Abstract
Incidental recharge occurs when normal irrigation operations result in infiltration of water to an underlying aquifer. The effect (groundwater recharge) is generally unintended but has profound impact. In the Bitterroot Valley there are more than 500 miles of canals and laterals that divert about 374,000 ac-ft/yr of surface water to irrigate about 85,000 acres. Most of the diverted water, as much as 267,000 ac-ft/yr, either evaporates, runs off to the river, or infiltrates (as conveyance losses and irrigation returns). The component that infiltrates becomes incidental groundwater recharge.
Surface-water diversions, and the associated groundwater recharge, have been taking place for more than 100 years in Bitterroot Valley, creating artificial hydrologic conditions that many consider to be “normal” but are not necessarily natural. The results are observable throughout the valley and include creation of aquifers and wetlands in arid upland areas, formation of lush riparian areas, and enhanced late-season stream flows.
Understanding the nature and magnitude of incidental recharge in the Bitterroot Valley is important because rapid population growth and associated land-use changes have the potential to stress and/or alter the hydrologic system. Population growth has increased the demand on groundwater resources as demonstrated by the proliferation of private domestic wells—so called “exempt wells”. The number of domestic wells more than doubled between 1990 and 2010; well densities exceed 300 per square mile in places, the highest in the state. The population growth is also driving landuse change from agricultural to residential, reducing irrigated acreage, and potentially reducing the amount of recharge to the shallow groundwater system.
Despite the increase in groundwater use and land-use change from agricultural to rural residential, long-term groundwater level and stream flow data do not show measurable impacts, on a basin-wide scale, to groundwater storage or Bitterroot River baseflows.
Start Date
5-3-2010 3:00 PM
End Date
5-3-2010 3:30 PM
Document Type
Presentation
Incidental Recharge and Exempt Wells in the Bitterroot Valley, Southwest 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 has profound impact. In the Bitterroot Valley there are more than 500 miles of canals and laterals that divert about 374,000 ac-ft/yr of surface water to irrigate about 85,000 acres. Most of the diverted water, as much as 267,000 ac-ft/yr, either evaporates, runs off to the river, or infiltrates (as conveyance losses and irrigation returns). The component that infiltrates becomes incidental groundwater recharge.
Surface-water diversions, and the associated groundwater recharge, have been taking place for more than 100 years in Bitterroot Valley, creating artificial hydrologic conditions that many consider to be “normal” but are not necessarily natural. The results are observable throughout the valley and include creation of aquifers and wetlands in arid upland areas, formation of lush riparian areas, and enhanced late-season stream flows.
Understanding the nature and magnitude of incidental recharge in the Bitterroot Valley is important because rapid population growth and associated land-use changes have the potential to stress and/or alter the hydrologic system. Population growth has increased the demand on groundwater resources as demonstrated by the proliferation of private domestic wells—so called “exempt wells”. The number of domestic wells more than doubled between 1990 and 2010; well densities exceed 300 per square mile in places, the highest in the state. The population growth is also driving landuse change from agricultural to residential, reducing irrigated acreage, and potentially reducing the amount of recharge to the shallow groundwater system.
Despite the increase in groundwater use and land-use change from agricultural to rural residential, long-term groundwater level and stream flow data do not show measurable impacts, on a basin-wide scale, to groundwater storage or Bitterroot River baseflows.