No abstract

Increasing demands on western water are causing a mounting need for the conjunctive management of surface water and ground water resources. Under western water law, the senior water rights holder has priority over the junior water rights holder in times of water shortage. Water managers have been reluctant to conjunctively manage surface water and ground water resources because of the difficulty of quantification of the impacts to surface water resources from ground water stresses. Impacts from ground water use can take years to propagate through an aquifer system. Prediction of the degree of impact to surface water resources over time and the spatial distribution of impacts is very difficult. Response functions mathematically describe the relationship between a unit ground water stress applied at a specific location and stream depletion or aquifer water level change elsewhere in the system. Response functions can be used to help quantify the spatial and temporal impacts to surface water resources caused by ground water pumping. This paper describes the theory of response functions and presents an application of transient response functions in the Snake River Plain, Idaho. Transient response functions can be used to facilitate the conjunctive management of surface and ground water not only in the eastern Snake River Plain basin, but also in similar basins throughout the western United States.

Declining ground-water levels and spring discharges have heightened water user concerns about the sustainability of the Snake River Plain aquifer in southern Idaho. Diminished recharge from surface water irrigation and increased irrigation pumping have been depleting the aquifer at a rate of about 350,000 acre-feet/year. Previously, aquifer conditions were treated as an uncontrollable consequence of weather and development activities. With increasing competition for available water, the State appears to be progressing through a three-stage process of recharge management. The first stage is that which has occurred historically, where recharge is largely an incidental effect of surface water irrigation. The second stage is the implementation of intentional recharge with little regard to identifying or maximizing benefits. Idaho has been at this stage for the past few years. The State is entering a third stage in which recharge sites will be located and designed to meet specific water user and environmental objectives.Preliminary estimates using numerical and analytical models demonstrate that managed recharge within a few miles of the river will result in short-term increases in spring discharge. More distant recharge sites are needed to provide longer-term benefits. The primary challenge facing implementation of the managed recharge program will be the balancing of economic and environmental costs and benefits and to whom they accrue.(KEY TERMS: water management; water policy; water development; aquifer recharge; artificial recharge; managed recharge; groundwater hydrology.)The appropriation of ground water and spring flows created a dependence of water users on the incidental recharge from surface water irrigation. Incidental recharge, however, is not controlled, nor is it 'Paper No.

Changes in irrigation and land use may impact discharge of the Snake River Plain aquifer, which is a major contributor to flow of the Snake River in southern Idaho. The Snake River Basin planning and management model (SRBM) has been expanded to include the spatial distribution and temporal attenuation that occurs as aquifer stresses propagate through the aquifer to the river. The SRBM is a network flow model in which aquifer characteristics have been introduced through a matrix of response functions. The response functions were determined by independently simulating the effect of a unit stress in each cell of a finite difference groundwater flow model on six reaches of the Snake River. Cells were aggregated into 20 aquifer zones and average response functions for each river reach were included in the SRBM. This approach links many of the capabilities of surface and ground water flow models. Evaluation of an artificial recharge scenario approximately reproduced estimates made by direct simulation in a ground water flow model. The example demonstrated that the method can produce reasonable results but interpretation of the results can be biased if the simulation period is not of adequate duration.