Month-by-month water storage in California from 2006 through 2014, estimated from the regional GPS network. The scale is the average water-layer thickness on a 25-kilometer grid patch. Blue represents higher than average water load, while red is lower than average. The summer months experience a loss of water in the Sierra Nevada and Central Valley that is replenished in the mountains during the wet winter months as reservoirs fill and snow accumulates. In the Central Valley, water storage has been continuously decreasing due to large scale agriculture. The seasonal change in mass deforms the crust and the associated stress changes modulate regional seismicity.