Glance up at the night sky, and you might see the movement of a celestial object belonging to a fleet of earth-observing satellites launched by the National Aeronautics and Space Administration (NASA). Eighteen earthfocused satellites complete their daily orbits around the earth, not to observe and understand the far reaches of space, but to observe and understand our changing planet. Satellite observatories have the distance from Earth to observe the big picture and the technological sophistication to track changes in the biosphere at high spatial and temporal resolutions. Their images awe the eye and shape our understanding. This article focuses on four earth-observing satellites that bring new data to the needs of water research, management, and decision-making.
NASA’s Gravity Recovery and Science Experiment (GRACE) has been called “the giant scale in the sky.” It measures discrepancies in Earth’s gravitational mass, which varies from location to location, determined in part by the height and thickness of the crust and the density of different materials. Scientists realized that shifts in Earth’s gravity occur primarily because water moves from place to place on and under land, in the ocean, and in the atmosphere. GRACE’s twin satellites circle the globe about 137 miles (220 km) apart. As the first satellite encounters a gravitational anomaly on the Earth below, it pulls away from the second. As the second satellite encounters the anomaly, it pulls away from the first. An onboard microwaveranging system makes precise, continuous measurements of these shifts in distance between the two spacecraft. In addition, Satellite Global Positioning System records the exact position of the satellite over the Earth to within a centimeter or less. By comparing monthly maps constructed with these measurements, it is possible to quantify how a region’s water storage changes over time.
GRACE has been able to quantify the volume of water needed to end the drought in California. According to a study released in July 2104, it would take over 33 million acre-feet of water to bring water storage in the state’s Sacramento and San Joaquin river basins up to normal seasonal levels. GRACE data has been able to demonstrate that the amount of water in storage has been steadily decreasing. Since 2011, the water volumes in the Sacramento and San Joaquin river basins have decreased by more than twelve million acre-feet each year. About two-thirds of the loss is a result of groundwater depletion below California’s Central Valley.
GRACE-derived water data is used to monitor monthly changes of water mass in the seven-state basin. A study conducted by NASA, in collaboration with University of California, Irvine, revealed that the basin lost nearly 53 million acre-feet of freshwater from December 2004 to November 2013. This is nearly double the volume of Lake Mead, the largest reservoir on the Colorado River system. More than three-quarters of this total, about 41 million acre-feet, was from groundwater. This information, added to other drought indicators, is used to determine a drought’s severity level.
Data from other satellites has been used to track changes in the northeastern reaches of Lake Powell, a critical part of the Colorado River basin system. NASA’s Landsat series of satellites enabled scientists to produce a series of natural-color images documenting the fluctuations of water present in the side canyons feeding Lake Powell and other features of the reservoir between 1999 and 2014 as the water volume in the lake dropped to 42 percent of its capacity.
The Landsat program is a joint mission of NASA and the United States Geological Survey (USGS), which manages the National Satellite Land Remote Sensing Data Archive that currently holds more than three million Landsat scenes. Since 2009, Landsat images have been made available to the public free of charge.
The data from the Landsat spacecraft has a record of over forty years of continuous coverage, which makes it a primary global reference for scientific issues related to land use and natural resources. “There are newer, fancier satellite imagers out there,” says researcher Eve Halper, of the Bureau of Reclamation, “but they lack the history.”
Landsat images are processed to establish vegetation ratings, at a 30 meter resolution, in order to compile a tree canopy data set. Tree canopy information is combined with local water use to analyze correlations between local residential outdoor water use, green canopy, and heat. Lack of vegetation can impact temperatures, comfort, and energy costs in low-canopy neighborhoods—often those with a vulnerable population due to poverty, says Halper.
Observations from Landsat-8, the newest in the Landsat series, have the spatial resolution to discern individual agricultural fields, each with its own attributes and behavior. Landsat-8’s thermal imager observes differences in temperature as fields cool due to evapotranspiration (ET) and can therefore estimate how much water that field consumed on any particular day. Calculating ET from individual fields is useful for agricultural water management. Landsat data has also been used to settle legal disputes over water rights where actual consumption is an issue.
SMAP, the Soil Moisture Active Passive project, is the most recent addition to NASA’s fleet of earth-facing observatories. Launched in January 2015, SMAP is designed to study a critical part of the water cycle: soil moisture and its freeze and thaw state. Using active radar and passive radiometry, SMAP allows mapping of water, frozen and liquid, in the top few inches of soil around the globe every three days even through cloud and moderate vegetation cover. Measurements from on-theground sensor stations validate the remote satellite estimations under the SMAP program. With these data points added together, SMAP can help researchers in weather forecasting and modeling for climate change and climate variability. SMAP applications will also improve the ability of farmers to assess crop productivity using measurements of root zone soil moisture. Global measurements of crop stress can help in the prediction of famine and allow aid organizations to position supplies of food. SMAP data will also help water resource managers to predict drought, map flood areas, and assess ecosystem health.
Launched in February 2014, the Global Precipitation Measurement (GPM) project, a joint mission of NASA and the Japan Aerospace Exploration Agency is on course to coordinate a dozen satellites measuring precipitation around the globe. In addition to the GPM Core Observatory, each partner satellite has its own orbit, allowing different portions of Earth’s surface to be viewed at the same time, every 30 minutes. The mission is capable of merging data from each of the satellites into a single map, called Imerg, or Integrated Multi-composite map, with a resolution of five by five miles. Data is available in strips, called swaths, which correspond to the satellites’ overpasses. This is the first mission to combine data from an international group of satellites to monitor rainfall and snow in near-real time.
The GPM project, managed by NASA, has allowed the space agency to produce amazing 3D maps of rain and snowfall around the world at both regional and global scales. A new video, showing precipitation from April to September, 2014, was released by the mission in February 2015. Precipitation is seen from 60 degrees north to 60 degrees south of the equator-roughly from the Northwest Territories of Canada to south of Argentina. Large frontal systems, which can last for days, can be seen in the video at middle latitudes, moving heat and water across the Atlantic and Pacific Oceans. Also visible are deep tropical convective storms popping up across the equator, which move heat from the ocean’s surface into the atmosphere, redistributing it throughout the Earth’s system.
Scientists have been studying the shapes of raindrops for years. Their shapes depend on size and the forces acting on them as they fall. The GPM mission is designed to investigate the size of rain drops, using an advanced radar instrument aboard the GPM Core Observatory. Knowing raindrop size and shape helps characterize rainfall patterns—larger, flatter drops are associated with heavier rain fall.
These four earth-facing satellite missions are part of NASA’s Earth Observing System that launched its first mission in 1997. Since NASA’s inception, all of the data received from its spacecraft projects has been archived and made available to the public including over 4 terabytes of new earth science data each day. These missions provide robust data to improve knowledge of our planet’s land surface, biosphere, atmosphere, and oceans helping scientists obtain long-term data critical to reaching a full understanding of the interactions between the Earth’s physical, biological, and socio-economic systems.