend test was carried out with %95 and %90 confidence levels. The results showed that none of the series, except for the high desertification degree, showed a significant trend at the level of 5%. However, severe and high desertification degrees time series with p-value, 0.090 and 0.050 values showed a significant trend at the level of 10%, respectively. Also, the spatial distribution map of the average changes in the trend of desertification index in different classes, showed that in total, about 81% of the region was in severe and high desertification classes with a significant increasing trend of desertification.The results showed a high desertification degree at %5 significant level, and a sever desertification degree at %10 significant levels, showing increasing desertification trend. Furthermore, the spatial distribution of average DDI index indicated that about %81 of the study area was in severe and high desertification classes with a significant increasing trend.