Abstract: Solar radiation plays a critical role in the process of energy and material exchange over land surface. It is not only the drive force of land surface physical process, but also the input parameter of Land Surface Model and Remote Sensing Model. Researchers focused on the semiarid and arid regions because of its special geographic environment. Therefore this paper took a typical arid area in the lower reaches of the Tarim River as a case to discuss the radiation balance over riparian forest belt and the measured radiation data, and to evaluate the applicability of FAO56 method used to calculate the net radiation in arid region. FAO56 method is used to calculate shortwave radiation through sunshine duration and longwave radiation through air temperature, humidity and cloud amount. Accuracy was evaluated by Model Efficiency (ME), Root Mean Square Difference (RMSD) and Mean Bias Error (MBE). Statistical results of measured values showed that the daily variation of radiation flux was in a typical normal distribution trend. The maximal values of solar radiation, reflected radiation and net radiation occurred at Beijing time 14:00 o’clock and were 629.96 W·m-2, 106.51 W·m-2 and 520.76 W·m-2 respectively. Maximal value of averaged ground surface longwave radiation occurred at Beijing time 16:00 o’clock and was 460.08 W·m-2. As a result, response of surface temperature to solar radiation lagged for about 2 hours. Seasonal difference was highly related to zenith angle and albedo. Annual variation of net radiation was unimodal with a slight interannual fluctuation, the net radiation increased gradually from January to June, reached its maximum in June, and decreased gradually from June to December. Daily total value of net radiation fluctuated between 0.5 MJ·m-2·d-1 and 18 MJ·m-2·d-1, and its average in full year was 7.81 MJ·m-2·d-1. Weather conditions, such as windy or cloudy weather, affected significantly the radiation flux. Wind and cloud could reduce solar radiation and correspondingly reflected radiation. Effective radiation was slightly affected, thus net radiation dropped to some extent compared with that on sunny day. The curve of daily albedo was Ushaped, it was high at sunrise and sunset but low around midday. Albedo was closely related to the stage of vegetation growth. The averaged albedo was 0.18 at vegetation growth stage and 0.20 at nongrowth stage. Simulated result showed that ME values of net shortwave radiation and net radiation were 0.87 and 0.80 respectively. The slopes of the correlation analysis for them were 0.95 and 0.89 respectively, for effective radiation, although ME was 0.19, the slope of the correlation analysis was 0.99. It could be concluded that FAO56 radiation simulation could be applied to calculate net radiation over the lower reaches of the Tarim River.

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