Watershed Modeling

1
Watershed Modeling

• - An overview of the SWAT Model -
• Julien Harou,
• Professor A. Lembo, CSS 620
• Spring 2001

2
Presentation based on

• Zhou, Y., Fulcher, C., A Watershed Management
  Tool using SWAT and ArcInfo, Esri User
  Conference, proceedings 1997.
• Khairy et al., Application of information systems
  in modelling of nonpoint pollution, 4th Int.
  Conf. On integrating GIS and Environmental
  Modeling (GIS/EM4)
• SWAT Model Manuals and web-site

3
Why Model Watersheds?

• To answer questions like
• What will be the response (flow, erosion,
  sedimentation) of a river to a 1 hour 50 year
  storm?
• What is the long-term affect (20 years) of
  urbanization on the water quality of a regions
  streams and reservoirs?
• Temporal and spatial scales
• of these questions vary radically !

4
Modeling Scale

• Modeling scale will vary in order to address
  these specific issues.
• The watershed discretization scheme (i.e., how
  the watershed is divided up spatially for
  modeling) will depend on the models objectives.

5
Different Watershed Discretization schemes

• Watershed can be divided into
• Grid cells
• Sub-watersheds
• Entire Watershed

6
Grid Cells

• - For physically based distributed models
• Time Scale single extreme
• events (storms, floods)
• Time step seconds - hours

7
Sub-watersheds

• - For semi-distributed
• models.
• - Flows must be routed
• from each sub-basin.
• Time scale Continuous
• time (pluri-annual
• time series)
• Time step days

8
Entire Watershed

• - For lumped or black box models.
• - Each variable gets 1 value for the whole basin.
  
• - Formulas tend to be of empirical nature.
• Time scale Continuous time
• (pluri-annual time series)
• Time step days months.

9
SWAT

• Soil and Water Assessment Tool
• Developed by USDA, from the 1980s today.
• Deterministic physically based model it models
  known physical processes
• Dynamic model differential equations are used
• (dt 1 day)
• Predictive model
• Basically, its a long-term
• hydrologic simulation model

10
Spatial Modeling in SWAT

• SWAT is also a distributed model. It has been
  integrated into several GIS (ArcView, ArcInfo,
  Grass).
• Discretization scheme sub-watersheds

11
HRUs

• Sub-basins are actually divided further into
  Hydrologic Response Units (HRUs).
• An HRU is a polygon containing a unique
  combination of soil and land-use.

Land Use layer
Soils layer
HRU layer
12
Watershed Hydrology

• All SWAT calculations are based on watershed mass
  balance calculations (i.e. water in water out).
  
• Watershed hydrology can be divided into 2 phases
• Land phase processes rainfall, evaporation,
  transpiration, infiltration, runoff,...
• Routing phase processes water, sediment and
  pollution flow in river.

13
Land Phase Modeling

• For each HRU, SWAT models
• Weather
• Hydrology
• Land cover
• plant growth
• Erosion
• Nutrients (N, P)
• Pesticides
• Agricultural
• management

14
Routing Phase Modeling

• SWAT models
• Routing in main channel
• water,
• sediment,
• nutrients,
• pesticides
• Routing in reservoirs
• water balance,
• sediment,
• nutrients,
• pesticides

15
Results

• Results are daily time series of modeled
  parameters (values can be averaged over months,
  years or over the whole simulation period).
• Results are available for different spatial
  identities
• HRUs,
• sub-basins,
• channel reaches.

16
Sample Output Sub-basin Hydrology

• Sub-basin
• outputs
• of yearly
• precip.
• runoff
• and ET

17
Sample Output - Reach

• Sub-basin
• channel reach
• outputs of
• yearly
• sediment
• N
• P