SWAT+ Documentation
  • INTRODUCTION
    • Introduction to SWAT+
    • Watershed Configuration
      • Spatial Objects
    • Calibration
  • SWAT+ Input Files
    • Input File Format
    • Master File (file.cio)
    • Simulation Settings
      • object.cnt
      • time.sim
        • day_start
        • yrc_start
        • day_end
        • yrc_end
        • step
      • print.prt
        • nyskip
        • interval
        • aa_int_cnt
        • hydcon
        • object
      • object.prt
        • obj_typ
        • obj_typ_no
        • hyd_typ
    • Climate
      • weather-sta.cli
        • name (weather_sta.cli)
        • wgn
        • pcp
        • tmp
        • slr
        • hmd
        • wnd
        • pet
        • atmo_dep
      • weather-wgn.cli
        • name (weather_wgn.cli)
        • yrs_pcp
        • tmp_max_ave
        • tmp_min_ave
        • tmp_max_sd
        • tmp_min_sd
        • pcp_ave
        • pcp_sd
        • pcp_skew
        • wet_dry
        • wet_wet
        • pcp_days
        • pcp_hhr
        • slr_ave
        • dew_ave
        • wnd_ave
      • pcp.cli and pcp data files
      • tmp.cli and tmp data files
      • hmd.cli and hmd data files
      • slr.cli and slr data files
      • wnd.cli and wnd data files
      • atmo.cli
        • timestep
        • num_aa
    • Basin
      • codes.bsn
        • pet
        • crack
        • swift_out
        • rte_cha
        • nostress
        • carbon
        • lapse
        • uhyd
        • tiledrain
        • wtable
        • soil_p
        • gampt
        • qual2e
        • gwflow
      • parameters.bsn
        • sw_init
        • surq_lag
        • adj_pkrt
        • prf
        • orgn_min
        • n_uptake
        • p_uptake
        • n_perc
        • p_perc
        • p_soil
        • p_avail
        • rsd_decomp
        • pest_perc
        • msk_co1
        • msk_co2
        • msk_x
        • nperco_lchtile
        • evap_adj
        • denit_exp
        • denit_frac
        • adj_uhyd
        • cn_froz
        • dorm_hr
        • plaps
        • tlaps
        • n_fix_max
        • rsd_decay
        • urb_init_abst
        • uhyd_alpha
        • splash
        • rill
        • surq_exp
        • cov_mgt
        • co2
    • Landscape Units
      • ls_unit.def
        • id (ls_unit.def)
        • name (ls_unit.def)
        • elem_tot
        • elements
      • ls_unit.ele
        • id (ls_unit.ele)
        • name (ls_unit.ele)
        • obj_typ
        • obj_typ_no
        • bsn_frac
    • Routing Units
      • rout_unit.rtu
        • id (rout_unit.rtu)
        • name (rout_unit.rtu)
        • define
        • topo
        • field
      • rout_unit.def
        • id (rout_unit.def)
        • name (rout_unit.def)
        • elem_tot
        • elements
      • rout_unit.ele
        • id (rout_unit.ele)
        • name (rout_uni.ele)
        • obj_typ
        • obj_id
    • Hydrologic Response Units
      • hru-data.hru
        • id (hru-data.hru)
        • name (hru-data.hru)
        • topo
        • hydro
        • soil
        • lu_mgt
        • soil_plant_init
        • surf_stor
        • snow
        • field
      • hru-lte.hru
        • id (hru-lte.hru)
        • name (hru-lte.hru)
        • area
        • cn2
        • cn3_swf
        • t_conc
        • soil_dp
        • perco_co
        • slp
        • slp_len
        • et_co
        • aqu_sp_yld
        • alpha_bf
        • revap
        • rchg_dp
        • sw_init
        • aqu_init
        • aqu_sh_flo
        • aqu_dp_flo
        • snow_h2o
        • lat
        • soil_text
        • trop_flag
        • grow_start
        • grow_end
        • plnt_typ
        • stress
        • pet_flag
        • irr_flag
        • irr_src
        • t_drain
        • usle_k
        • usle_c
        • usle_p
        • usle_ls
    • Hydrology
      • topography.hyd
        • name (topography.hyd)
        • slp
