ارزيابي ساختار مكاني بارش نيم قرن اخير دشت مشهد

The Evaluation of the Spatial Structure of Mashhad Plain Precipitation in the Last Half-Century

تعامل عميق، پيچيده و مداوم بارش با ساير عناصر و عوامل اقليمي، سبب تغيير و تنوع اين عنصر در بعد زمان و مكان شده است. يكي از رويكردهاي مطالعاتي جديد در اقليم شناسي، توصيف تنوع مكاني بارش بر اساس آماره هاي مكاني است. هدف مطالعه ي حاضر آن است كه با استفاده از روش هاي آمار مكاني، رفتار عمومي بارش دشت مشهد در امتداد مكان ارايه گردد. در اين راستا از داده هاي بارش روزانه 34 ايستگاه همديد، اقليم شناسي و باران سنجي طي دوره ي آماري (1392-1342 هجري خورشيدي) استفاده شده است. در ابتدا بر اساس روش هاي آمار كلاسيك، پراكندگي مكاني و زماني بارش مورد مطالعه و سپس سه مشخصه ي ميانگين مركزي، فاصله استاندارد و توزيع جهت دار مورد بررسي قرار گرفتند. نتايج و بررسي ها، نشان داد كه مركز ثقل (گرانيگاه) بارش هاي سالانه نيم‌قرن اخير دشت مشهد 83/3 كيلومتر جابجايي داشته است و توزيع جهت دار گوياي بزرگي اثر شيب و جهت گيري آن برميزان بارش است. همچنين فاصله استاندارد بارش دشت مشهد در دهه ي پنجم (1392-1382) نسبت به دهه ي اول (1352-1342) به ميزان 57/1254 واحد تغيير نموده است. اين عامل دليلي بر ناپايداري روابط خطي عوامل مكاني در توليد بارش در دشت مشهد است. بر اين اساس ناهمواري ها و ارتفاع بيشترين نقش را در الگوي مكاني بارش در دشت مشهد ايفا مي‌كنند.

Introduction The meaningful, complex, and ongoing connection between the rainfall and other climatic elements causes diversity in space and time. A new approach in climatology is to describe the spatial variability of the rainfall based on the spatial statistics. Unlike the classical statistics, the spatial statistics shows the statistical data on a map. Therefore, the attention and emphasis on the spatial differences and the identification of the specific and unique points or homogeneous regions will be provided. Modeling of the rainfall behavior is one of the main foundations in any climate research. In this regard, two major efforts are of interest to climatologists. One of them is the precipitation zoning. The other one is the analysis of the spatial temporal variations of the precipitation. This analysis is important for weather forecasting and a wide range of decision makers, including hydrologists, farmers, and industrialists. Methodology Using statistical methods, the present study aimed to introduce the fundamentals of the spatial data and the general precipitation behavior of Mashhad’s plain along the space. In this regard, the study used the daily precipitation data of 34 synoptic stations, climatology, and rain gauge during the survey period, 1963-2013. The study initially analyzed the spatial and temporal distributions of the precipitation based on the classical statistical methods. Then, it focused on the central average, standard distance, and directional distribution. In this research, the universal Moran method was used to calculate the spatial autocorrelation data. In addition, the central mean method was used to calculate the basin rainfall gradient. Finally, directional distribution was used to calculate the trend and direction of the precipitation distribution. Discussion The results showed that the gravity center, the centroid, of the annual rainfall during the last half-century sustains a displacement of 3.83 km where the distribution arrow demonstrates the magnitude of the tilt and orientation on the amount of the precipitation. Also, the standard distance of the precipitation in Mashhad, in the fifth decade (2003-2013) compared to the first decade (1963-1973), changed to 1254.57. This change can be one of the reasons of the instability of the linear relationships of the spatial factors and the rainfall in the plains of Mashhad. Conclusion The results showed that the center of the rainfall gravity of Mashhad plain was displaced over 3.3 km during the 50-year period. In addition, there was a change of 0.269 degrees in the direction of the distribution of the precipitation in the fifth period compared to the first period. Since, this shift was negative to areas with spatial dependence, it indicated a general drop in the rainfall in the Mashhad plain. The results also showed that the roughness and height might be two important factors affecting the spatial patterns of the rainfall in Mashhad Plain.