SGMA implementation is envisaged over a 20-year horizon. The Act enables the formation of local Groundwater Sustainability Agencies (GSAs), which oversee basins in their areas. Of more than 500 alluvial basins, SGMA has identified 21 that are critically overdrafted. An additional 94, representing 90% of total groundwater pumping in California, are considered high and medium priority. Currently, plans for 8 of the 21 critical overdraft basins have been approved.
SGMA defines sustainability in terms of six conditions to be avoided: 1) lowering groundwater levels, 2) reduction of groundwater storage, 3) seawater intrusion, 4) degraded water quality, 5) land subsidence, and 6) depletion of interconnected streams. As an example of land subsidence, Godwin told of the Friant-Kern Canal, which has had a 60% reduction in its ability to convey water, due to the partial collapse of land subsidence. He noted that when land above an aquifer starts to collapse, it threatens bridges, railways, pipelines and water pipes.
Godwin explained that determining the appropriate metrics for groundwater pumping, water levels and recharge is a complex process. Each local agency needs to evaluate the level of demand for water, sources of supply, risk of contamination, performance of local wells, and many other factors.
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