Introduction to SWAT

Léonard Bernard-Jannin

TRAINING SCHOOL

What is SWAT

• SWAT = Soil and Water Assessment Tool

• Agro-hydrological model

• Semi-distributed

• Physically based

• Continuous time step

• A tool

• simulate the quality and quantity of surface and ground water

• predict the environmental impact of land use, land management practices, and climate change.

What does it simulate ?

• Water balance (surface runoff, soil water, groundwater)

• Water quality

• Nutrients (N,P) /Pesticides/Bacteria/Carbon

• Algae/CBOD/DO in streams

• Erosion/Sediment transport

• Vegetation growth

• Management practices (crops, irrigation, septic tanks, urban areas, filter strips)

• Reservoirs

What spatial and temporal scales ?

• Spatial scale

• From small catchment (few km 2) to

large watershed (Amazon)

• Temporal scale

• Continuous time, run at daily or sub daily time step

• Better used for long period (need a “warm up” 2- 3 years)

0.00

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1 6 11 4 9 2 7 12 5 10 3 8 1 6 11 4 9 2 7 12 5 10 3 8 1 6 11 4 9 2 7 12 5 10 3 8 1 6 11 4 9 2 7 12 5 10 3 8 1 6 11 4 9 2 7 12 5 10 3 8 1 6 11 4 9 2 7 12 5 10 3 8 1 6 11 4 9 2 7 12 5 10

1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

Flo

w (m

3/s

)

Observed Streamflow (m3/s) SWAT Streamflow (m3/s)

How does SWAT work ?

Slope (DEM )

Land uses

Soils

1- Hydrological

response unit

(HRU)

Watershed

2-Subbasins

+

+

=>

3-Watershed

The spatial discretization

Stream

Plant

growthHydrology

Nutrient

cycling

1-LAND PHASE

(HRU)

2- ROUTING PHASE

(subbasin)

In-stream

transformations

Lake and

reservoir

processes

Wastewater

discharges

1-1 Computation of land phase for each HRUs

1-2 All the HRUs outputs are added at the subbasin level

subbasin

HRU

watershed

2 main phases

INPUT

OUTPUT

Water discharge

Sediment loads

Concentrations, …

Climate

Human management

Water routing in

channel

1-Land phase: water balance is the driving force

Computed for each HRU

1-Land phase : Nutrients

1-Land phase : Pesticides

1-Land phase : Bacteria

2 – wastewater treatments works1 - Fertilization and grazing

(diffuse loads)

Foliage and soil decay Transport in runoff

Leaching to groundwater

Stream decay

Transport in stream

2-Routing phase : water2 - Routing through the river network

(based on manning equation and

kinematic waves)

Bank storage

Evaporation

Irrigation

Flow out

Flow in =Upstream reach +

Sum of HRUs water yield +

Point source

1 – Sum all HRUs at subbasin scale

HRU 1

HRU 2

HRU 3

2-Routing phase : sediments/ nutrients/ contaminants

Data requirement

• Digital Elevation Model (DEM)

• Land use map

• Soil map

• Meteo data (rain, and parameter for ETP)

• Management practices (crop rotation)

• Observations (Water discharge and element concentrations)

HRUs creation

Watershed

delineation

Model Input

Calibration/validation

How to use SWAT ?

• One website : https://swat.tamu.edu/• Download the model, interfaces and tools

• Detailed documentation• SWAT2012 Input/Output: full description of model input/output

• SWAT2009 Theoretical : describe equations

• Support (User groups)

• Interface with QGIS (QSWAT) and ArcGIS (ArcSWAT) to facilitate model setup

• SWAT-CUP, a calibration tool, for advanced users only

Step 1 : Catchment Delineation

DEM 2- Subbasins and

watershed creation

3-Reservoirs (optional)

subbasin

HRU

watershed

1- Streams creation

Step 2: HRUs creation

Slope (DEM) Land uses Soils

subbasin

HRU

watershed

• define all possible HRUs

• use thresholds to eliminate small areas

• One HRU per subbasin

• …

Different options for HRUs creation

+ +

Hydrologic Response Unit (HRU) = Unique combination of slope/land use/soil

Step 3: Climate input

• Daily input of • Rainfall (can be subdaily)

• Temperature (min max)

• Humidity relative

• Wind speed

• Solar radiation

• Missing data can be simulated : Weather generator• Use long term statistics (average, standard deviation, …)

to fill data gaps (see documentation for detailed format, https://swat.tamu.edu/docs/)

PET calculation (several methods)

• Maximum of one station per subbasin (one station can be used by several subasin)

• The model we select the station the closest to the subbasin center point

• Stations can be located out of the watershed

• Possibility to use elevation bands (change of precipitation with elevation)

S1

S2

S3

sub1

sub2

sub3

sub4

Subbasin Meteo station

Sub1 S2

Sub2 S2

Sub3 S2

Sub4 S3

Step 3: Climate input

Step 4 : Run the model

Data preparation - QSWAT

• DEM/Land use/Soil• All data have to be projected in the same system

• Raster data (ArcSWAT can read shape file)

• Lookup tables: correspondence between pixel value and land use/soil type

• The original database is developed for USA soils and land uses

• Might need to adapt it to your study site

• Need to manually edit the access database (QSWATRef2012.mdb)

Original land use

crop

Added land use

Need to fill all the land use parameters: look in the SWAT 2012

input/output documentation (https://swat.tamu.edu/docs/) for

parameters description.

Same procedure for Soils

LANDUSE_ID,SWAT_CODE

1,URMD

2,AGRL

3,FRST

4,WTNI

5,RLEI

Lookup table:

Open

data

Europe World

DEM eu-dem

(Copernicus)

SRTM (CGIAR-CSI)

Land use Corine land cover Global Land Cover

Characterization (GLCC)

Soils European Soil

database

FAO world soil map

Climate ECA&D Global weather data for SWAT

(model reanalysis)

Data sources

If available, prefer local datasets

http://www.waterbase.org/

download_data.html