ارزيابي شاخص‌هاي هواشناسي و روش‌هاي درون‌يابي به‌منظور پايش و پهنه‌بندي خشكسالي در مناطق خشك و نيمه‌خشك (مطالعۀ موردي: حوزۀ آبخيز قم-كهك)

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

خشكسالي پديده‌اي آرام و خزنده محسوب مي‌شود كه با تأثير در بخش‌هاي كشاورزي، منابع آب و پوشش گياهي در نواحي مختلف، به‌خصوص مناطق خشك و نيمه‌خشك، مي‌تواند گسترش مناطق بياباني را تسريع كند. يكي از روش‌هاي بررسي وضعيت خشكسالي در هر منطقه، پايش و پهنه‌بندي خشكسالي با استفاده از شاخص‌هاي هواشناسي و سيستم اطلاعات جغرافيايي (GIS) است. در تحقيق حاضر به‌منظور بررسي و پايش خشكسالي در منطقۀ مطالعاتي قم-كهك طي دورۀ آماري 29ساله (1367ـ1368 تا 1395ـ1396)، شاخص‌هاي خشكسالي مانند شاخص بارش استانداردشده (SPI) ، درصد نرمال (PNI)، شاخص دهك (DI)، شاخص Z چيني (CZI) و شاخص نمرۀ Z (ZSI) بررسي شدند. سپس با هدف پهنه‌بندي خشكسالي، روش‌هاي مختلف درون‌يابي در تمامي سال‌هاي دورۀ آماري مورد مطالعه در محيط نرم‌افزار ArcGIS ارزيابي گرديدند. نتايج شاخص‌هاي مختلف نشان داد كه حالت نزديك به نرمال در همۀ ايستگاه‌ها، طي دورۀ آماري مورد مطالعه، داراي بيشترين فراواني است. بر اساس نتايج شاخص‌هاي SPI و CZI، بيشترين (7 سال) و كمترين (3 سال) تعداد خشكسالي به‌ترتيب در ايستگاه‌هاي كهك و جنت‌آباد اتفاق افتاده است؛ درحالي‌كه بر اساس شاخص PN بيشترين و كمترين تعداد خشكسالي به‌ترتيب در ايستگاه‌هاي قم (9 سال) و جنت‌آباد (3 سال) مشاهده شد. همچنين نتايج شاخص PN حاكي از عدم وجود دورۀ ترسالي طي دورۀ آماري مورد مطالعه بوده است. بر اساس شاخص ZSI خشكسالي بسيار شديد در منطقه رخ نداده و نتايج ZSI در خصوص فراواني طبقۀ ترسالي بسيار شديد، مشابه نتاج شاخص SPI است. در بين شاخص‌هاي مورد مطالعه، شاخص SPI مقارن با سال وقوع كمينۀ بارندگي در طول دورۀ آماري رخداد خشكسالي شديد و بسيار شديد را در تمام ايستگاه‌ها نشان داده، لذا به‌عنوان مناسب‌ترين شاخص براي پهنه‌بندي خشكسالي انتخاب شد. نتايج پهنه‌بندي نشان داد كه در سال‌هاي آبي 1368ـ1369، 1373ـ1374 و 1375ـ1376 بيشتر منطقه در وضعيت خشكسالي بسيار شديد تا خفيف قرار داشته است. در سال‌هاي آبي 1388ـ1389 و 1395ـ1396 نيز در بيشتر سطح منطقۀ مورد مطالعه، خشكسالي خفيف تا شديد رخ داده و تنها در قسمت شرقي منطقه، ترسالي خفيف مشاهده شد؛ درحالي‌كه طي سال‌هاي آبي 1370ـ1371، 1374ـ1375 و 1387ـ1388 منطقۀ مورد مطالعه، ترسالي متوسط تا شديد را تجربه كرده است. در سال آبي 1389ـ1390 نيمۀ شرقي منطقه، ترسالي متوسط و نيمۀ غربي منطقه، ترسالي خفيف را نشان مي‌دهد.

Introduction: Drought is recognized as a slow, creeping phenomenon that can accelerates the expansion of desertification in different regions especially in arid and semi-arid area with its effect on the agricultural sectors, water resources and vegetation cover. Positioning of Iran in the world arid and semi-arid 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 semi-arid 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 Qoum-Kahak study region we used annual rainfall data from four stations (i.e. Qoume, Kahak, Koohsefid and Janatabad) which are located inside and around the Qoume-Kahak 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 Z-Score 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 near-normal 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 1997-98. 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 2016-2017 and 1997-98 respectively. Extremely wet class was occurred also for Kahak and Janatabad stations during the water years of 2009-2010 and 1992-1993 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.