That is why plants look green to our eye, as this is where they reflect the most radiation in the visible part of the spectrum. The structure of the leaves account for this relative increase in reflectance in the near-infrared. Because of these properties, measurements of reflectance in the red and NIR can be used to determine differences in crop canopy densities. Thermal Remote Sensing Another area of the spectrum that is useful for assessing crop conditions is the thermal portion of the spectrum. Measures of radiance in this area can be used to derive the surface temperature of the crop.
Plants take up water from the soil and then release it from their leaves back to the air.
If a crop cannot get enough water, its surface temperature will increase, so the plant's surface temperature can tell us something about how healthy it is. For more information on applications of thermal data to agriculture, see the sections on crop water stress detection and large area evapotranspiration estimation. Jackson, RD. 1984. Remote sensing of vegetation characteristics for farm management . Detection and Evaluation of plant stresses for crop management decisions .