Assessing stress by using remote sensing
- 1. 1 PRESENTED BY, Ch. Allaylay Devi PhD. (Hort.) 1st Year Dept. of FSc.
- 2. Introduction • Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object and thus in contrast to on-site observation. • Remote sensing is used in numerous fields, including geography, land surveying and most Earth Science disciplines (for example, hydrology, ecology, oceanography, glaciology, geology) • It also has military, intelligence, commercial, economic, planning, and humanitarian applications. 2
- 3. Remote sensing • “Remote sensing" generally refers to the use of satellite- or aircraft-based sensor technologies to detect and classify objects on Earth, including on the surface and in the atmosphere and oceans, based on propagated signals (e.g. electromagnetic radiation). • It may be split into active and passive remote sensing • “Active" remote sensing (i.e., when a signal is emitted by a satellite or aircraft and its reflection by the object is detected by the sensor). • “Passive" remote sensing (i.e., when the reflection of sunlight is detected by the sensor)
- 4. Techniques of remote sensing and its applications • Conventional radar is mostly associated with aerial traffic control, early warning, and certain large scale meteorological data. • Doppler radar is used by local law enforcements’ monitoring of speed limits and in enhanced meteorological collection such as wind speed and direction within weather systems in addition to precipitation location and intensity. • Other types of active collection includes plasmas in the ionosphere. • Interferometric synthetic aperture radar is used to produce precise digital elevation models of large scale terrain (RADARST, TerraSAR-X, Magellan). • Laser and radar altimeters on satellites have provided a wide range of data. By measuring the bulges of water caused by gravity, they map features on the seafloor to a resolution of a mile or so.
- 5. • By measuring the height and wavelength of ocean waves, the altimeters measure wind speeds and direction, and surface ocean currents and directions. • Ultrasound (acoustic) and radar tide gauges measure sea level, tides and wave direction in coastal and offshore tide gauges. • Light detection and ranging (LIDAR) is well known in examples of weapon ranging, laser illuminated homing of projectiles. It is used to detect and measure the concentration of various chemicals in the atmosphere, while airborne LIDAR can be used to measure heights of objects and features on the ground more accurately than with radar technology. Vegetation remote sensing is a principal application of LIDAR.
- 6. • Radiometers and photometers are the most common instrument in use, collecting reflected and emitted radiation in a wide range of frequencies. The most common are visible and infrared sensors, followed by microwave, gamma ray and rarely, ultraviolet. They may also be used to detect the emission spectra of various chemicals, providing data on chemical concentrations in the atmosphere. • Stereographic pairs of aerial photographs have often been used to make topographic maps by imagery and terrain analysts in trafficability and highway departments for potential routes, in addition to modelling terrestrial habitat features.
- 7. • Simultaneous multi-spectral platforms such as Landsat have been in use since the 1970s. • These thematic mappers take images in multiple wavelengths of electro- magnetic radiation (multi-spectral) and are usually found on Earth observation satellites, including (for example) the Landsat program or the IKONOS satellite. • Maps of land cover and land use from thematic mapping can be used to prospect for minerals, detect or monitor land usage, detect invasive vegetation, deforestation, and examine the health of indigenous plants and crops, including entire farming regions or forests. • Landsat images are used by regulatory agencies such as KYDOW to indicate water quality parameters including Secchi depth, chlorophyll a density and total phosphorus content. Weather satellites are used in meteorology and climatology.
- 8. • Hyperspectral imaging produces an image where each pixel has full spectral information with imaging narrow spectral bands over a contiguous spectral range. Hyperspectral imagers are used in various applications including mineralogy, biology, defence, and environmental measurements. • Within the scope of the combat against desertification, remote sensing allows to follow up and monitor risk areas in the long term, to determine desertification factors, to support decision-makers in defining relevant measures of environmental management, and to assess their impacts
- 9. Methods for monitoring indicators of vegetation condition Spatial and temporal scales of monitoring • It is a site-based monitoring programmes having a long a history of application (e.g. Lawley et al., 2013 ; Sinclair, 2005), and still being commonly used today. • It involves selecting sites from within homogenous patches of vegetation of the same community type and site history. • The majority of site-based assessments are quadrat-based where detailed information is collected about the compositional, structural, and functional attributes of a site. • In some cases landscape metrics have also been added into overall measures, and benchmarked scores are combined to give an overall condition index for individual patches of sampled vegetation (Department, 2011; Michaels, 2006; Parkes et al., 2003). • It predominantly measure structural and compositional indicators of vegetation condition, with less emphasis on indicators of function.
