Remote sensing for irrigation using unmanned aircraft systems By Kevin Yemoto and Huihui Zhang, PhD


Remotely sensed data is valuable for irrigation management use and can be acquired at various spatial and temporal scales. Sensors used for remote sensing can be left in the field for around-the-clock, in situ monitoring, or they can be used to periodically survey a field. Although they provide a wealth of information, sensors left in the field for in situ monitoring are limited to point measurements of the local area surrounding the sensor. For more complete information, sensors left in the field are used to ground truth information collected during periodic surveys.


Satellite and aerial imagery has been used for agricultural water studies. NASA’s Landsat series satellites are the most commonly used satellite-based remote sensing platforms. This is due in part to their free and readily available spatial data. Challenges to using satellite imagery for daily irrigation decisions


are its relative low spatial and temporal resolutions, including meteorological interference.


Airborne remote sensing platforms, such as piloted aircrafts, offer an alternative to satellite-based remote sensing platforms, offering the ability to adjust the timing and frequency of data collection and increased flexibility of sensor payloads and spatial resolution. However, the problems with airborne platforms still include less flexibility of flight schedules, specialized training and licensing to operate, and high initial operational and maintenance costs.


The UAS alternative


Now, these problems can be overcome by using low-cost and flexible unmanned platforms such as unmanned aircraft systems [UAS], commonly known as drones. Helicopter, multi-rotor or fixed-wing unmanned aircraft, integrated with imaging, ranging sensors and GPS, are able to acquire remote sensing data in the visible, near-infrared and thermal infrared electromagnetic regions at better spatial and temporal resolutions compared to airborne or satellite-based remote sensing.


Obtaining a remote pilot certificate to operate a UAS commercially is required and is significantly cheaper than manned aircraft systems. Federal Aviation Administration restrictions for flying UAS privately have been relaxed. Learning to fly UAS can typically be learned in an afternoon. Many of the UAS systems can either be flown manually to fully autonomously under operator


supervision, and they can be launched and retrieved from the same or adjacent fields being surveyed. Mission plans can also be reused allowing UAS platforms to be launched at a moment’s notice to take advantage of short windows of opportunities such as breaks in weather.


Different cameras are currently available for unmanned aerial platforms to acquire real-time crop information to estimate crop water use for irrigation management.


RGB images


RGB (red, green, blue) images, also called true-color images, are images that use the RGB color model. Color images are displayed as a mixture of red, green and blue light. UAS-mounted RGB cameras produce high-resolution images (sub- decimeter per pixel). An orthomosaic is a series of individual photos that are programmatically matched up so that they form a new composite image. Full- field orthomosaics can be produced by stitching multiple rectified images using GPS positional data.


Many UAS with RGB cameras are able to send live-feed video back to the ground station. Among other uses, live-feed video and/or images from RGB cameras can be useful for visual inspections. Visual inspections of fields can be helpful in locating trouble spots such as irrigation leaks, standing water or weeds. RGB images can also give a visual representation of crop health. The images from an RGB camera can also be used to calculate canopy cover, which in turn can help determine crop water use.


This color infrared image of a cornfield at the Limited Irrigation Research Farm illustrates the false-color image where green vegetation appears red. The plots are comprised of different varieties of corn in which varied levels of irrigation is applied to test plant response and yield loss. This image was taken on July 21, 2017, which was the 79th day after the corn was planted.


18 Irrigation TODAY | January 2018


NEW YEAR, NEW TECHNOLOGIES


Photo credit: Kevin Yemoto


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