The Future of Earth Observation in Hydrology

Abstract

In just the past five years, the field of Earth observation has progressed beyond the offerings of conventional space agency based platforms to include a plethora of sensing opportunities afforded by CubeSats, Unmanned Aerial Vehicles (UAVs), and smartphone technologies that are being embraced by both for-profit companies and individual researchers. Over the previous decades, space agency efforts have brought forth well-known and immensely useful satellites such as the Landsat series and the Gravity Research and Climate Experiment (GRACE) system, with costs typically on the order of one billion dollars per satellite and with concept-to-launch timelines on the order of two decades (for new missions). More recently, the proliferation of smartphones has helped to miniaturise sensors and energy requirements, facilitating advances in the use of CubeSats that can be launched by the dozens, while providing ultra-high (3-5 m) resolution sensing of the Earth on a daily basis. Start-up companies that did not exist five years ago now operate more satellites in orbit than any space agency, and at costs that are a mere fraction of the cost of traditional satellite missions. With these advances come new space-borne measurements, such as real-time high-definition video for tracking air pollution, storm-cell development, flood propagation, precipitation monitoring, or even for constructing digital surfaces using structure-from-motion techniques. Closer to the surface, measurements from small unmanned drones and tethered balloons have mapped snow depths, floods, and estimated evaporation at sub-meter resolutions, pushing back on spatio-temporal constraints and delivering new process insights. At ground level, precipitation has been measured using signal attenuation between antennae mounted on cell phone towers, while the proliferation of mobile devices has enabled citizen-scientists to catalogue photos of environmental conditions, estimate daily average temperatures from battery state, and sense other hydrologically important variables such as channel depths using commercially available wireless devices. Global internet access is being pursued via high altitude balloons, solar planes, and hundreds of planned satellite launches, providing a means to exploit the Internet of Things as an entirely new measurement domain. Such global access will enable real-time collection of data from billions of smartphones or from remote research platforms. This future will produce petabytes of data that can only be accessed via cloud storage and will require new analytical approaches to interpret. The extent to which today's hydrologic models can usefully ingest such massive data volumes is unclear. Nor is it clear whether this deluge of data will be usefully exploited, either because the measurements are superfluous, inconsistent, not accurate enough, or simply because we lack the capacity to process and analyse them. What is apparent is that the tools and techniques afforded by this array of novel and game-changing sensing platforms present our community with a unique opportunity to develop new insights that advance fundamental aspects of the hydrological sciences. To accomplish this will require more than just an application of the technology: in some cases, it will demand a radical rethink on how we utilise and exploit these new observing systems to enhance our understanding of the Earth and its linked processes.

Figure 1.

The state of play in space today. Estimates are based on the Union of Concerned Scientists satellite database, updated from 1/1//2017 (see http://www.ucsusa.org/nuclear-weapons/space-weapons/satellite-database). In terms of the sectors operating Earth Observing systems (right panel), another 5% include shared systems between those listed.

Figure 2.

An Earth observing “System of Systems” for revolutionizing our understanding of the hydrological cycle. This multi-scale, multi-resolution observation strategy is not really a concept, as the technology exists and is largely in place now. Supporting traditional space based satellites, there are now a range of orbital options from commercial CubeSats to demonstration sensors on-board the International Space Station. Beyond orbiting EO systems, technological advances in hardware design and communications are opening the skies to stratospheric balloons and solar planes, as well as an explosion of UAV-type platforms for enhanced sensing. At the ground level, the ubiquity of mobile devices are expanding traditional in-situ network capacity, while proximal sensing and signals of opportunity are opening up novel measurement strategies.

Figure 3.

Employing a UAV to retrieve high-resolution multispectral information on the land surface for hydrology and related applications over an Australian rangeland site located near Fowler’s Gap in New South Wales. Retrieved products include: a) a false-colour infrared image; b) a reconstructed digital surface model using visible imagery and structure-from-motion techniques; and c) an optimized soil adjusted vegetation index (OSAVI) derived from the 4-band multispectral image. Images were captured using a MicaSense/Parrot Sequoia sensor on-board a 3DR Solo quadcopter. The UAV was flying at a height of 40 m, providing a ground sampling distance of approximately 3 cm. Imagery provided by the University of Tasmania’s TerraLuma Research Group.

Figure 4.

Multi-scale capabilities of state of the art sensing optical satellites. Image illustrates the expanding resolution options available from both commercial and government satellites. A) Planet CubeSat at 3 m ground sampling distance over the Tawdeehiya Farm in Al Kharj, Saudi Arabia. Center pivot irrigated fields dot the landscape, with dimensions approaching 800 m. The inset in A) is zoomed to show the resolution advantages offered by the next generation of sensing solutions over B) Landsat-8 at 30 m, with C) Sentinel-2A at 10 m and D) Planet imagery at 3 m providing enhanced details. All images are false colour representations of NIR, Red and Blue in RGB bands. Sentinel-2A and Landsat-8 images were acquired on December 4th, 2016, while the Planet data were captured on December 5th, 2016.

Figure 5.

Worldwide global system for mobile communication (GSM) coverage for the year 2013. The GSM network does not include the growth of related 3G or 4G networks. The image is derived from Figure 2 in Overeem et al. (2016).

Multimedia 1.

On-board the International Space Station, the UrtheCast IRIS high-resolution camera (HRC) captures colour video at 3 frames per second for a duration of 60 seconds. Here we see an example of the HD Video over the Burj Khalifi in Dubai. The tracking of vehicles on roads is analogous to monitoring flow in rivers or the speed of moving clouds, while the capacity to extract 3D structure of the underlying terrain provides opportunities in dynamic monitoring of surfaces.