Estimation of daily average net radiation from MODIS data and DEM over the Baiyangdian watershed in North China for clear sky days
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Daily average net radiation (DANR) is a critical variable for estimation of daily evapotranspiration (ET) from remote sensing techniques at watershed or regional scales, and in turn for hydrological modeling and water resources management. This study attempts to comprehensively analyze physical mechanisms governing the variation of each component of DANR during a day, with the objective to improve parameterization schemes for daily average net shortwave radiation (DANSR) and daily average net longwave radiation (DANLR) using MODIS (MODerate Resolution Imaging Spectroradiometer) data products, DEM, and minimum meteorological data in order to map spatially consistent and reasonably distributed DANR at watershed scales for clear sky days. First, a geometric model for simulating daily average direct solar radiation by accounting for the effects of terrain factors (slope, azimuth and elevation) on the availability of direct solar radiation for sloping land surfaces is adopted. Specifically, the magnitudes of sunrise and sunset angles, the frequencies of a sloping surface being illuminated as well as the potential sunshine duration for a given sloping surface are computed on a daily basis. The geometric model is applied to the Baiyangdian watershed in North China, with showing the capability to distinctly characterize the spatial pattern of daily average direct solar radiation for sloping land surfaces. DANSR can then be successfully derived from simulated daily average direct solar radiation by means of the geometric model and the characteristics of nearly invariant diffuse solar radiation during daytime in conjunction with MCD43A1 albedo products. Second, four observations of Terra-MODIS and Aqua-MODIS land surface temperature (LST) and surface emissivities in band 31 and band 32 from MOD11A1, MYD11A1 and MOD11_L2 data products for six clear sky days from April to September in the year 2007, are utilized to simulate daily average LST to improve the accuracy of estimates of DANLR. Comparisons of the DANLR estimates from the proposed four observation-based method and that from an existing one observation-based method, against that from the Penmen equation solely using routine meteorological data indicates that the proposed method is capable of more accurately simulating DANLR than is the one observation-based method. Using the Penman equation as a reference, results show that overall the proposed method has a bias of 2.7 W m-2 and a root mean square error (RMSE) of 12.8 W m-2, whereas the one observation-based method has a bias of -33.3 W m-2 and a RMSE of 39.6 W m-2 across 18 weather stations for six tested days. In general, simulated DANR is shown to be reasonable over the entire study watershed for the six clear sky days as a result of the improvement in the parameterization schemes of DANSR and DANLR. The resulting DANR would serve well as a critical variable linking instantaneous evaporative fraction to the estimates of daily ET primarily from remotely sensed data. 2010 Elsevier B.V. All rights reserved.
