Evaluating a Water Conservation Response to Climate Change in the Lower Boise River Basin

Aquifers created or sustained by seepage losses from Bureau of Reclamation (Reclamation) Projects extend over vast areas of western states. Yet agricultural water conservation measures such as canal lining top the list of State and Federal policies for mitigating the effect of water shortages brought about by climate change. Cost benefit analysis (CBA) of new Reclamation water conservation infrastructure such as canal lining or piping is too often Project-specific, and detached from basin hydrology. The value of canal seepage as a positive externality is thus ignored in CBA. A basin-wide approach to hydro-economic modeling that accounts for the externalized costs and benefits of both canal seepage and new canal lining conservation insures that incidental aquifer recharge is recognized in CBA of Federally financed irrigation water conservation measures. Integrated hydrologic and partial equilibrium models are employed in the Lower Boise River basin to calculate the foregone benefit to non-project groundwater and drain water irrigation of a hypothetical Boise Project canal lining response to projected climate change water shortages. Basin-wide hydrologic response data is used to compute shifts in non-project groundwater supply functions and drain water supply constraints, and a base-case water supply scenario is compared to six climate change scenarios in which projected water shortages are offset by lining of project canals. The foregone net benefit to non-project groundwater and drain water irrigation resulting from elimination of the canal seepage externality (US$4.4–22.6million depending on the scenario) outweighs the increase in net benefit to Boise Project irrigation by canal lining (US$1.4–19.3million). On average, foregone groundwater and drain water irrigation benefit exceeds restored canal irrigation benefit by about 38%. Canal lining conservation is unable to restore total basin-wide irrigation net benefit to the base-case level in any of the climate change scenarios; rather it shifts the foregone benefit of climate change shortages from project canal irrigation to non-project groundwater and drain water irrigation. The canal lining CPA is not a complete accounting of either costs or benefits of canal lining conservation. On the cost side, only the foregone benefits of eliminating the positive canal seepage externality are calculated; construction and maintenance costs of canal lining are omitted. On the benefit side, Arrowrock canal irrigators are assumed to be the sole beneficiary of reduced seepage losses.