When you dive into that salad full of lettuce grown in the
American West, there's a good chance you are enjoying the
product of irrigation from an underground water source. These
hidden groundwater systems are precious resources that need
careful management, but regulatory groups have a hard time
monitoring them, owing to a lack of accurate data.  Now,
scientists at Stanford have found a way to cheaply and
effectively monitor aquifer levels in agricultural regions using
data from satellites that are already in orbit mapping the shape
of Earth's surface with millimeter precision.  The amount of
water in a groundwater system typically grows and shrinks
seasonally. Rainfall and melted snow seep down into the system
in the cooler months, and farmers pull water out to irrigate
their crops in the warmer, drier months.  In agricultural
regions, groundwater regulators have to monitor aquifer levels
carefully to avoid drought. They make do with direct
measurements from wells drilled into the aquifers, but wells are
ge