عنوان مقاله [English]
Drought is one of climate hazards that over time brings a lot of damage on human life and natural ecosystems. Commonly Droughts are divided to four main groups of meteorological, hydrologic, agricultural and socio-economic .All Types of droughts are different from each other significantly. In another sense, the occurrence of an event of drought would be the cause of another draught. Various methods have been used for the analysis and assessment of drought and its impacts on human activities and natural resources. Statistics, Synoptic, Remotely sensed methods, and several types of models such spatial, dynamic and statistics models can be seen in the most studies related to drought. The zoning of drought using spatial-statistics indices and generally the spatial zoning and regional distribution of dry periods is one of important features that makes a better understanding towards the phenomenon of drought and a closer consideration of the effects of it. In the past four decades remote sensing widely provide drought monitoring tools, and many drought monitoring model is presented, which is generally based on vegetation and thermal indices especially Normalized Difference Vegetation Index (NDVI), land surface temperature (LST), moisture and reflection at the visible and infrared areas.
Golestan province is located in the North East of Iran in the neighborhood of the Caspian Sea and the northern slopes of the Alborz Mountains, Which following local conditions of the Caspian coast line and high peaks, with increasing altitude, vegetation diversity there are certain bands. From the peaks over 1500 meters to foothills is covered with dense forest broadleaf. Field craps are the dominant vegetation’s From Foothills to the plain. From North Gorganrood to border of Turkmenistan due to rare moisture Resources vegetation is thinner than the southern and central provinces.
The research in terms of nature and methods in theoretical basis is parts of the descriptive researches and due to relationship and impact is an applied one. In the present study for following meteorological drought and ecological in Golestan province, two types of data were used. 72 pluviometry stations on monthly rainfall data (period 1971-2010) and remote sensing data in the three periods (July, 1975, 1987 and 2000), derived from Landsat satellite images. Standard Precipitation Index (SPI) is used to identify and to zoning meteorological drought and the Normalized Difference Vegetation Index (NDVI) is used for the detection of the plant tension that were affected by drought. In the following the results of two parameters and their relationships were compared to each other.
Examining maps of drought frequency and severity of droughts indicates that most of frequency droughts of stations occurred in the North and North-East Province. And gradually the intensity and frequency of droughts reduced to the South and South West. The most severe drought that can be seen in parts of the North East belongs to the Hutan station. However the lowest number has occurred in the central and western regions of Golestan province. According to table (3) correlation between both NDVI and SPI are positive in three years. In all the years of study SPI index-correlation with NDVI was fairly good together. Overall the parts of Northern Province and East Sea have most drough in the selected three years. While most of the wet years are related to the southern part of the province (northern slopes of the Alborz mountain range).
According to conducted research either in the country or abroad, it seems that the use of SPI index as representative of meteorological drought and NDVI index as representative of indicators satellite drought are appropriate for monitoring this kind of droughts. According to the study, two SPI drought index and NDVI due to adaptation with each other are proposed for drought monitoring and meteorological satellite in Golestan province.