Regionalization of Susceptibility to Drought in Najaf Abad Basin

Introduction
Drought as a complex phenomenon involves extensive research. Therefore, being aware of the drought situation in high risk areas can reduce the damage and problems and enhance the ability to manage natural and agricultural resources. Fortunately, the advent of new technologies for identifying and zoning in high risk areas made it possible to conduct extensive research in this field. Isfahan province is located in the Central Plains of Iranwith dry weather conditions. Decrease in rainfall, over exploitation of wells and misuse of water resources play an important role recently drought in the province. Najaf Abad Plain is one of the Basins located in this province. Placement of a modern irrigation network, a sharp drop in groundwater levels and reduction of water discharge are important factors in this area. Thus, this area is selected as study area in this research. Unfortunately, frequent droughts in parallel with drying up of Zayanderood River brought about undesirable effects on the local economy in the recent years. The aim of this study is to help the researchers and decision makers choose proper management decisions of water resources in this area by identifying the areas susceptible to drought.
Materials and methods
In this study, we have used meteorological data and satellite images in a 25-year period. According to rainfall data from weather stations, three years of high rainfall (1995), low rainfall (2008) and normal year (2015) were selected and the amount of actual evapotranspiration was calculated using SEBAL algorithms on the ETM+ images and Penman-Monteith method on meteorological data. For this purpose, 36 images of landsat5, and 8 ETM+ were downloaded from the earth explorer site in these years. The period covered by each image found and the amount of monthly reference evapotranspiration was calculated using Penman-Monteith and meteorological data. Monthly reference evapotranspiration were multiplied by the daily evapotranspiration values and monthly actual evapotranspiration. Annual evapotranspiration was obtained by monthly actual evapotranspiration values. Then, the rainfall zoning map was prepared by interpolation of rainfall data from weather stations. Fuzzy Method and weighted overlay are a method to determine the areas susceptible to drought. Fuzzy method was used in this study because Fuzzy method shows better and clearly results. Finally, the sensitive areas were identified by overlaying fuzzy maps of rainfall and evapotranspiration in these three years.
Results and Discussion 
Comparing the results of Penman-Monteith and SEBAL algorithm showed that the root mean square error of these two methods is about 0.21 and 0.73, respectively. In other words, 73 percent of evapotranspiration in Penman-Monteith method can justify the changes resulted from the SEBAL method and so can be trusted by regression equation. Because the determination coefficients of the regression are high, so it can show changes in dependent and independent variables. Results indicate that the evapotranspiration value highest in 2008 and lowest in 1995. The result shows that South and Southeast regions of Najaf Abad are more sensitive than other parts and they have higher risk of drought in 1995. Because in this part evapotranspiration is high and rainfall is low. Therefore, the drought sensitive areas have been identified. In 2008, the probability of drought in those regions has continued. The difference is that the number of pixels in 1995 showed less sensitive to drought than those in 2008. While at the same area in 2008 more pixels have been involved in drought with large scale mapping of sensitive areas. In addition to the central and western regions of Eastern areas, there are also many other susceptible areas to drought. This result matches with the result of Standardized Precipitation Index (SPI). Because the Standardized Precipitation Index (SPI) shows that intense meteorological drought has occurred in this area. However, the zoning map shows sensitive area. The southern and southeastern areas are out of drought situation and they have favorable conditions in 2015. This result shows that rainfall is better in this part of the basin.
Conclusion
The results of this research help experts identify the areas prone to drought events. We need proper planning to reduce the effects of drought in the areas with high risk more than before based on the map resulted in this research. It is recommended that the methods used for irrigation and cropping patterns are according to region and the effects of drought.