Tech Corner

Remote Sensing in Irrigation: From Landsat to UAS By José L. Chávez, PhD, and Christopher M.U. Neale, PhD

Remote sensing of irrigated areas has mainly used satellite images for monitoring and modeling. There are multiple satellite-based remote sensing platforms and instruments available, such as the National Aeronautics and Space Administration’s Advanced Space- borne Thermal Emissions and Reflection Radiometer, Moderate Resolution Imaging Spectroradiometer, and the National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer.

The most commonly used satellite-based remote sensing platform for monitoring irrigated areas is the NASA Landsat series satellites. The Landsat satellites have maintained continuous multispectral and thermal imagery coverage of the earth’s surface since July 1978.

algorithms, due to their inexpensive (free, in the case of Landstat), spatially distributed and readily available multispectral imagery. However, there are major drawbacks to using satellite remote sensing data for precision irrigation applications.

Satellite remote sensing platforms formed the basis for many of the current evapotranspiration (ETc

The spatial resolution of the Landsat thematic mapper instrument is 30 meters for the visible or optical and near infrared bands while 100 – 120 meters for the thermal infrared band. The relatively low spatial resolution of the RS data limits the development of man- agement zones of less than 100 meters by 100 meters for use in variable rate irrigation systems, for instance.

Satellite RS platforms have a fixed overpass temporal resolution, 16 days for Landsat, which can cause multiple issues due to spatially and temporally variable meteorological conditions. With

) and soil water content (SWC)

the fixed overpass temporal resolution, weather condition, such as cloud cover or atmospheric haze, can render the data unsuitable for use with ETc

and SWC algorithms.

In order to use the satellite-based remote sensing in SWC algo- rithms, the actual ETc

between overpasses must be interpolat-

ed from the RS data of the current and previous overpasses. In semi-arid and arid environments, or water-limited irrigation systems, the large temporal resolution does not allow the required accuracy to independently manage a precision irrigation system.

In the case of changing meteorological and surface (wetting events) conditions between overpasses, the errors associated with interpolating SWC between overpasses greatly increases.

Additionally, satellite remote sensing data must be corrected for the atmospheric conditions (water vapor content, particulate concentration, etc.) during the overpass. In contrast, airborne multispectral remote sensing platforms, in terms of availability or operability, are more flexible and responsive to meteorological conditions and irrigation managers’ needs.

The flexibility of airborne remote sensing platform’s payloads affords operators the ability to customize the data collected to fit the analytical needs of the end user. The operational flexibility of airborne remote sensing platforms allows the rapid deployment of the platforms in response to changing weather conditions and/ or data requirements.

The ability to adjust the timing and frequency of overpasses with the airborne systems are a significant advantage over the satellite remote sensing platforms. Not only can data collection occur

30 Irrigation TODAY | October 2016

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