ارزیابی شاخص‌های هواشناسی و روش‌های درون‌یابی به‌منظور پایش و پهنه‌بندی خشکسالی در مناطق خشک و نیمه‌خشک (مطالعۀ موردی: حوزۀ آبخیز قم-کهک)

خشکسالی پدیده ای آرام و خزنده محسوب می شود که با تاثیر در بخش های کشاورزی، منابع آب و پوشش گیاهی در نواحی مختلف، به خصوص مناطق خشک و نیمه خشک، می تواند گسترش مناطق بیابانی را تسریع کند. یکی از روش های بررسی وضعیت خشکسالی در هر منطقه، پایش و پهنه بندی خشکسالی با استفاده از شاخص های هواشناسی و سیستم اطلاعات جغرافیایی (gis) است. در تحقیق حاضر به منظور بررسی و پایش خشکسالی در منطقۀ مطالعاتی قمکهک طی دورۀ آماری 29ساله (1367-1368 تا 13951396)، شاخص های خشکسالی مانند شاخص بارش استانداردشده (spi) ، درصد نرمال (pni)، شاخص دهک (di)، شاخص z چینی (czi) و شاخص نمرۀ z (zsi) بررسی شدند. سپس با هدف پهنه بندی خشکسالی، روش های مختلف درون یابی در تمامی سال های دورۀ آماری مورد مطالعه در محیط نرم افزار arcgis ارزیابی گردیدند. نتایج شاخص های مختلف نشان داد که حالت نزدیک به نرمال در همۀ ایستگاه ها، طی دورۀ آماری مورد مطالعه، دارای بیشترین فراوانی است. بر اساس نتایج شاخص های spi و czi، بیشترین (7 سال) و کمترین (3 سال) تعداد خشکسالی به ترتیب در ایستگاه های کهک و جنت آباد اتفاق افتاده است؛ درحالی که بر اساس شاخص pn بیشترین و کمترین تعداد خشکسالی به ترتیب در ایستگاه های قم (9 سال) و جنت آباد (3 سال) مشاهده شد. همچنین نتایج شاخص pn حاکی از عدم وجود دورۀ ترسالی طی دورۀ آماری مورد مطالعه بوده است. بر اساس شاخص zsi خشکسالی بسیار شدید در منطقه رخ نداده و نتایج zsi در خصوص فراوانی طبقۀ ترسالی بسیار شدید، مشابه نتاج شاخص spi است. در بین شاخص های مورد مطالعه، شاخص spi مقارن با سال وقوع کمینۀ بارندگی در طول دورۀ آماری رخداد خشکسالی شدید و بسیار شدید را در تمام ایستگاه ها نشان داده، لذا به عنوان مناسب ترین شاخص برای پهنه بندی خشکسالی انتخاب شد. نتایج پهنه بندی نشان داد که در سال های آبی 1368-1369، 1373-1374 و 1375-1376 بیشتر منطقه در وضعیت خشکسالی بسیار شدید تا خفیف قرار داشته است. در سال های آبی 13881389 و 1395-1396 نیز در بیشتر سطح منطقۀ مورد مطالعه، خشکسالی خفیف تا شدید رخ داده و تنها در قسمت شرقی منطقه، ترسالی خفیف مشاهده شد؛ درحالی که طی سال های آبی 13701371، 13741375 و 13871388 منطقۀ مورد مطالعه، ترسالی متوسط تا شدید را تجربه کرده است. در سال آبی 13891390 نیمۀ شرقی منطقه، ترسالی متوسط و نیمۀ غربی منطقه، ترسالی خفیف را نشان می دهد.

Evaluation of Meteorological Indices and Interpolation Methods to Drought Monitoring and Zoning in Arid and Semi-Arid Regions (A Case Study: Qoum-Kahak Basin)

Introduction: Drought is recognized as a slow, creeping phenomenon that can accelerates the expansion of desertification in different regions especially in arid and semiarid area with its effect on the agricultural sectors, water resources and vegetation cover. Positioning of Iran in the world arid and semiarid belt is exposed to drought and water crisis and desertification. Drought monitoring and zoning with the goal of planning and water resources management are very important in different area especially in arid and semiarid regions. One of the methods for drought study is monitoring and mapping of drought using meteorological indices and Geographic Information System techniques. Materials and Methods: In order to investigate and drought monitoring in present research during the period of 29 years (1989 to 2017) in QoumKahak study region we used annual rainfall data from four stations (i.e. Qoume, Kahak, Koohsefid and Janatabad) which are located inside and around the QoumeKahak catchment. Then we applied meteorological indices such as Standard of Precipitation Index (SPI), Percent of Normal Index (PNi), Deciles Index (DI), Chinese Z Index (CZI) and ZScore Index (ZSI) for drought monitoring during the study period. Afterward for drought mapping, four interpolation methods included Simple Kriging (SK), Ordinary Kriging (OK), Inverse Distance Weighting (IDW) and Radial Basis Functions (RBF) were used and compared. After determination of best method in each year, yearly drought map were produced based on the best interpolation methods in ArcGIS software environment.Results: According to the result of meteorological indices the nearnormal class occurred more than other classes in study stations. SPI results indicated that Jannatabad and Kahak stations have the highest and lowest frequency of extremely dry class during the study period, respectively. The most extremely dry class with the SPI of 2.35 was occurred in Janatabad station in the year of 199798. Based on the results of PNI and ZSI indexes there was no occurred extremely dry class in all stations. The result of PNI index indicated that Koohsefid station has the highest frequency of very dry class between all stations. According to the result of CZI index the extremely dry class was occurred only for Kahak and Janatabad stations in the water years of 20162017 and 199798 respectively. Extremely wet class was occurred also for Kahak and Janatabad stations during the water years of 20092010 and 19921993 respectively. The results of DI index illustrated that the class of very much below normal was occurred in Qoume and Koohsefid stations 4 times and in Kahak and Janatabad stations 3 times during the study period. We applied geostatistical analyst tools in ArcGIS software to produce zoning maps for rainfall and drought. Its result showed that ordinary kriging with RMSE of 28 mm is the best method to create rainfall map. Accordingly mean annual rainfall was 136 mm/year during the study period. We selected SPI index for drought monitoring and zoning because of according to the results of all meteorological indices,only for this index the extremely and severely drought classes for all stations have occurred Simultaneous with the minimal rainfall during the study period. Afterward we compared different interpolation methods to produce drought zoning maps for each year during the study period. Based on the results of RMSE ordinary kriging and simple kriging were the best methods for most years. Discussion and Conclusion: The results of all meteorological indices suggested that in all stations the nearnormal class has the most frequency during the study period. According to the results of SPI and CZI indices, the highest (7 years) and lowest (3 years) number of drought were occurred in Kahak and Jannatabad stations, respectively. While according to PN index the highest (9 years) and lowest (3 years) number of drought were observed in Qoum and Jannatabad stations, respectively. Also the results of PN index illustrated that there was no wet period during the study period. According to the ZSI Index, the extremely dry class did not occurred in the region and the ZSI results about the frequency of extremely wet class was similar to the SPI results. The results of drought zoning indicated that in the years of 1990, 1995 and 1997, most of the region has been in a state of extremely to slightly drought. In the years 2010 and 2017, slight to sever drought classes were occurred in most parts of the study area and the slight wet class was observed in the eastern part of region. While during the years of 1992, 1996 and 2009 whole of the study area has experienced moderate and severe wet conditions. In the year of 2011, half of the study area which is located in the eastern part of the region has been in a state of moderate wet condition.