The common prescription for dealing with limited supplies of water is to improve irrigation efficiency, that is, to reduce gross delivery to farm fields while maintaining full crop production. The public and some policy makers continue to assume that the water thus saved constitutes a new supply that may be applied to other uses. Scientists and hydrologists have long understood that the nonconsumed fraction of applied water often becomes the source for another human or ecosystem purpose after leaving the field, and irrigation improvements interrupt these uses. Thoughtful researchers have provided valuable guidance in conceptual frameworks and analysis procedures to address this issue. Researchers have also noted empirically that total consumptive use often increases when efficiency improves, and have cited case‐specific reasons that this occurs. This paper shows it is a general case arising from rational producer behavior in equating the marginal cost of a production input (irrigation water) with its marginal benefit. At any nonzero marginal cost of water, improving irrigation efficiency enables the irrigator to be willing and able to purchase a quantity of irrigation water that sustains more consumptive use than was possible with the prior, less efficient system.A case study indicates that improving efficiency from 60 to 80% reduces field delivery of irrigation water by 15% but increases consumptive use by 3%. Copyright © 2013 John Wiley & Sons, Ltd.

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Managed aquifer recharge (MAR) is typically used to enhance the agricultural water supply but may also be promising to maintain summer streamflows and temperatures for cold-water fish. An existing aquifer model, water temperature data, and analysis of water administration were used to assess potential benefits of MAR to cold-water fisheries in Idaho’s Snake River. This highly-regulated river supports irrigated agriculture worth US $10 billion and recreational trout fisheries worth $100 million. The assessment focused on the Henry’s Fork Snake River, which receives groundwater from recharge incidental to irrigation and from MAR operations 8 km from the river, addressing (1) the quantity and timing of MAR-produced streamflow response, (2) the mechanism through which MAR increases streamflow, (3) whether groundwater inputs decrease the local stream temperature, and (4) the legal and administrative hurdles to using MAR for cold-water fisheries conservation in Idaho. The model estimated a long-term 4%–7% increase in summertime streamflow from annual MAR similar to that conducted in 2019. Water temperature observations confirmed that recharge increased streamflow via aquifer discharge rather than reduction in river losses to the aquifer. In addition, groundwater seeps created summer thermal refugia. Measured summer stream temperature at seeps was within the optimal temperature range for brown trout, averaging 14.4 °C, whereas ambient stream temperature exceeded 19 °C, the stress threshold for brown trout. Implementing MAR for fisheries conservation is challenged by administrative water rules and regulations. Well-developed and trusted water rights and water-transaction systems in Idaho and other western states enable MAR. However, in Idaho, conservation groups are unable to engage directly in water transactions, hampering MAR for fisheries protection.

Water supply uncertainty continues to threaten the reliability of regional water resources in the western United States. Climate variability and water dispute potentials induce water managers to develop proactive adaptive management strategies to mitigate future hydroclimate impacts. The Eastern Snake Plain Aquifer in the state of Idaho is also facing these challenges in the sense that population growth and economic development strongly depend on reliable water resources from underground storage. Drought and subsequent water conflict often drive scientific research and political agendas because water resources availability and aquifer management for a sustainable rural economy are of great interest. In this study, a system dynamics approach is applied to address dynamically complex problems with management of the aquifer and associated surface‐water and groundwater interactions. Recharge and discharge dynamics within the aquifer system are coded in an environmental modeling framework to identify long‐term behavior of aquifer responses to uncertain future hydrological variability. The research shows that the system dynamics approach is a promising modeling tool to develop sustainable water resources planning and management in a collaborative decision‐making framework and also to provide useful insights and alternative opportunities for operational management, policy support, and participatory strategic planning to mitigate future hydroclimate impacts in human dimensions.