        • slp_len
        • lat_len
        • depos
      • hydrology.hyd
        • name (hydrology.hyd)
        • lat_time
        • lat_sed
        • can_max
        • esco
        • epco
        • orgn_enrich
        • orgp_enrich
        • cn3_swf
        • bio_mix
        • perco
        • lat_orgn
        • lat_orgp
        • pet_co
        • latq_co
      • snow.sno
        • name (snow.sno)
        • fall_tmp
        • melt_tmp
        • melt_max
        • melt_min
        • tmp_lag
        • snow_h2o
        • cov50
        • snow_init
      • field.fld
        • name (field.fld)
        • len
        • wd
    • Soils
      • soils.sol
        • name (soils.sol)
        • hyd_grp
        • dp_tot
        • anion_excl
        • perc_crk
        • texture
        • bd
        • awc
        • soil_k
        • carbon
        • clay
        • silt
        • sand
        • rock
        • alb
        • usle_k
        • ec
        • caco3
        • ph
      • nutrients.sol
        • name (nutrients.sol)
        • exp_co
        • lab_p
        • nitrate
        • fr_hum_act
        • hum_c_n
        • hum_c_p
    • Landuse and Management
      • landuse.lum
        • name (landuse.lum)
        • cal_grp
        • plnt_com
        • mgt
        • cn2
        • cons_prac
        • urban
        • urb_ro
        • ov_mann
        • tile
        • sep
        • vfs
        • grww
        • bmp
      • plant.ini
        • name (plant.ini)
        • plnt_cnt
        • rot_yr_ini
        • plnt_name
        • lc_status
        • lai_init
        • bm_init
        • phu_init
        • plnt_pop
        • yrs_init
        • rsd_init
      • management.sch
        • name (management.sch)
        • op_typ
        • op_data1
        • op_data2
        • op_data3
      • cntable.lum
        • name (cntable.lum)
        • cn_a
        • cn_b
        • cn_c
        • cn_d
      • cons_practice.lum
        • name (cons_practice.lum)
        • usle_p
        • slp_len_max
      • ovn_table.lum
        • name (ovn_table.lum)
        • ovn_mean
        • ovn_min
        • ovn_max
    • Decision Tables
      • "name".dtl
        • var
        • obj
        • obj_num
        • lim_var
        • lim_op
        • lim_const
        • alt
        • act_typ
        • obj
        • obj_num
        • name
        • option
        • const
        • const2
        • fp
        • outcome
    • Management Practices
      • harv.ops
        • name (harv.ops)
        • harv_typ
        • harv_idx
        • harv_eff
        • harv_bm_min
      • graze.ops
        • name (graze.ops)
        • fertname
        • bm_eat
        • bm_tramp
        • man_amt
        • grz_bm_min
      • irr.ops
        • name (irr.ops)
        • irr_eff
        • surq_rto
        • irr_amt
        • irr_dep
        • irr_salt
        • irr_no3n
        • irr_po4
      • chem_app.ops
        • name (chem_app.ops)
        • app_eff
        • inject_dp
        • surf_frac
      • fire.ops
        • name (fire.ops)
        • chg_cn2
        • frac_burn
      • sweep.ops
        • name (sweep.ops)
        • swp_eff
        • frac_curb
    • Structural Practices
      • tiledrain.str
        • name (tiledrain.str)
        • dp
        • t_fc
        • lag
        • rad
        • dist
        • drain
        • pump
        • lat_ksat
      • filterstrip.str
        • name (filterstrip.str)
        • fld_vfs
        • con_vfs
        • cha_q
      • grassedww.str
        • name (grassedww.str)
        • mann
        • sed_co
        • dp
        • wd
        • len
        • slp
      • bmpuser.str
        • name (bmpuser.str)
        • sed_eff
        • ptlp_eff
        • solp_eff
        • ptln_eff
        • soln_eff
        • bact_eff
      • septic.str
        • name (septic.str)
        • typ
        • yr
        • operation
        • residents
        • area
        • t_fail
