Tulare County has two primary water sources: surface water and groundwater. The specific mix of surface to groundwater used during any year depends on precipitation: much more surface water is used during wet years, and much more groundwater is used during dry years.
During wet years (precipitation index greater than 100%), more surface water is used, reducing groundwater pumping and withdrawals from deep percolation . During dry years, the inverse occurs.
Figure 2. Groundwater vs. Surface Water Supply Use in a Dry vs. Wet Year
Figure 3. Changes in Groundwater Elevations (Water Years 2011-2016)
These five counties, with very high water demands in critically overdrafted groundwater basins, need to become drought resilient as soon as possible. All have experienced substantial land subsidence due to over-pumping of groundwater basins, are contending with significant water quality concerns due to decades of agricultural runoff carrying fertilizers and pesticides into groundwater basins and into natural waterways, and have had significant dry hydrology over the past ten years.
Note that except for a very small blue area in Kern County and several green areas in Tulare and Kern, groundwater elevations decreased considerably since Water Year 2011 (red areas).
Of the five South San Joaquin Valley counties, Tulare experienced the most serious drought impacts:
- Tulare has little diversity in its water supply portfolio, meeting most of its urban water demand with groundwater.
- About 44% of the County’s residential customers – 205,000 - are served by 41 small community water systems.  Ninety nine percent of the water provided to residents by community water systems is groundwater. 
- The State Water Resources Control Board (SWRCB) found that 40% of tested wells by community water systems exceeded the Maximum Contaminant Level (MCL) for nitrates.
- Tulare had a further daunting challenge: thousands of residents who relied upon a private well as their sole water resource had no drinking water when wells went dry.
These factors, combined with very low annual precipitation over the past ten years, created serious problems for the County and its residents. Residents in remote areas that historically provided their own water supplies had no groundwater to pump. The challenges to East Porterville, an unincorporated area of the county adjacent to the City of Porterville, were well publicized, both locally and nationally. The state Office of Emergency Services (OES) trucked water to residents left without enough water to meet critical needs for drinking, cooking, and sanitation. While the state has worked closely with local governments to expedite connection of residents to municipal water systems, water deliveries continue today to some communities.
 “QuickFacts.” U.S. Census Bureau. July 1, 2017. https://www.census.gov/quickfacts/fact/table/tularecountycalifornia/PST045217
 “Deep percolation” refers to water that percolates the ground beyond the lower limit of the root zone of plants into groundwater. Source: DWR Water Supply & Balance Data Interface Tool, LITE v.9.1.
 “Land subsidence is a gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials.” Source: “Land Subsidence in California.” U.S. Geological Survey. July 8, 2018. https://ca.water.usgs.gov/land_subsidence/
 California Health and Safety Code Section 116275(i) defines a “community water system” as a public water system that serves at least 15 service connections used by yearlong residents or that regularly serves at least 25 yearlong residents of the area served by the system.
 State Water Resources Control Board Report to the Legislature. Communities that Rely on a Contaminated Groundwater Source for Drinking Water. January 2013.