Title
Long-Term Trends in Base Flow, Precipitation, and Groundwater Development in the Bitterroot Watershed
Abstract
Most municipalities and residences in the Bitterroot watershed rely on groundwater. Between 1970 and 2012, the number of wells in the watershed increased eight-fold to more than 20,000 (GWIC, http://mbmggwic.mtech.edu/). Well densities on non-federal land are approximately 30 wells per square mile, the highest in the state. Groundwater pumping must be balanced by either decreased groundwater storage or a combination of increased recharge and/or decreased groundwater discharge (capture). Usually, the groundwater discharge component (often seen as baseflow in streams) is captured by groundwater development.
To assess potential capture of Bitterroot River flow by groundwater development, the authors calculated daily maximum, median, and mean flows; 7-day minimum flows; and long-term flow trends for the 1935-2013 period of record (USGS gauge 12352500, Bitterroot River at Missoula) using Exploration and Graphics for River Trends (EGRET) software. Seasonal trend analysis and Loess smoothing were used to determine the long-term trends in baseflow during December-February periods, which should be the most sensitive to decreased groundwater discharge.
Although there are more than 20,000 wells in the Bitterroot Valley, groundwater use has not produced measurable basin-wide impacts to Bitterroot River baseflow. Stream-flow is highly correlated with precipitation and relatively uncorrelated with increased groundwater development. Long-term groundwater monitoring that began in 1993 shows slight declining water-level trends in 20 of 27 shallow wells. The declines may reflect: 1) decreased groundwater storage due to pumping, 2) decreased recharge due to changing irrigation or other land-use practices, or 3) decreased recharge due to climate variability. The decline in groundwater levels may eventually decrease groundwater discharge to the Bitterroot River.
Start Date
24-4-2015 12:00 PM
End Date
24-4-2015 2:00 PM
Document Type
Poster
Long-Term Trends in Base Flow, Precipitation, and Groundwater Development in the Bitterroot Watershed
Most municipalities and residences in the Bitterroot watershed rely on groundwater. Between 1970 and 2012, the number of wells in the watershed increased eight-fold to more than 20,000 (GWIC, http://mbmggwic.mtech.edu/). Well densities on non-federal land are approximately 30 wells per square mile, the highest in the state. Groundwater pumping must be balanced by either decreased groundwater storage or a combination of increased recharge and/or decreased groundwater discharge (capture). Usually, the groundwater discharge component (often seen as baseflow in streams) is captured by groundwater development.
To assess potential capture of Bitterroot River flow by groundwater development, the authors calculated daily maximum, median, and mean flows; 7-day minimum flows; and long-term flow trends for the 1935-2013 period of record (USGS gauge 12352500, Bitterroot River at Missoula) using Exploration and Graphics for River Trends (EGRET) software. Seasonal trend analysis and Loess smoothing were used to determine the long-term trends in baseflow during December-February periods, which should be the most sensitive to decreased groundwater discharge.
Although there are more than 20,000 wells in the Bitterroot Valley, groundwater use has not produced measurable basin-wide impacts to Bitterroot River baseflow. Stream-flow is highly correlated with precipitation and relatively uncorrelated with increased groundwater development. Long-term groundwater monitoring that began in 1993 shows slight declining water-level trends in 20 of 27 shallow wells. The declines may reflect: 1) decreased groundwater storage due to pumping, 2) decreased recharge due to changing irrigation or other land-use practices, or 3) decreased recharge due to climate variability. The decline in groundwater levels may eventually decrease groundwater discharge to the Bitterroot River.