Spectral characterization of water stress impact on some agricultural crops: III. Studies on Sudan grass and other different crops using handheld radiometer

摘要

Vegetation monitoring has been one of the major targets of remote sensing studies. Remotely sensed reflectance concerning the impact of environmental factors upon crop vegetative cover can be predicted from two combinations of spectral bands as a ratio or as normalized vegetation indices. The most common spectral bands used lie in the red and infrared region (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 Sudan grass and other crops using a handheld radiometer and assess the spectral correlation with the ground-truth; (2) to evaluate the applications of a Hyperspectral Structure Component Index (HSCI) developed by Shakir and Girmay-Gwahid in 1998; and (3) to evaluate the application of Index of Relative Stress (IRS) proposed by Shakir and Girmay-Gwahid in 1998. The experiment was designed to collect reflectance data from Sudan grass and other crops planted at the Blythe Research Station, California in rows. The size of the plots for Sudan grass was in rows, the unstressed mature stands were 9 feet tall, and the stressed mature stands were 5 feet tall. The other fields are in nearby and planted with cotton crops in different stages of maturity. 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 DayTime (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 ranges, the stressed Sudan grass stands showed lower reflectance than unstressed stands. However, the reflectance of stressed Sudan grass stands 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 same pattern was found in the cotton crop. The analysis of data using the (HSCI) model showed that the stressed Sudan grass stands have values less than 1 and under unstressed Sudan grass stands have the value greater than 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. Furthermore; the Index of Relative Stress (IRS) showed that the unstressed vegetation stands is higher in values than in the stressed.