Organic carbon and nitrate transfers at a watershed scalewith the SWAT+ model using landscape units: applicationto a large watershed in France

Clément Fabre, Sabine Sauvage, Raghavan Srinivasan, José Miguel Sánchez Pérez

Presented by Clément Fabre

Riparian zones : Powerful water cleaners

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5(CH2O) + 4NO3- + 4H+ → 5CO2 + 2N2 + 7H2O

Riparian zones impacts on the nitrogen cycle (Pinay et al., 1998 ; Sánchez-Pérez et al., 2003 ; Sun et al., 2015)

Dilution

Root absorption

Denitrification

Groundwaterlevel

Non SaturatedZone

SaturatedZone

Riparian zones : 40% of nitrates consumed by denitrification at a watershed scale(Seine : Pacsy et al., Garonne : Sun et al., 2015)

Objectives

• Characterize hotspots of denitrification in the alluvial plain of an agricultural catchment

• Quantify the effect of riparian zones on the nitrogen cycle

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The Garonne River

• Area : 55 000 km²

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1000

2000

3000

4000

5000

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

Observed discharge (m3.s-1)

The Garonne River

• Average discharge : 650 m3.s-1

• Nitrate concentration : 9.3 mg.L-1

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5

10

15

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2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

Observed [NO3] (mg.L-1)

Portail Adour-Garonne

The Garonne River : An agricultural catchment

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Land use type Coverage

Agriculture 43 %

Forest 32 %

Pasture 16 %

FAO

SWAT+ advantagesfor this study

• Delineation of the floodplain

directly in the setup

of the project

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SWAT + advantagesfor this study

• 3 methods

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SWAT + advantagesfor this study

• 3 methods

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SWAT + advantagesfor this study

• 3 methods

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Method 1

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Method 2

Fluvisols

Alternative :SWAT - LUD

• Landscape Units as in

SWAT + with Darcy’s law

12LU 1 LU 2 LU 3

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Alternative : SWAT - LUD

• Landuse and soil dependant

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Alternative : SWAT - LUD

• Landuse and soil dependant

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Standard

LUD

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Standard

LUD

DECREASING

DECREASING & DELAY

Daily time step

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Standard

LUD

Daily time step

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Standard

LUD

Perspectives : Amazon River

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Standard

LUD

Perspectives : Amazon River

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Standard

LUD

DECREASING & DELAY

Conclusion

• Each model(SWAT, SWAT_LUD, SWAT+) different responses

• Spatially different hotspots

• Different methodologies A whole spectre of the effective alluvial zones

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Thank you for your attention