Page 30 - Beaumont Basin Watermaster

Beaumont Basin Watermaster
2013 Reevaluation of the Beaumont Basin Safe Yield 3-Apr-15

4.0 Water Balance

The water balance is an accounting of all of the sources of groundwater inflow to and outflow
from the basin. Inflow terms are sources of groundwater recharge and include subsurface
underflow from upgradient areas (i.e. Edgar Canyon), mountain front recharge, areal recharge
from precipitation, infiltration of streamflow within unlined drainages, managed recharge in
spreading basins, return flow from agricultural and municipal irrigation in urban areas, and
return flow from individual septic systems. Groundwater outflow includes groundwater
pumping, evapotranspiration, and subsurface outflow. A summary of the water balance for the
Study Area for the period between 1972 and 2012 is provided in Table 2 and is shown
graphically on Figure 28.

4.1 Groundwater Recharge

4.1.1 Areal Recharge

A portion of precipitation falling on the undeveloped portions of the Study Area infiltrates past
the root zones of the plants and becomes deep percolation and groundwater recharge. Initial
estimates of areal recharge in the Study Area were developed based on the USGS’s rainfall-
runoff model Infil v3, which was originally developed and described in Rewis et al. (2006) and
has been updated with precipitation data through December 2012 (Hevesi, in Review). The
rainfall-runoff model includes the San Timoteo Creek and San Gorgonio River surface water
drainage basins, which fully encompass the Study Area (see Figure 10). A full description of the
rainfall-runoff model is provided in Rewis et al. (2006).

The updated rainfall-runoff model provided by the USGS (Hevesi, in Review) included a range
of potential recharge based on a range of assumed shallow soil hydrologic properties. Initial
estimates of areal recharge were based on the set of shallow soil properties that resulted in the
closest recharge to the original model (Rewis et al., 2006). These recharge estimates were then
adjusted during the groundwater model calibration process.

4.1.2 Groundwater Recharge within Unlined Stream Channels

Under natural pre-developed conditions, all of the streams in the Study Area are ephemeral,
meaning water flows in the channels only during precipitation events. Noble Creek, Little San
Gorgonio Creek, Marshall Creek, Smith Creek, and portions of Cooper’s Creek remain
ephemeral under developed conditions. Water in San Timoteo Creek and the portion of
Cooper’s Creek downstream of the City of Beaumont’s Wastewater Treatment Plant flows
perennially as a result of wastewater discharges and groundwater discharging at the land surface.

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