resourceful. naturally. evaluating watershed recharge and implications for supporting surface water uses SWCS Annual Conference Greg Wilson, Evan Christianson, Eric Novotny Barr Engineering Company
Dec 05, 2014
resourceful. naturally.
evaluating watershed recharge and implications for supporting surface water uses
SWCS Annual ConferenceGreg Wilson, Evan Christianson, Eric Novotny
Barr Engineering Company
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background
• competing demand for groundwater resources– land use/land cover conversion; ↑ popula on
– pattern of increasing drought frequency
– ↑demand to appropriate water for irrigation and waterworks
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challenge for watershed management
• how to provide groundwater protection and preservation for beneficial purposes?– wildlife and aquatic life– recreation– domestic & municipal consumption– agriculture– industry and power– navigation
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regulation of groundwater appropriation
• MN Dept. of Natural Resources– 1969 permit program to regulate large volume (>million gals a year) groundwater appropriations
– SWUDS database tracks monthly appropriations for each permit (by crop type, where applicable) since 1988
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challenge in developing TMDLs for stressors
the maximum amount of a specific pollutant that can
be discharged to a waterbody and still meet water quality standards
EPA does not believe flow, or lack of flow, is a pollutant
as defined by CWA
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Little Rock Creek
• protected trout stream– impairments for coldwater fish assemblage and nitrate– 45% row crops– mostly corn– 36% irrigated– sand plain (west)
– downstream impoundment
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biotic impairment for fish—stressors addressed by TMDL
• groundwater withdrawal has increased significantly over the last decade, likely altering stream flow during the summer months and exacerbated stressors for trout:– dissolved oxygen– nitrate– temperature
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TMDL endpoints for each stressor
• Use existing water quality standard for dissolved oxygen (> 7.0 mg/L as a daily minimum)
• Use 10 mg/L nitrate concentration for TMDL endpoint, based on drinking water standard; biotic integrity standard is under development
• Comparison of available temperature data to EPA guidance for chronic and acute effects on trout
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nitrate
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Summary of WQ monitoring relative to standards—DO
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demand for groundwater appropriations
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demand for groundwater appropriations
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demand for groundwater appropriations
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limitations of typical watershed models used for TMDLs
• Hydrologic Response Units (HRUs) or model segments are lumped and model inputs averaged in each subwatershed
• recharge is not spatially referenced to the receiving waters
• groundwater is simulated in one dimension based on average parameters/homogeneous media; assumed to follow surface divides
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representation of groundwater in typical watershed models used for TMDLs
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what we need from watershed models used for TMDLs
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importance of linked surface water-groundwater modeling
• MODFLOW—poor job with surface runoff processes
• SWAT—has “empirical” groundwater equations–MODFLOW accounts for “physics” of flow with finite difference methods
• linked modeling provides best spatial/temporal representation of what is occurring
• recharge for MODFLOW derived from SWAT
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description of model linkage
SWAT w/ MODFLOW
Groundwater pumping (can account for spatial locations)
Irrigation
Groundwater Storage
Stream Loss
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Note: Filled circles indicate permits issued since 2000.Positive values indicate an increase in pumping
Difference of 1997-1999 Average Pumping Compared to 2006-2008 Average Pumping
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hypothetical simulations
• two differences—amount of groundwater pumping and wells being pumped–1997‐1999 average monthly pumping vs. 2006‐2008 average monthly pumping
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Note: Filled circles indicate permits issued since 2000.Positive values indicate an increase in pumping
Difference of 1997-1999 Average Pumping Compared to 2006-2008 Average Pumping
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hypothetical simulations
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implementation strategies
• need to restore groundwater flow and WMA connectivity
• increase irrigation efficiency
• reduce appropriations and loads
• DO, nitrate & temperature
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implementation strategies
• Need to restore groundwater flow and WMA connectivity
• Temperature
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summary
• significant increasing trend in appropriations– impacts volumes/flows available for lakes/ streams at critical times
– impacts assimilation capacity/aquatic life, along with recreation/navigation
– sensitivity of surface waters varies significantly throughout the state
• managing use support requires better data and tools/modeling to ID sources and prioritize
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questions?Greg [email protected]