پهنه بندي مناطق حساس در برابر خشكسالي در دشت نجف آباد با استفاده از تصاوير ماهواره اي

Regionalization of Susceptibility to Drought in Najaf Abad Basin using Remote sensing

در تحقيق حاضر، با استفاده هم زمان از داده هاي ايستگاهي بارندگي و تبخير تعرقِ واقعيِ حاصل از تصاوير ماهواره اي و روش فازي، كوشش شده است تا مناطق حساس در برابر خشك سالي در دشت نجف آباد واقع در استان اصفهان شناسايي شود. براي دست يابي به اين هدف از داده هاي ايستگاه هاي هواشناسي و تصاوير ماهواره اي در يك دوره 25 ساله استفاده شد. در اين پژوهش سال هاي 1995، 2008، و 2015 به عنوان نمونه انتخاب شد و با اجراي الگوريتم سبال بر روي تصاوير ماهواره اي لندست تبخير- تعرقِ واقعي محاسبه شد. سپس، با هم پوشاني فازي نقشه هاي بارندگي و تبخير تعرق مناطق حساس شناسايي شد. نتايج حاصل گوياي آن است كه در سال 1995 نواحي جنوب و جنوب شرقي و بخش هايي از شرق دشت نجف آباد حساسيت بيشتري نشان داده اند. اين در حالي است كه در سال 2008، علاوه بر نواحي فوق الذكر، نواحي شرقي، مركزي، و غربي نيز درگير خشك سالي شده اند. در سال 2015 بخش هاي جنوب و جنوب شرقي به سبب برخورداري از بارش بيشتر از وضعيت خشك سالي خارج و از شرايط مطلوب تري برخوردار شده اند. ويژگي ممتاز اين تحقيق كاربرد پارامترهاي تكميلي در سنجش حساسيت به خشك سالي است.

Drought has always been a heavy financial burden on the country's economy. Therefore، being aware of the drought situation in high risk areas can reduce the damages، Drought management، reduce problems and increase the ability to manage natural resources and agricultural resources. Fortunately، the advent of new technologies to identify and zoning in high risk areas caused done extensive research in this field. Isfahan province is located in the Central Plains of Iran. It has the arm and dry weather. Unfortunately، decreasing rainfall، over exploitation of wells and misuse of water resources play an important role in the province's drought recently. Najaf Abad Plain is one of the Basins which is located in this province. Placement of a modern irrigation network، a sharp drop in groundwater levels and reduce of water discharge are reason of important this area. So this area was chosen in this study. Unfortunately، frequent droughts in parallel with drying up of Zayanderood River brought about undesirable effects on the local economy in the current years. The aim of this study is to help the researchers and decision makers for a proper management of water resources in this area by identifying areas susceptible to drought. Materials and methods To reach this goal، meteorological data and satellite images in a 25-year period were used. In this study، 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 landsat ETM+ were downloaded from the earth explorer site in these years and daily evapotranspiration was calculated. 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 in the daily evapotranspiration values and monthly actual evapotranspiration was calculated. Annual evapotranspiration was obtained the 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 method for determination of 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 justifies the changes resulting from the SEBAL method and so can be trusted regression equation. Because the determination coefficients of the regression are high so Changes dependent and independent variables show well. Studies show that 2008 is the most evapotranspiration and 1995 is the lowest evapotranspiration. The result shows that South and Southeast regions of Najaf Abad are more sensitive than other parts and they are higher risk for 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 pixels in 2008. While at the same area in 2008 more pixels have been involved in drought and large scale mapping of sensitive areas. In addition to the central and western regions of Eastern have susceptible to drought also. This result matches with 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 sensitive area is represented the issue. The southern and southeastern areas 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 to identify areas susceptible to drought. We need proper planning to reduce the effects of drought in areas with high risk more than before based on the resulting map. It is recommended that the methods used for irrigation and cropping patterns according to region and the effects of drought reduced greatly. Key Words:Drought، evapotranspiration، Rain fall، Remote sensing، GIS، Najaf Abad Plain.