California's hydrology is complex, involving interactions between geology, climate, plants, animals, and humans. Geographers and hydrologists study these relationships and how water moves through watersheds, entering and exiting the system through precipitation, evaporation, runoff, and groundwater. They work to help manage water resources and improve living conditions. On average, 200 million acre-feet of water falls as precipitation in California each year, but human water diversions and projects have significantly altered natural hydrologic cycles and watersheds to supply water for urban and agricultural uses across the state.