Abstract:
Drought is one of the natural phenomena which occurs in all climates in different parts of the world. Iran is located in the dry belt of the world. The increase of desertification, drought reoccurrence, and destruction by human in this geographical region needs more studies on spatial and temporal trend of rainfall. In this study, trends of climatic drought during 1975-76/ 2004-05 in seasonal and yearly time scales were evaluated at 50 synoptic stations in Iran using a drought index, RDI (Reconnaissance Drought Index), and Mann-Kendall non-parametric test, at 90 and 95% confidence levels. Results showed that among studied stations, in which RDI was calculated for, 89% had an ascending trend, and results of Mann-Kendall non-parametric test on annual values of RDI showed that among 21 stations, 76% of them had a negative trend and 24% had a significant positive trend. Based on results of this study, there exists an increasing drought trend at all-time scales in Iran.
Machine summary:
Therefore, in this study, new drought index, RDI, is used because this index uses precipitation and potential evapotranspiration for calculation of different severities of drought, so it has more accurate scientific basis than other indices which use only rainfall as input data (Tsakiris and Vangelis, 2005).
So, this study tries to show the trends of climatic drought in Iran using RDI and other indices during 1975-76 / 2004- 05 in seasonal and yearly time scales.
In this study, considering various criteria in selection of stations with long-term data, less incomplete data, and suitable distribution, among meteorological data of 50 synoptic stations of Iran, monthly rainfall, monthly mean, minimum, and maximum temperature were used for calculation of potential evapotranspiration using Hargreaves-Samani method (Hargreaves and Samani, 1982) and RDI.
First, using RDI, various drought severities were calculated at annual and seasonal time steps, then by the use of Mann-Kendall non parametric test, trend analysis of time changes was performed.
4. Results and Discussion First, various drought severities were calculated for various stations of Iran at annual and seasonal time steps.
According to these figures, because Yazd station in the last quarter of water year (July-September) in most years, had no rainfall, RDI values in this period are not logical and calculation of trend is not significant.
The spatial distribution of RDI index trend in autumn in Iran (View the image of this page) Fig. 7.
Results of Mann-Kendall non-parametric test on annual values of RDI showed that among 21 stations, 76% of them had a negative trend and 24% had a significant positive trend.