Aquifer inflows in balance with outflows.
There are several reaches of the Snake River, as well as small streams, that are hydraulically connected with the Snake River Plain aquifer. Ground water pumping from the aquifer initially causes a localized decline in the water table. That decline, or cone of depression, propagates progressively outward until it encounters hydraulically connected surface water bodies. The surface water bodies are subsequently depleted as a result of the pumping. There are four primary points to recognize about the effects of pumping on surface water in the eastern Snake River Plain. The reader should recognize that throughout this discussion the focus is on ground water pumping, but the same concepts, in reverse, apply to aquifer recharge. For more information on these concepts consult Surface Water and Ground Water Interactions.
1) Pumping effects propagate in all directions through the aquifer, not just down-gradient. This means it is possible for a down-gradient water user to affect stream flow in the upper reaches of the plain. This appears to be in contrast to the logic that "water flows downhill", but in fact it is not. Consider the analogy of an water tank in which water is entering from two sources on one end and continuously discharging from an overflow weir on the other end, as shown in the illustration. The water in the tank represents the Snake River Plain aquifer. The one source of fill is a faucet that does not make direct contact with the water level in the tank, therefore it cannot siphon. This represents recharge to the aquifer resulting from perched streams, precipitation, and irrigation sources. If the water level in the tank (i.e. aquifer) changes, it has no effect on this recharge. The second source of fill is a pipe connected below the water surface in the tank. This source represents hydraulically connected stream and river channels. If the water level in the tank drops, more water will flow out of the pipe and into the tank. If a pump is introduced to extract water from the middle of the tank, what will happen? Water levels in the tank will decline, causing the outflow over the weir to be reduced (similar to Thousand Springs). In addition, the inflow from pipe connected beneath the water surface will increase or outflow will decrease (similar to river connection in the Henrys Fork and the upper Snake River). This increase occurs despite the fact that water is flowing through the tank from the inlet to the outlet sides. In the Snake River Plain aquifer, if water is pumped, or recharged, in the center of the plain, gains and losses of the Snake River may be affected at many locations, not just along the flow lines. Flow lines in an aquifer have limited meaning when evaluating the propagation of pumping or recharge effects; they are significant with respect to water quality considerations. See linked image.
The radial propagation of pumping effects is evident from depletion graphs generated from a numerical model of the Snake River Plain aquifer. It is apparent from the graphs superimposed on the map of the Snake River Plain that effects do not preferentially propagate along the flow lines in the aquifer (see figure). Although pumping sites in the upper portion of the basin are aligned with flow lines that discharge near Thousand Springs, the majority of the impact is expected near American Falls. The simulations did not include effects of the hydraulic connection of the Henrys and upper Snake River, although they may be significant.
2) The total volume of water pumped and consumptively used from the Snake River Plain aquifer will ultimately be depleted from surface water sources and cause a reduction in ground water storage. It is obviously true that water pumped and consumptively used is water that would otherwise have gone somewhere else. If we again consider the water tank above, when our pump is taking water out of the tank, the discharge of the tank is diminished and the inflow from the submerged pipe is increased. The pumping has not exceeded the rate of recharge to the tank, but it has impacted outflow (analogous to springs) and inflow (analogous to a hydraulically connected river reach). In the Snake River Plain aquifer, the entire volume of water pumped and consumptively used will either be depleted from spring discharge, cause a corresponding increase in river losses, or cause a corresponding decrease in river gains. We can neither create nor destroy water in the process of pumping.
Pumping and aquifer recharge effects on surface water are often greatly attenuated.
Even though the entire volume of water consumptively pumped throughout the Snake River Plain aquifer will ultimately be drawn from surface water sources and ground water storage, that depletion may be distributed over time periods ranging from days to decades. The attenuation of the effects is related to the proximity of the pumping location and surface water body, and the hydraulic properties of the aquifer and stream. The expected approximate attenuation of pumping or recharge effects within about 25 miles of the Snake River in the Thousand Springs area is illustrated graphically. The effects of a 30 day pumping or recharge event are seen to become more attenuated as the site becomes progressively more distant from the river. The illustration uses aquifer properties typical of the Thousand Springs area that may not be representative of other locations within the Snake River Plain aquifer. At greater distances, the effects are much more attenuated. The effects of a 30 year pumping event in the northeast portion of the plain are expected to continue for decades even after the pumping ceases.
4) Our ability to estimate ground water pumping impacts on surface water resources is limited. Although this page has presented many illustrations that quantitatively relate consumptive ground water pumping or recharge to surface water depletion, these illustrations are approximate. They are the result of analytical and numerical models, but those models are the current representations of our limited knowledge about the real system. More detail can be found in the section on "Evaluation Methods".
The above points are extremely significant to water management in the Snake River and the Snake River Plain aquifer. Some of the primary management considerations are:
1) Negative impacts can result from consumptive ground water pumping, even though the rate of pumping does not exceed natural recharge. This is because other users or system needs are dependent on the aquifer.
2) There is no "no-impact" consumptive pumping. Every gallon of water consumptively used is not available somewhere else in the system where it would otherwise have existed.
3) Conjunctive administration of water rights under the Prior Appropriation Doctrine will be an involved process. Complication results from the propagation of effects to changes in consumptive ground water pumping or managed recharge in all directions and the attenuation of those effects in the aquifer and to the stream, and due to our limited knowledge about the aquifer.
aquifer recharge can offset some of the adverse economic and environmental impacts
of consumptive ground water pumping.
Information supplied by Idaho Water Resource Research Institute, University of Idaho December 1998
Authors: Dr. Gary Johnson, Donna Cosgrove, and Mark Lovell.
Graphics: Sherry Laney and Mark Lovell of Idaho Water Resources Research Institute.
All State of Idaho images and graphics created with GIS files obtained through Idaho Department of Water Resources Public Domain GIS unless otherwise noted.