And Space Systems
Ray A. Williamson, PhD, is editor of Imaging Notes and Executive Director of the Secure World Foundation, an organization devoted to the promotion of cooperative approaches to space security (SecureWorldFoundation.org).
Water. I’ve been thinking a lot about water recently, especially since my wife and I completed our house in the high desert plateau of Southwestern Colorado. Water is always in short supply in the desert Southwest, where annual rainfall seldom exceeds 12 or 13 inches (30 or 33 cm). Coping with recent heavy snows has obscured the basic fact that the landscape is generally parched, and if the spring and summer rains don’t come at the right times, unirrigated dryland crops fail. Lately, perhaps as a result of climate change, those rains have come late and sparsely.
The supply of clean potable water, always an issue in traditionally dry parts of the planet, has grown more uncertain throughout the world as the climate changes and countries continue to degrade and pollute freshwater supplies. Water experts such as Peter Gleick of the Pacific Institute in Oakland, California, warn of impending water shortages and increasing conflict as a result.
“There is a long history of conflict over the world’s scarce freshwater,” notes Gleick. “Identifying and addressing the risks of violence over these resources is critical to understanding how to reduce such risks in the future as populations grow, climates change, and pressures on limited water resources worsen.”
In theory, satellite imagery should be of great assistance in monitoring water availability throughout the world. Satellites orbiting in polar orbit are the most efficient and accurate way to acquire a consistent, calibrated set of data across the entire globe. In fact, the data acquired from satellites is very important for certain aspects of hydrology, the study of the distribution, movement, and quality of water.
Digital terrain maps of a region, combined with information on the distribution of buildings, pavement, and trees and other plant species on the landscape, provide very powerful means of estimating water sources and sinks throughout the area. Satellite imagery combined with other measurements can also assist in estimating the amount of water contained in snowpack, lakes and rivers. Such information is important for agriculture.
For agriculture, especially in most arid parts of the world, however, the measurement of moisture in the soil is generally even more important. And it is not just total yearly precipitation that is significant, but also when and at what rate it occurs. Moisture has to be there at the right time to germinate seeds and promote growth. For government officials, such information provides estimates of future regional food availability. For farmers, knowledge of current and potential future soil moisture helps them plan their planting and irrigation strategies for the season’s crops.
Soil moisture is also a key element of the scientific understanding of the global water cycle, from moisture content of the atmosphere to precipitation to eventual uptake of moisture into the atmosphere again.
Until recently, satellites have done a poor job of measuring soil moisture accurately because such measurements depend on very weak signals detected by means of a microwave radiometer. Scientists and farmers have depended largely instead on direct in-situ measurements of the water content of soil. That works at the scale of the average farm field, but provides only spotty estimates across large areas.
Although these in-situ measurements can be supplemented with data garnered from microwave instruments on the NOAA series of National Polar-Orbiting Environmental Satellites (NPOES), Eumetsat’s METOP satellite, and the more sensitive instrumentation aboard NASA’s experimental AQUA satellite, such data can provide estimates only and are not sufficiently sensitive for use in agricultural models.
The launch of ESA’s latest Earth observing spacecraft, the Soil Moisture and Ocean Salinity (SMOS) satellite seeks to change that. Part of ESA’s Earth Explorer series of satellites, SMOS is likely not only to revolutionize the study of Earth’s water cycle, but also to provide key operational information for farmers in Europe and around the world. Its revolutionary interferometric radiometer, the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), will capture two-dimensional images of the Earth’s surface, measuring changes in land surface moisture by observing variations in the natural microwave emission radiating from the surface.
Soil moisture is an important component of Earth’s water cycle, and it influences regional weather patterns. Hence, measurements of this element will contribute to scientists’ understanding of Earth’s hydrology and help to track desertification.
MIRAS will also measure variations in the salinity of seawater across the planet, measurements that are important for better understanding of ocean currents and their role in moderating climate. Salinity and temperature together determine the density of seawater, which is a factor in creating ocean currents. Like soil moisture, ocean salinity is poorly understood throughout the world.
The measurements from SMOS could provide not only important data for understanding the global water cycle, but also crucial information for managing Earth’s dwindling sources of fresh water. If fully successful, it will point the way toward the development of more sensitive future soil moisture and salinity instruments.
Two more scientific satellites are expanding our understanding of Earth’s water resources. NASA’s twin Gravity Recovery and Climate Experiment (GRACE) satellites, launched in 2002, were designed to generate gravity maps of Earth. Among other things, the twin Grace satellites monitor tiny month-to-month changes in Earth’s gravity field caused primarily by the movement of water in Earth’s land, ocean, ice and atmosphere reservoirs.
As announced in a press conference at the American Geophysical Union’s annual meeting, scientists Jay Famiglietti, of the University of California Center for Hydrological Modeling, and colleagues Sean Swenson, NCAR and Matt Rodell, NASA Goddard Space Flight Center, have discovered that between 2003 and 2009, the volume of water resources stored in the Sacramento and San Joaquin River Basins of Central California decreased by some 31.3 cubic kilometers. Most of that decrease comes from losses in groundwater storage. In the opinion of these researchers, such losses are unsustainable.
This fall, ESA launched its own gravity satellite, the Gravity field and steady-state Ocean Circulation Explorer (GOCE). Over the next 20 months, this satellite will map global variations in Earth’s gravity field with extreme detail and accuracy, which will add to the mass of information about global groundwater resources.
The data acquired from all three satellites will contribute to our ability to accomplish climate studies and to understand the global water cycle. However, they are not enough. As global warming becomes more of a factor in determining the frequency and global distribution of precipitation, it is crucial to be able to monitor the performance of Earth’s water cycle operationally and to develop better management of existing water resources. Satellites can assist with that. However, collecting and putting satellite data to use will require new, improved systems and the institutional mechanisms needed to turn those data into operation for the benefit of humankind. That is a challenge not only for the space agencies of the world but also for the governments that supply the means to improve water resources management.