واسنجی و ارزیابی مدل های مختلف تخمین تابش خورشیدی روزانه در مقیاس های زمانی فصلی و سالانه در منطقه شیراز

تابش خورشیدی رسیده به سطح زمین دارای کاربرد بسیار وسیعی در مسایل هیدرولوژی، کشاورزی و هواشناسی می‌باشد. تابش خورشیدی از پارامترهای مهم مدل های برآورد تبخیرتعرق پتانسیل گیاه مرجع مانند معادله پنمن ماتیت می باشد، اما اندازه گیری آن در تعداد کمی از ایستگاه‌های هواشناسی ایران انجام می‌شود. با توجه به آن‌که اندازه گیری این پارامتر پرهزینه است، تاکنون مدل‌های متعددی جهت برآورد آن در اقلیم‌های مختلف پیشنهاد شده است. در این تحقیق از داده‌های اندازه‌گیری شده روزانه تابش خورشیدی در ایستگاه شیراز استفاده شد. از آمار سال‌های 1385 تا 1387 برای واسنجی و از آمار سال‌های 1388 تا 1389 برای ارزیابی چهارده مدل مختلف برآورد تابش خورشیدی در مقیاس‌های زمانی فصلی و سالانه استفاده شد. مدل‌های استفاده شده در این تحقیق شامل سه دسته وابسته به ساعات آفتابی، وابسته به دمای هوا و وابسته به ترکیب ساعات آفتابی و دمای هوا بودند. برای ارزیابی مدل‌های مختلف تخمین تابش خورشیدی از ترکیب فرمول‌های آماری و همبستگی خطی استفاده شد و مقدار میانیگن مربع انحراف (msd) محاسبه گردید. متوسط مقدار msd برای چهارده مدل انتخابی در فصل‌های بهار تا زمستان به ترتیب برابر 16/24، 42/20، 08/4 و 19/16 و در مقیاس سالانه برابر 40/15 شد. لذا نتایج نشان داد که در مجموع استفاده از تخمین تابش خورشیدی برای فصل پاییز و مقیاس زمانی سالانه مناسب‌تر است. از طرف دیگر متوسط مقدار msd برای مدل‌های وابسته به ساعات آفتابی، وابسته به دمای هوا و وابسته به ترکیب ساعات آفتابی و دمای هوا در مقیاس سالانه به ترتیب برابر 82/14، 40/17 و 88/14 شدند. لذا نتایج نشان داد که مدل‌های وابسته به دمای هوا برای تخمین تابش خورشیدی در منطقه شیراز مناسب نیستند و استفاده از ساعات آفتابی برای تخمین تابش خورشیدی در این منطقه ضرورت دارد.

Calibration and Evaluation of Different Estimation Models of Daily Solar Radiation in Seasonally and Annual Time Steps in Shiraz Region

Introduction: Solar radiation on the earth surface has a wide range of applications in hydrology, agriculture and meteorology. Solar radiation is an important parameter of estimated models of reference crop potential evapotranspiration such as the Penman–Monteith equation. Also, total sunshine hours are one of the most important factors affecting climate and environment, and its longterm variation is of much concern in climate studies. Reference crop potential evapotranspiration is one of the most important parts of water cycle in the nature but, direct measurement of this crop parameter is so difficult and not practical. Therefore, equations that can estimate the value of evapotranspiration only by using meteorological data are necessary. As mentioned before, the Penman–Monteith equation can be used for estimating reference crop potential evapotranspiration, however this equation needs solar radiation data, and the measurement of solar radiation is done in a limited numbers of weather stations in Iran, and also in Fars province, south of Iran. Since, the measurement of solar radiation is expensive, therefore many models have been derived for its estimation in different climates of the world., Many investigators also have been tried to estimate solar radiation for different locations of the world based on more simple measured weather data such as air temperature (minimum, maximum or mean) and sunshine hours. Hence, the derived equations for estimating solar radiation based on other weather data can be used for estimating reference crop potential evapotranspiration with the Penman–Monteith equation. Materials and Methods: In this study, solar radiation was estimated in Shiraz, central part of the Fars province in south of Iran. For this purpose, the daily measured of solar radiation data in Shiraz synoptic station were used. Also, other needed weather data were used. All available data was for the years 2006 to 2010. Measured data of years 2006 to 2008 were used for calibrating fourteen estimated models of solar radiation in seasonally and annual time steps and the measured data of years 2009 and 2010 were used for evaluating the obtained results. The equations were used in this study divided into three groups contains: 1) The equations based on only sunshine hours. 2) The equations based on only air temperature. 3) The equations based on sunshine hours and air temperature together. On the other hand, statistical comparison must be done to select the best equation for estimating solar radiation in seasonally and annual time steps. For this purpose, in validation stage the combination of statistical equations and linear correlation was used, and then the value of mean square deviation (MSD) was calculated to evaluate the different models for estimating solar radiation in mentioned time steps. Results and Discussion: The mean values of mean square deviation (MSD) of fourteen models for estimating solar radiation were equal to 24.16, 20.42, 4.08 and 16.19 for spring to winter respectively, and 15.40 in annual time step. Therefore, the results showed that using the equations for autumn enjoyed high accuracy, however for other seasons had low accuracy. So, using the equations for annual time step were appropriate more than the equations for seasonally time steps. Also, the mean values of mean square deviation (MSD) of the equations based on only sunshine hours, the equations based on only air temperature, and the equations based on the combination of sunshine hours and air temperature for estimating solar radiation were equal to 14.82, 17.40 and 14.88, respectively. Therefore, the results indicated that the models based on only air temperature were the worst conditions for estimating solar radiation in Shiraz region, and therefore, using the sunshine hours for estimating solar radiation is necessary. Conclusions: In this study for estimating solar radiation in seasonally and annual time steps in Shiraz region, three groups of equations were used (1: based on only sunshine hours. 2: based on only air temperature, and 3: based on sunshine hours and air temperature). Final results of this study for estimating solar radiation in Shiraz region were: 1) For autumn season the best equation was based on the extraterrestrial radiation, the ratio of daily actual sunshine hours to daily maximum sunshine hours, and minimum and maximum air temperatures. 2) For annual time step the best equation was based on the extraterrestrial radiation and the ratio of daily actual sunshine hours to daily maximum sunshine hours.