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f1 \12 '" ')8 uu u" .... , classified as dry or critic;a1, project recov-. ery. would occur. (These dry and critical y~ar hydrologic conditions occur nearly 30 percent of the time.) Project recovery would be accomplished.)Jy groundwater substitution. Groundwater substitution ~0U:1d necessitate that surface water us- ers forgo a portion of their surface water,., ' supply, thereby 1eavtng it in the river for use biothers. These repuctions in surface . witter supply would be made up by ex- . tracting additional groundwater that was placed in the'aquifer system during prior recharge years. .... One of the biggest issues facing any' . conjunctive use project in the Sacramento Valley is the "real water" issue. Will a con- junct~ve use project develop water that would not be available otherwise, or wtll it deplete Uows from adjacent rivers so tlie net available water supply is about the sarne? These are difficult questions to an- swer, but preliminary modeling studies suggest that these projects do create a~ additional water supply during dry peri- ods. Additional work remains before a de- finitive answer to these questions can be given. DWR is currently involved in activt~ ties to better answer these questions. , An~ther important issue facing conjunc- tive use projects ill the Sacramento Valley is tne potential ,for land subsidence occur- ring'during project recovery years, when increased demands are placed on the groundwater system. In the lower Sacra- mento Valley, soine areas have experienced more than six feet of permanent land sur- face lowering as a result of historic groundwater wtthdrawal. To obtain baseline subsidence conditions in the proj- ect area. an extenso meter was installed in 1993 and has been operational since. (This is one of three such instruments DWR op- erates in the Sacramento Valley.) J An extenso meter is a specially designed water well that extends belqw the typical groundwater extraction level in the region. The extensometer used for the American River basin site is about 1,000 feet deep. At the bottom of the well, a smal\ diameter steel pipe that extends to the surface is fixed in place. As aquifer sedirnents con- solidate (causing land subsidence). the pipe app~ars to protrude from'the top of the well casing. By accurately measuring the distance from the top of the pipe to the ground surface, a record of subsidence can be made. The instrument at the Amer- ican Basin site is capable of accurately recording subsidence to less than one-one hundredth of an inch. " J The extensometer has shown that the ground surface responds to changes in ' groundwater levels, although almo$t imperceptibily, and that essentially no permanent land subsidence has occurred at the American Basin site sin~e the instrument was installed. Figu~e 2 shows ~ the changes in groundwater levels at the extensometer site and the corresponding change in ground surface elevation. As noted on Figure 1. water levels in four dif- ferent aquifers are monitored. At this par- ticular, site. all four aquifers show similar . trends in water levels~a rise-'in water lev- els in the winter rffiny season and a de- cline during the summer inigation season, Any questions regarding the American Basin conjunctive use project or subSi- dence in the Sacramento Valley may be directed to Toccoy Dudley at the Central District (916) 227-7590. or John Fielden at the Divtsion of Planning (916) 653-9495. == . " /Check,olit the California Water Plan News at its website: http://rubicon.water.ca.gov .15 ,