Victor M. Ponce

♦ Fact No. 1 ♦

All groundwater reservoirs are temporarily holding water in transit from a place of recharge to a place of discharge.

Water originating in precipitation and snowmelt seeps into the ground and moves in a general downward direction until it reaches the water table. Then it moves along a nearly horizontal hydraulic gradient to discharge naturally somewhere downstream, where the water table intersects lakes, streams and rivers in the vicinity. Thus, groundwater is essentially borrowed surface water. If groundwater is pumped and consumed (not returned through recharge), it will result in a decrease of the natural discharge, i.e., in the decrease of the flow entering neighboring surface waters.

 Typical pattern and direction of groundwater flow

Fig. 1   Typical pattern and direction of groundwater flow.

♦ Fact No. 2 ♦

A pristine groundwater reservoir is in steady state, with inflows equal to ouflows.

In a groundwater reservoir that is not subject to exploitation, inflows through natural recharge equal outflows through natural discharge. Therefore, net recharge is zero.

♦ Fact No. 3 ♦

All pumping comes from capture; the greater the intensity of pumping, the greater the capture.

Every gallon of water pumped from a well comes from the capture of [increased] recharge and [decreased natural] discharge, and in cases of groundwater depletion, it comes increasingly from aquifer storage. Thus, pumping results in more water flowing into the well and less water flowing out [of the groundwater system] as aquifer discharge. In extreme cases, pumping comes from depletion of the water stored in the aquifer. Capture is directly proportional to the intensity of pumping; therefore, heavy, long-lasting pumping can result in the sequestering of all natural discharge.

♦ Fact No. 4 ♦

Capture comes from decreases in natural discharge and increases in aquifer recharge.

The decrease in natural discharge caused by capture impairs neighboring freshwater bodies and groundwater-dependent ecosystems. Capture has wide-ranging effects, including the drying out of lakes and wetlands, the loss of baseflow, and the loss of riparian ecosystems.

♦ Fact No. 5 ♦

Natural discharge supports riparian, wetland, and other groundwater-dependent ecosystems, and the baseflow of streams and rivers.

Baseflow is surface water that originates in groundwater exfiltration as natural discharge. Therefore, reducing natural discharge reduces baseflow. Reducing natural discharge may also affect wetlands and riparian ecosystems. Thus, pumping and capture directly affect the volume of surface water and the ecosystems dependent on shallow groundwater.

Geometric model of a groundwater reservoir

Fig. 2   Geometric model of a groundwater reservoir.

♦ Fact No. 6 ♦

The traditional concept of safe yield, which equates safe yield with natural recharge, is flawed and has been widely discredited.

Safe yield cannot be equated with natural recharge, because in nature, the latter goes on to form part of natural discharge. Thus, in nature, net recharge is zero, and the theoretical safe yield would have to be zero, i.e., no pumping allowed. Since the concept of safe yield implies the possibility of pumping, an inconsistency arises which cannot be resolved.

♦ Fact No. 7 ♦

Sustainable yield depends on the amount of capture, and whether this amount can be socially accepted as a reasonable compromise between a policy of no use and the use of all natural discharge.

A certain amount of "sustainable" yield may be accepted to avoid the sequestration of all natural discharge, which would effectively impair all natural-discharge-dependent ecosystems.

♦ Fact No. 8 ♦

Capture depends on usage and is not related to size or hydrogeological characteristics of the aquifer, or to the natural recharge.

Capture does not depend on aquifer size (hydrogeology) or amount of natural recharge (hydrology). Thus, there is no physical relation between [capture or] sustainable yield and natural recharge. However, for practical purposes, a working value of sustainable yield may be expressed as a percentage of natural recharge.

♦ Fact No. 9 ♦

Sustainable yield is a moving target, to be determined after judicious study and appraisal of all issues regarding groundwater utilization.

There is no consensus on how to determine sustainable yield. Studies to be made on a local basis will determine the optimum or reasonable compromise between conflicting interests. In addition to hydrogeology, the disciplines of surface-water hydrology and ecology, and the related socioeconomic and legal aspects have a bearing on the analysis.

♦ Fact No. 10 ♦

A reasonably conservative estimate of sustainable yield takes up all deep percolation as sustainable yield.

Deep percolation is the groundwater that flows directly into the ocean, bypassing the surface waters. Therefore, it does not affect natural discharge. Any amount of deep percolation captured by pumping will not affect the inland fresh surface waters. Reasonable care is necessary in coastal areas due to the possibility of saline intrusion.

Global components of precipitation

Fig. 3   Global components of precipitation.

♦ Fact No. 11 ♦

Sustainable yield may also be expressed as a percentage of natural recharge.

A working value of sustainable yield can be taken as a suitable percentage of natural recharge. Conservative values are around 10%, while average values are around 40%.

♦ Fact No. 12 ♦

Interdisciplinary studies are needed to develop more experience in [sustainable] yield-to-recharge percentages applicable on a local, subregional, and regional basis.

There is an urgent need for studies to develop working values for [sustainable] yield-to-recharge percentages on a local, subregional, and regional basis. These studies must be interdisciplinary in nature.

♦ Fact No. 13 ♦

Sustainability may be fostered by enlightened management seeking to: (a) capture rejected recharge, (b) encourage clean artificial recharge, and (c) limit negative artificial recharge.

Groundwater yield can be increased by the capture of additional surface water [capturing rejected recharge], increasing human-induced recharge, and decreasing [or limiting] the possibility of additional recharge losses to surface water [negative artificial recharge].

♦ Fact No. 14 ♦

Baseflow conservation is being considered as the standard against which to measure groundwater sustainability.

Groundwater sustainability can be assessed through baseflow conservation. Thus, sustainable yield is that which sustains the conservation of a selected baseflow level.

♦ Fact No. 15 ♦

Sustainability reflects resource conservation policy; the more conservative a policy, the more sustainable it is likely to be.

Sustainability goes hand-in-hand with conservation. A strong policy of resource conservation is a strong policy of resource sustainability.