Monday, December 10, 2007

Improving drought forecasting

Drought is called "the creeping disaster" because most people are unaware that their local water supplies are shrinking until the alarm goes off, announcing conservation measures and high-level concern with the impending crisis. With the prospect that more areas around the world may be subject to drought conditions - long range or short - we need better forecasting techniques for earlier detection of approaching drought. Such early detection will allow preparations to be made well in advance of the actual water shortage.

This article from Sign of the Times describes a focus on streamflow measurement and cloud reflectivity as a means of discovering higher risk for drought. One interviewee was Dr. Ashutosh Limaye, a hydrologist at the National Space Science and Technology Center (NSSTC) in Huntsville, Alabama. He described the stream flow part.

Streamflow is a term used by water management specialists to mean, very simply, the amount of water in streams and rivers. Areas of drought have reduced streamflow, and experts believe they can better forecast droughts by studying this key indicator of dry conditions.

"Streamflow is always changing, from day to day and even minute to minute, for a wide variety of reasons: evaporation from the soil and from bodies of water, runoff from rainfall and snowmelt, transpiration by plants and trees, and other natural and human influences," [Limaye] explains. National Weather Service River Forecast Centers have to consider all of these factors when they forecast streamflow.

"If we can help forecasters estimate any of these elements more accurately, they can better predict drought conditions months in advance," says Dr. Limaye. "These predictions are critical because they influence important decisions about measures like withholding water in reservoirs and restricting water use."

The reflectivity part? Dr. Mike Smith of the National Oceanic and Atmospheric Administration (NOAA) suggests that satellite measurement of cloud cover could provide closer estimates of evaporation rates that directly affect stream flow.

Why clouds? "Because most of the water that falls on the ground goes up in evaporation, evaporation is a huge component of the total surface water," explains Limaye. "So it's important to get those numbers right. Clouds affect radiation, which has a big influence on evaporation."

National Weather Service cloud cover estimates from the 1960s to the 1990s went like this: A trained technician literally walked outside, tilted his or her head back, eyeballed the sky like an old farmer, and rated the cloud cover on a 1-8 scale.

In the 90s, these manual observations were replaced by a device called a "ceilometer," part of the Automated Surface Observing System (ASOS), which has a laser beam that aims at the sky. Returns from this beam are used to detect clouds.

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