Evaluation of Drought Hazard Area of GharehAghaj Basin in Iran, Using GIS ()
1. Introduction
Drought risk area, by nature, is a result of a variety of factors. Drought in general originates from less precipitation over an extended period of time. These include occurrence of no rain in the rainy season, number and amount of rainfall event and other climatic anomalies. Ahmadi used such a method has been conventionally used for preparing the hazard and risk maps for different types of land degradation [1] [2] . In an operational definition of drought it identifies drought from impact data (i.e. crop damage). It is widely accepted that combination of physical nature of area, amount of rainfall and water resource development leads to identify the drought pattern. The shortage of the rainfall, the erratic distribution of the rainfall, highs evapotranspiration, water erosion, low water holding capacity of soil that all of them are the major causes of drought. Anukularmpai applied gamma distribution to predict minimum monthly rainfall in Thailand [3] .
Department of Environmental Planning and Policy established the drought risk areas in the northeast using GIS for spatial overlay of variable layers: rainfall index, soil water holding capacity, irrigated area, ground water yield, rainfall probability and land use [4] . Feiznia et al. and Grunblatt et al. reported the frequency rainfall occurrence in the northeast and mapped along with a measure of their variability [5] [6] . Koonthanakulwong concluded the drought on the basis of the number of days with rainfall was less than specified threshold [7] .
Masoudiand Kumar assessment the hazard of ground water resource degradation using GIS in Mond Miyani basin, Iran he used five indicator to this study [8] [9] .The eighth National Economic and Social Development Plan called for achievement of water resource sustainability [10] . Palmer identified monthly index values for past dry periods to yield an equation for calculating drought severity in four classes [11] . Singh et al. used similar method for assessment Desertification map in Western Rajasthan [12] . Another study of rainfall in the northeast of Thailand made by Siripon and Mongkolsawat concludes that the unevenly distributed rainfall during the rainy season is found extensively and extends longer period in the southwest and the central part of the region [13] .
Result of this research shown that severe and moderate hazardous areas it is concluded that the areas under severe hazard cover about 13% of the total plains, while those under moderate hazard have a greater spread (70%). Zareiee has shown that moderate hazard areas (63% of the basin) are much widespread than areas under severe hazard (37% of the basin) of this region but they used only meteorological indicator [14] . Zehtabian and Jafari used similar factor for evaluation of water resources degradation in Kashan area, but have emphasis on desertification model [15] .
This phenomenon is frequently occurred in second half of June and of September for the areas in southwest and northeast of the region respectively. The information obtained from limited studies still requires a more detail identification of spatial pattern of drought. This is to support the government in allocating water for rural consumption more accurately and at the right place. Computer-based analysis and GIS can addresses this issue with higher accuracy, based on the integration of meteorological, hydrological, physical, socioeconomic data of the areas. The purpose of this study is to model drought hazard area using GIS with a set of data layers empirically evaluated.
2. Material and Methods
2.1. Study region
GharehAghaj watershed, is one of the main watersheds in Fars province in southwest of Iran. Ghareh Aghaj borders on the Sepidan in the north and the Hormozgan province in the south and Boshehr province in the west and kazeron in the east. This region covers about one-ninth of the total area of the province. Area of this region is 1,264,900 hectare. It lies between Latitude 28˚22'11"N and 29˚55'23"N and Longitude 51˚48'15"E and 54˚24'43"E (Figure 1). Mean annual rainfall is about 320.54 (mm/annual) that main period of precipitation is during winter (60% of total rainfall). Mean annual temperature is 18.2 centigrade. The climate in most part of the basin is arid and semi arid. Total length of rivers in this basin is over than 360 km and average of drainage density in this basin is 300 m/km2. GharehAghaj watershed contains one-sixth of the total population of the Fars province (about 1.3 × 106 in 2010). Over 60% of the population is engaged in agriculture which dominated by irrigated cultivation and secondary stage by dry cultivation production. At the present time, less than 10% of the cultivated land in the ranges is dry cultivation. In addition, water shortage for domestic consumption is usually identified as principal constraint for the people during the dry season. Lack of the water or drought in the region has profound impact that can be listed as economic, social and environmental (Figure 1).
2.2. Methodology
The data obtained were of two types 1) numerical data and 2) thematic maps, but mainly in the map format useful for the GIS analysis. All such relevant data were obtained from the local and main offices and institutes of
Figure 1. Location of study area and monitoring sections.