Every drop counted

NASA and its international partners operate several Earth observing satellites that closely follow one another along the same orbital track. This coordinated group of satellites is called the Afternoon Constellation, or the A-Train, for short. Four satellites currently fly in the A-Train: Aqua, CloudSat, CALIPSO, and Aura. GCOM-W1 and OCO-2 are scheduled to join the configuration in 2012 and 2013, respectively.

Once rain falls, where does it go? How much is absorbed into plants or the soil, and how much of that is evaporated again? How much flows into rivers, or forms clouds and then becomes rain again?

These are some of the key questions Dr Matt McCabe and his team of researchers are answering at the UNSW Water Research Centre, in the School of Civil and Environmental Engineering, in Australia’s largest hydrological research program. Using satellites and a vast array of ground equipment, they are gradually piecing together a picture of how water travels through the earth’s  hydrological cycle, and then developing models to simulate the cycle and explore how changes in the system will affect it. “Improving our capacity to observe and model these systems is not just a research question,” he says. “Australia’s water security is of major national importance and enhancing our ability to simulate system behavior and response has many practical implications. Water resource management and supply, irrigation scheduling, drought monitoring, and flood forecasting and prediction all require an accurate quantification of the state of the hydrological system. None of this can be achieved without first being able to measure and model these interlinked processes.”

Satellites collect data about large movements of water (such as rainfall, soil moisture and cloud development), which can then be used to evaluate model predictions. At the UNSW Baldry Hydrological Observatory in central-west NSW, Matt and the team use radar to track rainfall, infrared scintillometers for measuring heat exchange between the surface and atmosphere, cosmic ray soil moisture systems and laser-based isotope instruments that can be used to track the origin and fate of atmospheric and liquid waters. Below-ground depth profiles show soil moisture, temperatures and ground heat flux, as well as monitoring the water table. “The site and equipment will ultimately be used to address one of the grand challenges in hydrology: observationally based closure of the hydrological cycle,” Matt says.

In multi-disciplinary research with the Australian Nuclear Science and Technology Organisation, the UNSW researchers are using stable water isotopes to understand how much water is evaporated from the soil and how much is transpired by the plant itself. The experiment involves cutting-edge application of a range of instrumentation: radon detectors for measuring flux concentrations, LIDAR units for measuring the height of the boundary layer and eddy covariance systems for surface heat fluxes.

Matt is also deeply involved with an international multi-institution effort supported by the World Climate Research Programme to produce a global climatology of evapotranspiration. “It is the hydrological consequences of climate change that will most directly impact society,” he says. “If we are to appreciate these impacts and subsequently adapt to potential changes, a firm understanding of how the water cycle will be affected is paramount.”