        • dp_bioz
        • thk_bioz
        • cha_dist
        • sep_dens
        • bm_dens
        • bod_decay
        • bod_conv
        • fc_lin
        • fc_exp
        • fecal_decay
        • tds_conv
        • mort
        • resp
        • slough1
        • slough2
        • nit
        • denit
        • p_sorp
        • p_sorp_max
        • solp_slp
        • solp_int
    • Databases
      • plants.plt
        • name (plants.plt)
        • plnt_typ
        • gro_trig
        • nfix_co
        • days_mat
        • bm_e
        • harv_idx
        • lai_pot
        • frac_hu1
        • lai_max1
        • frac_hu2
        • lai_max2
        • hu_lai_decl
        • dlai_rate
        • can_ht_max
        • rt_dp_max
        • tmp_opt
        • tmp_base
        • frac_n_yld
        • frac_p_yld
        • frac_n_em
        • frac_n_50
        • frac_n_mat
        • frac_p_em
        • frac_p_50
        • frac_p_mat
        • harv_idx_ws
        • usle_c_min
        • stcon_max
        • vpd
        • frac_stcon
        • ru_vpd
        • co2_hi
        • bm_e_hi
        • plnt_decomp
        • lai_min
        • bm_tree_acc
        • yrs_mat
        • bm_tree_max
        • ext_co
        • leaf_tov_min
        • leaf_tov_max
        • bm_dieoff
        • rt_st_beg
        • rt_st_end
        • plnt_pop1
        • frac_lai1
        • plnt_pop2
        • frac_lai2
        • frac_sw_gro
        • aeration
        • rsd_pctcov
        • rsd_covfac
      • urban.urb
        • name (urban.urb)
        • frac_imp
        • frac_dc_imp
        • curb_den
        • urb_wash
        • dirt_max
        • t_halfmax
        • conc_totn
        • conc_totp
        • conc_no3
        • urb_cn
      • tillage.til
        • name (tillage.til)
        • mix_eff
        • mix_dp
      • fertilizer.frt
        • name (fertilizer.frt)
        • min_n
        • min_p
        • org_n
        • org_p
        • nh3_n
      • pesticide.pes
        • name (pesticide.pes)
        • soil_ads
        • frac_wash
        • hl_foliage
        • hl_soil
        • solub
        • aq_reac
        • aq_volat
        • mol_wt
        • aq_resus
        • aq_settle
        • ben_act_dep
        • ben_bury
        • ben_reac
      • septic.sep
        • name (septic.sep)
        • q_rate
        • bod
        • tss
        • nh4_n
        • no3_n
        • no2_n
        • org_n
        • min_p
        • org_p
        • fcoli
    • Aquifers
      • aquifer.aqu
        • id (aquifer.aqu)
        • name (aquifer.aqu)
        • aqu_init
        • gw_flo
        • dep_bot
        • dep_wt
        • no3_n
        • sol_p
        • carbon
        • flo_dist
        • flo_max
        • alpha_bf
        • revap
        • rchg_dp
        • spec_yld
        • hl_no3n
        • flo_min
        • revap_min
      • initial.aqu
        • name (initial.aqu)
        • org_min
        • pest
        • path
        • hmet
        • salt
    • GWFlow
    • Channels
      • channel-lte.cha
        • id (channel-lte.cha)
        • name (channel-lte.cha)
        • ini
        • hyd
        • nut
      • initial.cha
        • name (initial.cha)
        • org_min
        • pest
        • salt
      • hyd-sed-lte.cha
        • name (hyd-sed-lte.cha)
        • wd
        • dp
        • slp
        • len
        • mann
        • k
        • erod_fact
        • cov_fact
        • sinu
        • eq_slp
        • d50
        • clay
        • carbon
        • dry_bd
        • side_slp
        • bed_load
        • fps
        • fpn
        • n_conc
        • p_conc
        • p_bio
      • nutrients.cha
        • name (nutrients.cha)
        • plt_n
        • plt_p
        • alg_stl
        • ben_disp
        • ben_nh3n
        • ptln_stl
        • ptlp_stl
        • cst_stl
        • ben_cst
        • cbn_bod_co
        • air_rt
        • cbn_bod_stl
        • ben_bod
        • bact_die
        • cst_decay
