Introduction:
Part of the importance of surface and ground water interaction lies in water rights systems.  Surface and ground water in Idaho and many of the western states are managed under the Prior Appropriation Doctrine (Idaho Department of Water Resources and the National Water Rights Digest).  The doctrine generally follows the principle of  “first in time is first in right”.  That is, a priority is established for water rights that are based on the date that the water was first put to beneficial use.  Junior priority rights may be fully or partially curtailed in times of short water supply to ensure that senior appropriators (water right holders) receive their full appropriation (legal share).  The Snake River basin includes portions of Wyoming, Idaho, Nevada, Oregon, and Washington.  Each of these states applies the Prior Appropriation Doctrine within their respective boundaries.  The following discussion focuses on the Eastern Snake River Plain in Idaho due to the significance of ground water and surface water interaction in that area.

Application of the Prior Appropriation Doctrine to conjunctively managed surface and ground water systems is much more difficult than application to the exclusively surface water systems for which it was initially adopted.  Diversion of surface water from a stream impacts downstream users in an amount nearly equal to the rate of diversion, often within a period of a few days or less.  Ground water pumping effects propagate through an aquifer in all directions.  Ultimately, these effects may reach surface water bodies and result in depletion.  Rates of depletion, however, often are less than pumping rates and extended over much longer periods of time.  Graphs have been developed illustrating the attenuation of continuous pumping effects in the Snake River Plain aquifer (to see illustration click here).

Conjunctive Management Rules
 In general, surface water rights on the Snake River and tributaries were developed before water rights for irrigation wells.  Consequently, ground water pumping rights for irrigation are typically junior to surface water rights.  Idaho’s conjunctive management rules  hold junior ground water users (excluding domestic use) partially responsible for spring and river depletion that potentially results in injury to senior surface water right holders.  Many unanswered questions still exist relative to conjunctive water management in Idaho.  These questions include:

1) To what degree does depletion of surface water sources constitute injury to a surface water right holder?

2) To what individual degree are the many ground water users in a basin responsible for a depleted surface water supply?

3) How should depletion rates be determined?

4) How can the prior appropriation doctrine best be applied when the time lags from pumping to depletion effect are so long that curtailing pumping may have no effect on surface water supplies for years in the future?

5) Are junior ground water users responsible for depletion resulting from past pumping practices?

These and other questions are yet to be answered in Idaho.  The next decade will no doubt witness significant changes in water management and use in southern Idaho.