This document discusses evaluating watershed recharge and implications for supporting surface water uses. It notes increasing demand for groundwater resources from population growth and land use changes. This causes challenges in providing groundwater protection while meeting water needs. The document examines a case study on Little Rock Creek, a protected trout stream impaired by low flows from increased groundwater pumping. It advocates using linked surface water and groundwater models to better understand impacts and inform management strategies like restoring flows, improving irrigation efficiency, and reducing appropriations and pollutant loads.

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evaluating watershed recharge and
implications for supporting surface water uses
SWCS Annual Conference
Greg 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 Wilson
952-832-2672
gwilson@barr.com