Submitted to: American Society of Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/1998
Publication Date: N/A
Interpretive Summary: Farmers are constantly trying to manage limited resources to maximize their net profit. One limited resource, especially in the western U.S., is water. It is very important for a farmer to apply just enough water at the appropriate times to provide the highest possible crop yield. This requires an efficient way to determine when crops are experiencing water stress. One way is to use images of crop reflectance and temperature obtained from satellites. Use of crop temperature is a proven means of monitoring crop water stress, but many satellites do not carry the equipment for measuring crop temperatures. More satellites provide images of crop reflectance, but crop reflectance is less sensitive to crop stress conditions and it is often difficult to separate water stress from other properties of the crop such as the amount of crop cover or the wetness of the soil. This paper discusses the best ways to monitor early and extreme crop water stress with images from orbiting satellites. It provides numerous suggestions for further research into this problem which, when solved, will provide farmers an effective means to use our limited water resources efficiently while, at the same time, optimizing their net profit.
Technical Abstract: Most approaches for monitoring crop water stress with remote sensing techniques are based on plant, soil, and air temperatures. The most well- known of these approaches is the Crop Water Stress Index (CWSI), which has been implemented commercially with hand-held and airborne sensors. Unfortunately, there are few thermal sensors aboard the existing and planned satellites. Thus, there is a need to explore the use of other remote sensing methods to determine crop water stress from satellites. This calls for an understanding of how plants respond to water stress and how these adaptions affect spectral signals in the visible, near- infrared (near-IR), shortwave infrared (SWIR), and synthetic aperture radar (SAR) wavelengths that are commonly measured by sensors aboard satellites. Measurements of reflectance in the visible and near-IR spectrums have been correlated with such crop stress adaptations as leaf shedding, decreases in leaf expansion, leaf wilting, decreased cell turgor, leaf chlorosis, and hastened or delayed phenologic development. SAR backscatter has been found useful for detecting crop water stress due to the dielectric properties of water, but it is also sensitive to crop geometry, soil moisture, and soil roughness. The SWIR band is the most promising in detecting early water stress because SWIR reflectance can be directly correlated with leaf water content. An approach was proposed to combine red and near-IR reflectance with SWIR reflectance to account for variations in plant geometry and vegetation cover and improve the correlation of SWIR reflectance with early stages of crop water stress. This approach was demonstrated with irrigation treatments in wheat and alfalfa test plots.