Source: California Department of Water Resources.
Q & A
ON HOW MUCH GROUNDWATER TO PUMP
IN A SUSTAINABLE FASHION
Victor M. Ponce
WHY SHOULD GROUNDWATER USE BE LIMITED?
Because much of the groundwater is in transit to the surface waters.
Therefore, pumping groundwater and not returning it
to the ground (effectively, "consumptive use") amounts to capturing potential quantities of surface water.
When the surface water has been fully allocated, a conflict arises.
WHAT SHOULD BE THE LIMIT?
The amount of groundwater that can be pumped depends on its interaction with surface waters, and whether those surface waters
have been fully allocated or not. Special care is required when groundwater is pumped near streams.
A first approximation is to limit groundwater pumping to an amount equal to the deep percolation, globally
estimated at 2% of precipitation.
This limit may be increased in cases where detailed interdisciplinary studies warrant it.
WHY SHOULD THE USE OF GROUNDWATER BE REGULATED?
Groundwater is indeed the ultimate "commons." Therefore, Hardin's "Tragedy of the Commons," applies. In the absence of regulation, any natural resource
held in common would tend to be overexploited until it becomes unsustainable. Thus, it makes perfect sense to regulate groundwater use.
WHY PUMP GROUNDWATER AT ALL?
In regions where surface water is limited, the use of groundwater is often essential to support socio-economic development.
Thus, the only way to avoid depletion is to limit groundwater use to a reasonable, sustainable amount.
WHAT STUDIES ARE REQUIRED IN AN EVALUATION OF GROUNDWATER POTENTIAL?
First and foremost, an assessment of groundwater potential should evaluate the effect of pumping on the neighboring surface waters and
related groundwater-dependent ecosystems.
The sustainable amount to pump is not related to the aquifer volume or to the aquifer recharge,
but rather to the maintenance of baseflow in the local and regional vicinity. Therefore, baseline studies of baseflow should be
a required component of an evaluation of sustainable groundwater potential.
HOW IS BASEFLOW RELATED TO GROUNDWATER?
Baseflow is the dry-weather flow of streams and rivers.
It originates in groundwater; therefore, it is related to groundwater.
Depletion of groundwater results in the disappearance of baseflow.
This cause-effect relation has been thoroughly demonstrated.
HOW IS GROUNDWATER SUSTAINABILITY ASSESSED?
To assess groundwater sustainability, there is a need for interdisciplinary studies that consider the
geological, hydrogeological, hydrological, ecological, and economic relations.
Pending such studies,
the 2% rule
(groundwater use should not exceed 2% of precipitation) may be taken as a reasonable estimate of sustainable groundwater utilization.