Characteristics Of Solar Radiation And Its Distribution Above The Canopy Of The Pinus Tabulaeformis

In this paper, the Pinus tabulaeformis which is the main afforest vegetation in loess areas are selected for study, the method of field fixed-point observation by radiometer is used , the writer researched on the daily solar radiation energy changes of Pinus tabulaeformis canopy layer on typical weather, the daily value feature in different stages of the growing season, and the distribution of radiation energy in space and time. This paper provided a theoretical basis for making full use of solar radiation energy, improving Pinus tabulaeformis vegetation productivity and restoring the vegetation in the arid and semi-arid areas under special circumstances. Its main conclusions are as follows:Firstly, in the sunny weather conditions, the daily changes of the canopy layer's total radiation and net radiation are regular single peak changes in the growing seasons. For the specific performances: the solar elevating angle is minimum and the solar radiation is in zero when sunrise or sunset; the sun's elevating angle is maximum and has a value peak around 13:00 at noon. In the sunny weather conditions, the ratio of net radiation's daily changes trends are as follows: it is high at noon and it is low in the morning and at night, the net radiation during a day is relatively stable, the net radiation changes less differences in different stages of the growing seasons, it generally maintains between 77%~84%.Secondly, in the cloudy weather conditions, for the impact of other factors, such as the clouds, the changes of the canopy layer's total radiation and net radiation are multimodal changes regularity in the growing seasons, the general trend for change is lower in the morning or at night and higher at noon. But the weather has a greater impact on total radiation and net radiation in the different growing seasons; as a result, the peaks of radiation occur at the different time. Comparing with the sunny weather conditions, the change of the radiation is more significant, and it has a small value peak of radiation in some moments. The ratio of the net radiation has changed in the range of 26% to 89%.Thirdly, as the vertical height decreases, the changes of the total solar radiation and net radiation are of diminishing significantly. At the top of the canopy, the solar radiation intensity is maximization, the radiation will be weakened with the vertical height decreased, and the total solar radiation can be reduced 50 to 70 percent from the surface of the leaf canopy to the bottom of the leaf canopy. The leaf canopy plays a more significant role in the stop-flow and weakening the solar radiation.Fourthly, the solar radiation in the leaf canopy has a clear orientation rule, and different orientations show a law with clear differentiation. Since from the day at 8:00, the solar radiation in the leaf canopy has a descending trend from east to west, the difference between East-West significantly decreases with the time changes; after about 13:00 at noon, the solar radiation in the leaf canopy gradually has a descending trend from west to east at sunset; after 16:00, the margin also gradually increases. During the whole growing seasons, these different stages show some same rules, but the differences between South-North are more clearly than the differences between East-West.Fifthly, stand environment affects on the distribution of radiation energy too. Because the branches density at the north side of the test place is much smaller than the south's, the measuring values of radiation in the north in one day is larger than in the south. North-south differences also reached to the maximum when the solar elevating angle of the sun is maximization.Lastly, the daily changes of the infrared radiation in the leaf canopy are similar with its total changes regularity, and it shows a regular single peak change. The solar radiation value is small at sunrise or sunset; the elevating angle of the sun is maximization; and it has a peak value around 13:00 at noon. Dates have some fluctuations because of weather conditions and stand environments. The whole growing seasons show the single peak changes regularity. Just because of the changes of the solar elevating angle, the radiation peak values and the time are different; the value of radiation is the largest in mid-stage of the growing seasons, then the second -largest at the end of the growing seasons.