Agriculture Reference
In-Depth Information
uniform, the correlation became weak. A significant correlation was found between
SPAD readings and vegetation indices (NDVI, GNDVI) derived from SPOT images.
Because the correlation between SPOT images and SPAD readings was similar to
that between aerial images and SPAD readings, SPOT images are considered to have
a potential for assessing chlorophyll contents and nitrogen stress in corn.
9.2.4 R
ADAR
AND
L
IDAR
Besides satellite remote sensing and aerial photography, radar and lidar are also
applied for nutrition acquisition of crops. The use of radar is a feasible means of acquir-
ing remote sensing data especially when there is frequent cloud cover (Ahamed et al.,
2011). The radar remote sensing technique can collect ground characteristic data irre-
spective of weather or light conditions (Lu, 2006). Hutchinson (2003) investigated the
feasibility of generating accurate and spatially distributed estimates of soil moisture
using a time series of ERS-2 radar images for a tallgrass prairie ecosystem in north-
east Kansas. The investigation process included field data collection of soil moisture,
digital image interpretation of optical (NOAA AVHRR and Landsat TM) and radar
(ERS-2) imagery, and environmental modeling in a raster GIS and image processing
environment. This study determined the scattering effection of overlying vegetation, or
the contribution of vegetation backscatter (O° veg) to the total backscatter coefficient
(O° total), which was simulated using a modified water cloud model. By eliminat-
ing O° veg from O° total, the amount of backscatter contributed by the soil surface
(O° soil) was separated, and the linear relationship between O° soil and volumetric
soil moisture was determined. The correlation coefficient of 0.62 and 0.67 for a single
date was obtained for a burned and unburned watershed, respectively, within the study
area. Moreover, this study shows that ERS-2 data may have the capability to moni-
tor soil moisture conditions over even extremely dense natural grassland vegetation.
Lidar technique is another remote sensing tool to estimate the biophysical properties
of a canopy (Drake et al., 2002; Hyde et al., 2005; Lim et al., 2003). There are numer-
ous application fields using the lidar technique, such as estimation of forest biomass,
temperate mixed deciduous forest biomass (Lefsky et al., 1999a, 1999b), tropical forest
biomass (Drake et al., 2003), tree height, stand volume (Nilsson, 1996; Zimble et al.,
2003), and canopy structure (Lovell et al., 2003). As can be seen, wavelength radar
data have important roles in the yield estimation of crop under conditions of frequent
cloud cover. However, preprocessing is necessary for the data analysis, and skill and
time are needed for image processing. The main drawbacks in implementing space-
and airborne remote sensing methods include noise and temporal resolution for site-
specific management of crops, which requires near real-time data processing to adopt
precision farming and to find the optimum window of harvesting crops.
9.2.5 C
ROP
I
NFORMATION
A
CQUISITION
M
ETHODS
AND
I
NSTRUMENTS
Crop growing information is a systematic and complex system that includes nutri-
tional, physiological, and ecological information (He and Zhao, 2011).
Nutritional information is mainly about the nitrogen content, phosphorus content,
and potassium content, and also some trace elements, including boron, manganese,
