USA, 5 July 2006: For the first time, NASA scientists using space-based measurements have directly monitored and measured the complete cycle of water movement for an entire continent. Their research confirmed that the amount of water as rain or snow flowing into the continent from the marine atmosphere is in balance with the estimated amount of water returned to the ocean by the continent’s rivers.
The findings are significant because until now there had been no direct way to monitor continental water balance. Scientists had been estimating the balance through regional ground-based measurements and computer models. The findings are published in Geophysical Research Letters.
Using satellite data from three Earth-orbiting NASA missions — Quick Scatterometer (QuikScat), Gravity Recovery and Climate Experiment (Grace), and Tropical Rainfall Measuring Mission (TRMM) — a science team at NASA’s Jet Propulsion Laboratory, Pasadena, California, directly observed the seasonal cycling of water into and out of South America.
“Having a better understanding of the processes in which water is transported from Earth’s oceans to continental land masses is important to a variety of climate and ecology studies,” said Dr. Timothy Liu, science team leader at JPL. “We’ll have greater understanding of floods and drought, surface and ground water quality, and the availability of freshwater resources for agriculture and ecosystems.”
To calculate the continent’s overall water balance equation, Liu’s team compared the amount of water coming into the continent with that going out. A statistical method was developed to estimate water transport using QuikScat’s surface wind data and atmospheric water vapor data from microwave radiometers. Rainfall data from NASA’s TRMM were used to measure the rainfall over the continent. Water going out from the continent was measured by combining data from river flow gauges with projections from models that predict the amount of water discharged at the rivers’ mouths.
The river discharge rates were collected over periods ranging from a few years to a century, depending on the river basin and locality, and were averaged to determine an annual cycle. Scientists compared that estimate with the monthly changes in South America’s mass over two annual cycles, from August 2002 to July 2004, as measured by Grace. They determined that the seasonal mass change is dominated by changes in the amount of surface and underground water.
Liu said the study strongly validates the credibility of space-based measurements to study continental water balance, but is only a beginning. “Planned reprocessing of QuikScat, Grace and TRMM data to improve the data quality and resolution, when combined with data from planned future missions, promises to further enhance our understanding of water balance on a global basis,” he said. Those planned future missions include NASA’s Global Precipitation Measurement Mission, the European Space Agency’s Soil Moisture and Salinity Sensor and NASA’s Aquarius satellite.