The role of evapotranspiration in meteorological drought monitoring in some climatic regions of the country

Abstract
Background and objectives:
Water is one of the most basic human needs and other living creatures. Drought is a natural phenomenon and repeatable, which could be caused serious crisis. This phenomenon may be seen in any climate regime, even in humid regions, but the effects and its frequency are more observed in arid and semi-arid areas. The most important cause of drought is precipitation, but the increase or decrease in evapotranspiration can partly intensify or modify drought conditions. In order to drought monitoring are used different indices that generally can be based on precipitation or precipitation and evapotranspiration. On the other hand drought monitoring based on different indices maybe cause different results. Therefore, in this study, meteorological drought monitoring and evaluation of the effect of evapotranspiration on it were examined during a 50 years period from 1961 to 2010. Drought conditions are compared based on three drought indices of SPI, RDI and SPEI for Ramsar, Hamedan, Shiraz, Sabzevar, Bandar Abbas and Yazd synoptic stations, which respectively are located in Coastal Wet, Mountain, Semi Mountain, Semi Desert, Coastal Desert and Desert climates.
Materials and methods:
In this study, were used monthly statistics of meteorological parameters such as precipitation, mean temperature, minimum temperature, maximum temperature, relative humidity, sunshine hours and wind speed in six synoptic stations. Firstly, fulfilled initial statistical tests on monthly meteorological parameters. For this purpose, is used three known tests as Mann-Kendall test for the identification of Trend, Mann-Whitney test for detecting of Homogeneity and Grubbs-Beck test for detecting of Outlier. After verifying these data in terms of Homogeneity and Outlier, values of each of SPI, RDI and SPEI indices calculated in annual and seasonal scales.
Results:
Comparing results represented that in humid regions, there are not any significant differences between SPI index with RDI and SPEI indices which consider evapotranspiration. However; the difference between the indices is clear in arid regions. The least difference in terms of drought condition monitoring is between SPI and RDI in spring in Hamadan and the greatest difference between SPI and RDI indices is in the summer and the city of Yazd (desert climate). This shows the role of evapotranspiration and its impact on the drought, in the hot and dry season, especially in arid and desert regions. In autumn and winter seasons in every six studied stations, the least difference has happened between SPI with SPEI and SPI with RDI indices, so in this seasons, which evapotranspiration is lower, it has less important role in humidity condition (drought or wet).
Conclusion:
According to the results of this study, can be concluded that in most cases the indices performance are not the same in different regions and at different time intervals. On the other side, can be concluded that the fluctuations of humidity condition (wet and drought) is very high in studied stations and based on any of the examined indices. While drought severity and its frequency of occurrence are different according to different indices. It is recommended for drought monitoring in arid regions, which the precipitation is negligible and the other side the evapotranspiration is manifold of annual precipitation, are used indices which are based on precipitation and evapotranspiration. Whereas in humid regions (such as Ramsar) evapotranspiration phenomena does not make significant role in the drought and in order to simplification, meteorological drought can be monitored only by precipitation.