Spectral characterization of water stress impact on some agricultural crops: II. studies on alfalfa using handheld radiometer

摘要

Remotely sensed reflectance from stressed and non-stressed crop vegetative cover can be predicted from two combination of spectral bands as a ratio or as normalized vegetation indices. The most common spectral bands used lie in the red and infraredregion(350-800 nm) and are dominated by the absorption of chlorophyll and other accessory pigments. In addition, reflectance in the middle infrared is dominated by absorption from liquid water contained in plant's tissues. The objectives of the present work are: 1) to evaluate the reflectance data from frequently irrigated and water stressed alfalfa using a handheld radiometer and assess the spectral correlation with the ground-truth and; 2) to evaluate the applications of a Hyperspectral Structure Component Index (HSCI) proposed by Shakir and Girmay-Gwahid (1998). The experiment was designed to collect reflectance data from alfalfa (pure alfalfa stand and a plot where alfalfa was mixed with sedge grass) planted at the Blythe Research Station, California. The size of the plots was 30 X 50 ft~2. With a field spectrometer, the scan over each treatment was made at 1 hr intervals between 10:00 a.m. and 2:00 p.m. Pacific Day Time(PDT). Vegetative samples were taken from the two treatments during the initial sampling for purposes of conducting chemical analysis. Soil samples were collected to determine the amount of available soil moisture differences in the two treatments. The results of this experiment showed that in the 850 - 1150 nm wavelength range the stressed alfalfa plots showed lower reflectance than unstressed plots. However; the reflectance of stressed alfalfa was higher than the unstressed stands above the 1150 nm. This is probably due to the absorption from liquid water contained in the unstressed plant tissues. The analysis of data using the (HSCI) model showed that the stressed pure alfalfa plots have values < 1 and under unstressed alfalfa plots have the value > 1. This means that the model is differentiating between the stressed and unstressed vegetation. Additional work will evaluate the reflectance peaks and their relationship to other parameters that were collected and are relevant to the applications of the model.