Evaluating watershed recharge and implications

resourceful. naturally.

evaluating watershed recharge and implications for supporting surface water uses

SWCS Annual ConferenceGreg Wilson, Evan Christianson, Eric Novotny

Barr Engineering Company


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


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


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


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


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


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


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


nitrate


Summary of WQ monitoring relative to standards—DO


demand for groundwater appropriations


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


representation of groundwater in typical watershed models used for TMDLs


what we need from watershed models used for TMDLs


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


description of model linkage

SWAT w/ MODFLOW

Groundwater pumping (can account for spatial locations)

Irrigation

Groundwater Storage

Stream Loss


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


hypothetical simulations

• two differences—amount of groundwater pumping and wells being pumped–1997‐1999 average monthly pumping vs. 2006‐2008 average monthly pumping


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


hypothetical simulations


implementation strategies

• need to restore groundwater flow and WMA connectivity

• increase irrigation efficiency

• reduce appropriations and loads

• DO, nitrate & temperature


implementation strategies

• Need to restore groundwater flow and WMA connectivity

• Temperature


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


questions?Greg [email protected]

Evaluating watershed recharge and implications

Environment

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