Incidental Recharge in Montana

Incidental recharge occurs when normal irrigation operations result in infiltration of water to an underlying aquifer. The effect (groundwater recharge) is generally unintended but the impacts are profound. In Montana there are more than 7,000 miles of canals that divert about 11.6 million ac-ft/yr of surface water to irrigate 2 million acres; the average state-wide application rate of about 5.8 ac-ft/acre is greater than any other western state. In comparison, most Montana crops require less than 2 feet of water annually. Evaporation, conveyance losses, and irrigation returns, are therefore, as much as 7.6 million ac-ft/yr. Much of this water ends up as incidental recharge. To put this volume in perspective, total groundwater withdrawals in Montana (for irrigation, municipal, private domestic, industrial, and stock uses) are estimated to be about 305,000 ac-ft/yr, the lowest of any western state.

Surface-water irrigation has been practiced in Montana alluvial valleys for more than 100 years and has created artificial hydrologic conditions that many consider to be “normal.” These conditions are observable throughout Montana’s irrigated regions and include substantial recharge to shallow aquifers, wetlands creation, producing aquifers in arid upland areas, formation of lush riparian areas, and enhanced late season stream flows.

As land-use change (e.g. from agricultural to residential) and irrigation efficiencies (lining canals, converting from flood to sprinkler, etc.) change how people use water, it is important to recognize the hidden significance of incidental recharge. Changes that inadvertently reduce recharge may also cause unforeseen impacts to groundwater storage and decrease late-season stream flows resulting in adverse impacts to other water users and the environment. However, recognition of and accounting for incidental recharge within the water management infrastructure may provide beneficial management opportunities.