水稻族群植冠反射光譜之分析

Abstract

為瞭解田間水稻族群植冠反射光譜之特性與變化趨勢，本試驗特利用攜帶式輻 射光譜層析儀遙測1996年一、二期稻作全生育期植冠反射光譜，據以探討、分析各波段隨生育進展的演變、値被指数的改變、及近紅外光反射比與葉面稹指數的關係。解折反射的波譜範圍介於350〜1100 nm，包含紫外光、可見光及近紅外光等波段。根據試驗結果，水稻兩期作値冠反射光譜在紫外光及可見光波段(350〜700 nm)範圍内，除了生育初期及成熟期外，其餘時期反射比値皆小於15 %。近紅外光波段(740～1100 nm)反射比値於全生育期間維持在30〜55 %範圍之間，而以生育初期及後期之反射比値較低。由特定紅光(674 nm)及近紅外光(756 nm)波長所計算之植被指數如標準値被指數差（NDVI)、植被植數比(RVI)、土壤校正植被指數(SAVI)、及植被指數差(DVI)等四項發現，植被指數隨水稻生育之進展呈近似鐘形曲線，並可顯現出一、二期稻作間生長差異之時間落差。以NDVI爲例，若將生育日數經時間標準化轉換後，則兩期稻作之變化類似。水稻葉面積指數與近紅外光（以756 nm爲例）反射比關係以Mltscherlich function之對數函數表示之適用性甚佳，可利用於估水稻全生育期葉面憤指數的變化。 The reflectance spectrum of crop canopy changes with time and space and is a function of growth status. Analysis of reflectance spectra of a crop during the growing period can help in characterizing and evaluating the spectral property and growth performance of that crop. Field experiments were carried out to remotely measure and analyze the reflectance spectra, in the range of 350〜1100 nm, of rice canopy in the first and the second growing seasons of 1996. Data indicated that, except in the early growth and grain-filling stage, reflectance within 350 to 700 nm region was lower than 15% in both crops. Reflectance of wavelengths above 740 nm was maintained in 30〜55%. By using two specific wavelengths of 674 nm (red light) and 756 nm (near-infrared) in calculating the vegetation indices, it was found curves of NDVI (normalized difference vegetation index), RVI (ratio vegetation index), SAVI (soil adjusted vegetation index), and DVI (difference vegetation index) were bell-shaped after transplanting. Difference in vegetation index between two crops was due mainly to the growth rate of rice plants. When normalization with unit time, however, changes of NDVI were similar in both crops. Reflectance in the near- infrared (for example, 756 nm) was suitable for estimating LAI (leaf area index), and their relationship was properly fitted to the inverse of the Mitscherlich function.