تخمین تابش خالص روزانۀ حوضۀ آبریز دریاچۀ ارومیه در شرایط آسمان صاف با کاربرد داده‏ های سنجندۀ modis

تخمین تابش خالص روزانه در مدیریت منابع آب، به‏ویژه در برآورد میزان تبخیر و تعرق، کاربرد گسترده‏ای دارد. در این پژوهش از مدل رقومی ارتفاع سنجنده aster داده‏های ماهواره‏ای سنجنده modis و یک مدل هندسی (مبتنی بر شیب و جهت) در تهیه نقشه‏های تابش خالص روزانه (rn) در حوضه آبریز دریاچه ارومیه استفاده شد. در تخمین مولفه‏های تابش از محصولات مختلف سنجنده modis شامل آلبدو، ضریب گسیلندگی زمین و اتمسفر، دمای سطح زمین و هوا استفاده شد. به دلیل وسعت زیاد حوضه، مولفه‏های تابش فقط برای چهار تاریخ در شرایط آسمان صاف تهیه شد. برای اعتبارسنجی نتایج، از داده‏های تابش طول موج کوتاه ایستگاه‏های سینوپتیک ارومیه و تبریز استفاده شد و معیارهای خطای rmse و mae به‌ترتیب برابر با 30.29 و 19 وات بر مترمربع به‏دست آمد. مقادیر تابش خالص طول موج بلند روزانه (rnl) با روش چهارزمانه مشاهدات دمای سطح زمین برآورد شد. به دلیل فقدان داده‏های مشاهده‏ای تابش طول موج بلند، از روش فائو (پنمن) برای تولید مقادیر rnl در ایستگاه‏های سینوپتیک ارومیه، تبریز، بناب، مراغه، و سهند استفاده شد. نتایج نشان داد مقادیر تابش طول موج کوتاه 24ساعته ورودی در پیکسل‏های هموار و شیب‏دار دارای اختلاف زیادی است.

Estimation of Daily Net Radiation in the Urmia Lake Basin under Clearsky Conditions based on MODIS Data

Extended Abstract Introduction Daily average net radiation (DANR) is necessary for hydrological modeling, water resources management and planning, in particular for prediction of daily evapotranspiration (ET). Recently, some remote sensing based methods have been developed at watershed and regional scales to help researchers for estimating DANR and ER under both clear and cloudy conditions. DANR is a critical variable linking estimates of instantaneous latent heat flux (satellite overpass time) from energy balance based models. Radiation modeling based on ground or station data is actually point scale estimation. On the other hands, application of geostatistical methods to produce radiation maps at watershed scale, are often associated with remarkable errors due to the limited number of stations. In this study, net daily radiation maps was created based on newly developed geometric radiation model based on land and atmospheric MODIS data in the Urmia Lake Basin (ULB) under clearsky condition for four selected dates in 2006 and 2007. Materials and Methods In this study, the ULB, located in northwestern Iran, was selected as the study area that extending in latitude from around 35.67° to 38.47° N and in longitude from around 44.22° to 47.89° E. The basin has an area of approximately 52700 square kilometers. This study attempts to estimate the DANR maps for four selected days under clear sky. Long et al. (2010) developed a geometric model considering slope and aspect effects to improve parameterization schemes to estimate daily average net shortwave radiation (Rns) and daily average net longwave radiation (Rnl). In present study, the same methodology applying a characteristics model based (slope, azimuth and elevation) based on daily MODIS data products and the global ASTER DEM. The DANR is total of the daily net short wavelength radiation and daily net long wavelength radiation. Long wave radiation was estimated using MODIS products including surface albedo, land surface temperature (LST), atmospheric temperature, pressure and emissivity. A fourobservation based method using TerraMODIS and AquaMODIS LST data was applied to estimate daily average LST in order to improve the accuracy of estimates of Rnl. In the study, we applied the 24hour incoming direct solar radiation (Sin24) measurements of Tabriz and Urmia synoptic stations for validation. The RMSE and MAE measures were used for model evaluation. In addition, due to the lack of groundbased Rnl observations; we utilized the FAO (Penman) method to generate corresponding ground data for model validation. Results and Discussion In order to compare the amount of incoming Sin24 on flat and sloping surface, the whole study area was assumed a flat and the Sin24 maps were produced. The different between Sin24 on flat (Sin24,s) and sloping surfaces (Sin24,f) were calculated for all pixels. The histogram plot or frequency of Sin24,s Sin24,f quantity at different intervals of slope showed that by increasing slope degree, the frequency of pixels at all slope classes becomes greater. This reflects the importance of the slope for estimating Sin24. In addition, the values of Sin24 at flat are higher than sloping surfaces especially for where slopes < 30 degree. Aspect is another influencing factor on the amount of receiving and absorbed radiation. The RMSE and MAE for observed and estimated Sin24 values were 30.29 and 19w.m2, respectively. The models in average overestimate Sin24 about 10.5%. The results showed that the eastern and western aspects receive more daily net radiation during day and sunshine hours or day length has its impact on radiation. For Rnl estimates, the RMSE was 36w.m2 Conclusions In present study the Sin24 was parameterized by taking into account the effect of terrain factors, such as slope, azimuth and elevation on direct solar radiation. The sunrise and sunset angles for a given sloping surface (pixel) are different in geometric radiation model. Results indicate that the model has the capability to characterize the variability in Sin24. The incoming solar radiation for flat surface is higher than sloping surface. We founded that southeast and southwest aspects receive grater net radiation. The DANR values decrease with an increase in elevation. The geometric model used in this research needs to be evaluated in different watersheds in Iran without data availability problem especially daily Rns, Rnl and LST measurements, a high number of meteorological stations with proper spatial distribution. This research was done under limited data in the ULB.