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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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questions?Greg Wilson952-832-2672gwilson@barr.com