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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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