The 30-acre pear orchard in the Sacramento-San Joaquin River Delta has been in Brett Baker’s family since the end of the Gold Rush. After six generations, though, California’s most precious resource is no longer gold – it’s water. And most of the state’s freshwater is in the delta.  Landowners there are required to report their water use, but methods for monitoring were expensive and inaccurate. Recently, however, a platform called OpenET, created by NASA, the U.S. Geological Survey (USGS), and other partners, has introduced the ability to calculate the total amount of water transferred from the surface to the atmosphere through evapotranspiration. This is a key measure of the water that’s actually being removed from a local water system. It’s calculated based on imagery from Landsat and other satellites.   “It’s good public policy to start with a measure everyone can agree upon,” Baker said.