• SWAT: Soil and Water Assessment Tool

• Developed to predict the impact of land management practices on water, sediment, and agricultural chemical yields in large complex watersheds with varying soils, land use, and management conditions over long periods of time.

• USLE : 1960s , CREAMS 1970s, EPIC SWRRB 1980s GLEAMS WEPP, SWAT 1990s AGNPS

• Dr. Jeff Arnold : USDA Agricultural Research Service (ARS)
• SWAT has 35+ years of modeling experience incorporated in it
• 1545 Peer reviewed Journal Articles used SWAT

• physically based model
• readily available inputs (topo, soil, lulc,etc.)
• watersheds w/ no monitoring data can be modeled
• relative impact of alt input data (changes in charact) on flow or water can be quantified
• Computationally efficinet
• Continuous time model (long term yield) - Daily time step ...one day --> hundreds of yrs
• Distributed model. User can define nlimited # of subwatersheds
• can simulate various management practices

• watershed divided into subwatersheds
• subwatersheds have an outlet (Flow/WQ)
• subwatersheds divided into HRUs
• HRUs : lumped land areas w/in a subbasin comprised of unique land cover, soil, and management combos
• INPUT from other models
• Point Sources : treatment plans

1. Weather
2. Hydrology
3. Plant Growth
4. Sedimentation
5. Nutrient Cycling
6. Pesticide Dynamics
7. Management

The climate of watershed provides the moisture and energy inputs that control water balance

• Daily precipitation
• max/min (air) temp
• solar radiation
• wind speed
• rel. humidity

Weather generator: Swat uses a model (developed by Nicks 1974) to generate daily precipitation for simulations that dont read in measured data. It is also used to fill in missing data in the measured records.

Hydrology

Water balance = driving force behind everything happening in wshed

SW_t = SW₀ + Σ^t_i-1(R_day - Q_surf - E_a - w_seep - Q_gw)

• SW_t = final soil water content (mm H₂O)
• SW₀ = initial soil water content on day i  (mm H₂O)
• t = time (days)
• R_day = amount of precipitation on day i (mm H₂O)
• E_a = amt of evapotranspiration on day i (mm H₂O)
• Q_surf = amt of surface runoff on day i (mm H₂O)
• w_seep = amt of water entering the vadose zone from soil profile on day i (mm H₂O)
• Q_gw = amount fo return flow on day i (mm H₂O)

Foliar Application & Surface Application

    degredation             degredation      

                   Washoff

                   Infiltration              Runoff

                   Leaching 

• Irrigation
• Subsurface (tile) drainage
• Water impoundment (i.e. rice)
• Land Management : BMP

1. Tillage
2. Irrigation
3. Fertilizer
4. Pesticides
5. Grazing 
6. Rotations
7. Subsurface Drainage

• Urban Areas

1. Pervious/Impervious areas
2. Street Sweeping
3. Lawn Chemicals

1. Flood Routing

• variable storage
• muskingum

   2.Transmission Losses, Evaporation

   3. Sediment Routing

• degredation and deposition computed simultaneously

    4. Nutrients 

• modified QUAL2E/WASP 

    5. Pesticide

• Toxic balance developed @ Univ. Of CO 

1. Water Balance

• Inflow
• Evaporation
• seepage
• withdrawals
• outflow
• spillway control
• target volume approach

    2. Nutrient Balance

• Well mixed system
• N and P loss rates
• 2 settling periods per yr

    3. Pesticide Balance

• well mixed system
• toxic balance dev. @ Univ. of CO