Page 11 - Beaumont Basin Watermaster

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

Under pre-developed conditions, the groundwater basin would be in a state of equilibrium such
that the inflow and outflow would balance and there would be no significant long-term change in
storage. Under this condition, groundwater levels would be relatively stable (see Figure 3).

Under developed land use conditions, the water balance changes as groundwater is pumped from
the basin for irrigation and municipal supply (see Figure 4). Lowering of the groundwater table
resulting from pumping reduces the amount of groundwater that would otherwise leave the basin
and reduces evapotranspiration losses in areas of shallow groundwater (e.g. San Timoteo Creek).
Some of the pumped groundwater used for irrigation infiltrates past the roots of the plants and
returns to the groundwater as return flow. Groundwater return flow also occurs as a result of
discharges from individual septic systems. Other sources of recharge to the groundwater under
developed land use include wastewater treatment plant discharges and artificial recharge in
spreading basins. Finally, the balance of precipitation infiltration and runoff changes in areas of
buildings and roads that were previously native soil and vegetation.

The water balance equation for developed land use conditions can be modified as follows:

(I ss + I mfr + I pr + I str + I rf + I ar) – (O ss + O et + O p) = S (3)

Where:

I rf = Inflow from Return Flow

I ar = Inflow from Artificial Recharge
O p= Outflow from Groundwater Pumping

Under developed basin conditions, if the inflow terms exceed the outflow terms, then the
groundwater in storage increases (become positive) and groundwater levels rise. If the outflow
terms exceed the inflow, then the groundwater in storage decreases (become negative) and
groundwater levels drop.

The Safe Yield of a developed groundwater basin is the combination of pumping and recharge
under a given land use condition that results in no long-term change in groundwater storage in
the basin. The water balance equation can be rearranged and simplified to estimate Safe Yield:

3