Page 86 - Beaumont Basin Watermaster - 2015 Annual Report

Section 5

Land Subsidence

In the first ten years of operations under the Judgment, a temporary surplus was established
that allows up to 160,000 acre-ft of overdraft within the Basin. The purpose of the temporary
surplus was to create room for the safe storage of supplemental water and to reduce losses
from the basin. A major concern is that overdraft of the groundwater basin may lead to the
lowering of groundwater levels and, subsequently, to land subsidence and ground fissuring.
To proactively address this concern, the STWMA and the Watermaster developed a
monitoring program specifically to assess the occurrence of subsidence from past
groundwater pumping and future pumping. To implement this program, the STWMA, on
behalf of the Watermaster, successfully applied for an AB303 Grant from the California
Department of Water Resources (DWR)

The Subsidence Monitoring Program was established in 2005. Initially, ground level
information for the 1928 to 2000 period was analyzed. In mid to late 2006, 72 benchmark
monuments were installed across the Basin and in nearby basins and an initial ground-level
survey conducted to establish the initial elevations of all benchmarks. A second survey was
conducted in 2007. A comparison analysis of the two surveying efforts reveals little vertical
change; in addition, this minimum subsidence was fairly evenly distributed across the Basin.
According to the program, the ground level survey of all benchmarks was to be conducted on
a tri-annual basis with the next round of survey scheduled for the spring of 2009. The 2009
survey was not conducted by Watermaster since it was determined that the level of
subsidence was minimal. No additional surveys are scheduled at this time.

Beaumont Basin Watermaster 2015 Annual Report – Final – December 2016 5-1