- 10. Assessed stress by utilising remote sensing Detection of plant water stress using remote sensing These technologies acquire many hundreds of spectral bands across the spectrum from 400 nm to 2 500 nm, using satellite, airborne or hand-held devices. The spectral characteristics of vegetation are governed primarily by scattering and absorption characteristics of the leaf internal structure and biochemical constituents, such as pigments, water, nitrogen, cellulose and lignin (Asner, 1998; Coops et al., 2002). Pigments are the main determinants controlling the spectral responses of leaves in the visible wavelengths (Gaussman, 1977). Chlorophyll pigment content, in particular, is directly associated with photosynthetic capacity and productivity (Gaussman, 1977; Curran et al., 1992). Reduced concentrations of chlorophyll are indicative of plant stress (Curran et al., 1992).
- 11. Spectral indicators of plant chlorophyll content • In stressed vegetation, leaf chlorophyll content decreases, thereby changing the proportion of light-absorbing pigments, leading to a reduction in the overall absorption of light (Murtha, 1982; Zarco-Tejada et al., 2000). • These changes affect the spectral reflectance signatures of plants through a reduction in green reflection and an increase in red and blue reflections, resulting in changes in the normal spectral reflectance patterns of plants (Murtha, 1982; Zarco-Tejada et al., 2000). • Thus, detecting changes from the normal (unstressed) spectral reflectance patterns is the key to interpreting plant stress.
- 12. Spectral indicators of plant water content • Plant water content at the leaf and canopy scales is often estimated using specific spectral reflectance bands and spectral reflectance indices from near infrared, middle infrared (MIR) and short-wave infrared (SWIR) regions of the electromagnetic spectrum (Tucker, 1980; Hunt and Chun-Jiang et al., 2006). • NIR and MIR spectral bands are highly correlated to water content of vegetation and soils (Tucker, 1980; Hunt and Rock, 1989; Musick and Pelletier, 1986; 1988). • Spectral bands from these regions have been used to delineate stressed trees from non-stressed trees. • In these regions of the electromagnetic spectrum, leaf water content has been remotely assessed using bands 1 550 nm to 1 750 nm (Tucker, 1980) • MIR reflectance increased with decreasing leaf water content.
- 13. Predawn leaf water potential • Predawn leaf water potential measurements, often undertaken with a pressure chamber, are useful for determining plant water stress. • At predawn, xylem water potential has equilibrated with soil water potential after a night of negligible transpiration. • At this time, plant water potential is usually at its minimum for the day (Cleary and Zaerr, 1984). • The pressure chamber is most commonly used for estimating leaf water potential, having the advantage of simplicity, reliability, instantaneous measurements, low capital cost and portability (Scholander et al., 1965; Boyer, 1968; Ritchie and Hinckley, 1975). • It is commonly used as a plant water stress indicator and has also been used to describe the water status of different species within a habitat
- 14. Leaf chlorophyll fluorescence • Chlorophyll fluorescence measurements can be described using the typical phases of a temporary fluorescence signal or transient. • Therefore, this photosynthetic apparatus has been recognised as being a good indicator of stress and stress adaptation of a plant and is associated with the measurement of chlorophyll fluorescence (Salisbury and Ross, 1992; Strasser and Tsmilli-Michael, 2001; Strasser et al., 2001). • Also, because changes in chlorophyll fluorescence may occur before any physical signs of tissue or chlorophyll deterioration are manifested in the plant, stress can be detected before the onset of physical damage (Lichtenthaler et al., 2007).
- 15. Advantages and Disadvantages Govender, et al., 2009
- 16. Conclusion • With the launch and continuous availability of multi-spectral (visible, near-infrared) sensors on polar orbiting earth observation satellites (Landsat, SPOT, IRS, etc.) remote sensing (RS) data has become an important tool for yield modeling. • Remote sensing research has identified several individual spectral bands and vegetation spectral reflectance indices which have been used to detect plant water stress. • Depending upon the scale at which an investigation is being undertaken, it is recommended that a practical approach to assessing plant water stress is adopted through the use of at least one ground-based measurement.
Editor's Notes
- #16: Govender, et al., 2009