羊草草甸草原FPAR时间变化规律分析

Analysis on Temporal Variation of FPAR on Leymus Chinensis Meadow-steppes

FPAR是植被叶子在光合有效辐射(400～700nm)波段有多少太阳光能被吸收的一个度量,表示了植被冠层能量的吸收能力,是描述植被结构以及与之相关的物质与能量交换过程的基本生理变量。FPAR模型是否能真实反映植被冠层吸收光合有效辐射状况,将直接影响遥感估算草地NPP的不确定性程度。本文通过对长生季羊草草甸草原冠层PAR各分量的观测,研究了入射PAR、冠层反射PAR及透过冠层到达地面的PAR的时间变化规律,并以观测的PAR为基础计算分析了羊草草甸草原FPAR的时间变化规律。结果表明,羊草草甸草原入射PAR及透射PAR日变化规律明显,呈较标准的正弦曲线变化;晴天FPAR的日变化呈较标准的余弦曲线变化,FPAR在早晚值较高,最高值约0.81,日平均FPAR值可以用9:30/太阳天顶角为48°时瞬时的FPAR值或14:30/太阳天顶角为30°时瞬时的FPAR值表示。

FPAR is a measure of how large a fraction of the sunlight leaves absorb in the 400- 700nm spectrum and FPAR thus expresses a canopy＇s energy absorption capacity. FPAR is one of the key biophysical variables which describe canopy structure and are related to functional process rates of energy and mass exchange. While FPAR model can represent the vegetation absorbed photosynthetically active radiation on vegetation canopies, it will influence directly the uncertainty of NPP estimation using remote sensing data. On the basis of an observation to Leymus chinensis meadow-steppes canopy PAR during the growing season of 2007 in Hulunbeier, the incident PAR, the reflective PAR of the canopy, the transmittance of PAR through the canopy, the reflectance of land surface were studied respectively in this paper. The FPAR at an interval of one hour and the average of a day were calculated and analyzed according to each PAR components in canopy. The result showed that the incident PAR and the transmittance PAR diurnal variation was a standard curve of sine; the diurnal FPAR variation was a curve of cosine in clear day, FPAR was found to be highest in the morning and late afternoon due to large solar zenith angles and lowest around noon where the solar zenith angle is low, the highest value of a day is 0.81. The daily average of FPAR may be approximated the incident FPAR values when the solar zenith angle was 48° at 9：30 am or when the solar zenith angle was 30° at 14：30 pm.