The State of Idaho administers water rights according to the Prior Appropriation Doctrine (see Water Rights and Conjunctive Management).  Idaho fully recognized the need to implement conjunctive management of its water resources in 1984 when the Idaho Supreme Court determined that hydropower water rights of Idaho Power Company at Swan Falls Dam were not subordinated to junior upstream irrigation rights.  The case alerted water users in the basin that ground water pumping for irrigation was impacting spring discharge and flow in the Snake River, and that surface and ground water rights were to be jointly administered.  In 1992, a moratorium was imposed on new irrigation pumping on the eastern Snake River Plain (Idaho Department of Water Resources, 1996), which is still in place.  Subsequently, IDWR promulgated conjunctive management rules to provide a mechanism to stem conflicts between surface and ground water users when water supplies are limited.  IDWR has also formed water measurement districts in the Eastern Snake River Plain (see above figure) that require the measurement and reporting of ground-water pumping at rates exceeding 0.24 cfs, or irrigating areas greater than 5 acres.  

Depletion of spring flows and declining ground-water levels are a collective effect of drought, changes in surface-water irrigation acreage and practices, and ground-water pumping.  A recent model study (IDWR, 1997) indicates that the collective effects of all ground-water pumping within the boundaries of the eastern Snake River Plain depletes spring discharge and flow of the Snake River by about 900,000 acre-feet per year (1,200 cfs).  The same study projects that changes in surface water irrigation practices have depleted the spring discharge by about 500,000 acre-feet per year (700 cfs).  IDWR and the courts are placed in the position of determining the degree to which junior ground-water users have injured senior surface-water users.  Isolating cause and effect relationships on a case by case basis will be difficult and costly.

Although most water users and managers accept the concept that ground water use depletes surface water supplies, it is not necessarily accepted that depletion constitutes legal injury.  The conjunctive management rules provide for weighing the time of year in which depletion is experienced, the efficiency of use of the senior water users, and the maximum economic benefits of all uses, against the possibility of "futile call".

The State's conjunctive management rules allow junior priority water users to mitigate injury to senior surface and ground water users.  One of the mechanisms is to provide supplemental recharge to the aquifer.  Both surface and ground water users have embraced artificial or managed recharge as a means of avoiding future conflicts and litigation.