To Eastern Sierra residents, in most years, annual run-off means the streams and canals rise and pasture lands start to green-up. For Los Angeles Department of Water and Power, run-off is the city’s life’s blood, the calculation that figures into the department’s operations for the next 12 months. In years like 2017, run-off is an all-consuming 24/7 concern. So, how do they figure it out? Eric Tillemans, LADWP engineer, gave the Inyo County Board of Supervisors a beginner’s course in Run-Off 101 at a recent meeting. Here’s how it works.