Together, these regions accounts for about half of total groundwater depletion in the United States since 1900 (Scanlon et al., As shown in Figure 1.10, both systems have been subject to serious groundwater overdraft, with water tables lowered by up to fifty meters or more for some portions of the aquifer during the studied period. However, Figure 1.10 shows that the reduction in water levels depends on the location within the affected areas. Similarly in California, the southern San Joaquin (SJ) and Tulare (T) counties are the main hotspots for groundwater level reduction, while the northern part of the valley, with better water endowment and surface water, is not really subject to groundwater depletion.