        • nh3n_no2n
        • no2n_no3n
        • ptln_nh3n
        • ptlp_solp
        • q2e_lt
        • q2e_alg
        • chla_alg
        • alg_n
        • alg_p
        • alg_o2_prod
        • alg_o2_resp
        • o2_nh3n
        • o2_no2n
        • alg_grow
        • alg_resp
        • slr_act
        • lt_co
        • const_n
        • const_p
        • lt_nonalg
        • alg_shd_l
        • alg_shd_nl
        • nh3_pref
    • Reservoirs and Ponds
      • reservoir.res
        • id (reservoir.res)
        • name (reservoir.res)
        • init
        • hyd
        • rel
        • sed
        • nut
      • initial.res
        • name (initial.res)
        • org_min
        • pest
        • salt
      • hydrology.res
        • name (hydrology.res)
        • yr_op
        • mon_op
        • area_ps
        • vol_ps
        • area_es
        • vol_es
        • k
        • evap_co
        • shp_co1
        • shp_co2
      • sediment.res
        • name (sediment.res)
        • nsed
        • d50
        • carbon
        • bd
        • sed_stl
        • stl_vel
      • nutrients.res
        • name (nutrients.res)
        • mid_start
        • mid_end
        • mid_n_stl
        • n_stl
        • mid_p_stl
        • p_stl
        • chla_co
        • secchi_co
        • theta_n
        • theta_p
        • n_min_stl
        • p_min_stl
      • weir.res
        • name (weir.res)
        • linear_c
        • exp_k
        • width
        • height
    • Wetlands
      • wetland.wet
        • id (wetland.wet)
        • name (wetland.wet)
        • init
        • hyd
        • rel
        • sed
        • nut
      • hydrology.wet
        • name (hydrology.wet)
        • hru_ps
        • dp_ps
        • hru_es
        • dp_es
        • k
        • evap
        • vol_area_co
        • vol_dp_a
        • vol_dp_b
        • hru_frac
    • Initialization
      • soil_plant.ini
        • name (soil_plant.ini)
        • sw_frac
        • nutrients
        • pest
        • salt
      • om_water.ini
        • name (om_water.ini)
        • vol
        • sed
        • part_n
        • part_p
        • no3
        • solp
        • chl_a
        • nh3
        • no2
        • cbn_bod
        • dis_ox
        • sand
        • silt
        • clay
        • sm_ag
        • l_ag
        • gvl
        • tmp
      • pest_water.ini
      • pest_hru.ini
      • salt_water.ini
      • salt_hru.ini
    • Constituents
      • constituents.cs
    • Point Sources and Inlets
      • recall.rec
        • id (recall.rec)
        • rec_typ
        • file
      • 'filename'.rec
    • Connectivity
      • 'object'.con
        • id
        • name
        • gis_id
        • area
        • lat
        • lon
        • elev
        • 'obj'
        • wst
        • cst
        • ovfl
        • rule
        • out_tot
        • obj_typ
        • obj_id
        • hyd_typ
        • frac
      • aqu_cha.lin
      • chan_surf.lin
    • Water Allocation
      • water_allocation.wro
        • rule_typ
        • cha_ob
        • ob_typ (source)
        • limit_mon
        • ob_typ (demand)
        • withdr
        • amount
        • right
        • rcv_ob
        • rcv_num
        • srcs
        • src
        • frac
        • comp
    • Calibration
      • codes.sft
      • wb_parms.sft
      • water_balance.sft
      • cal_parms.cal
      • calibration.cal
  • SWAT+ Output Files
    • Output File Format
    • Debugging Outputs
    • Soil
    • Management
    • Flow Duration Curve
    • Water Balance
    • Nutrient Balance
    • Losses
    • Plant and Weather
    • Channel
    • Aquifer
    • Reservoir
    • Recall
    • Hydrographs
    • Routing Unit
    • Pesticides
    • Object Outputs
  • Theoretical Documentation
    • ☁️Section 1: Climate
      • 🟰Chapter 1:1 Equations: Energy
        • ☀️1:1.1 Sun-Earth Relationships
          • 1:1.1.1 Distance between Earth and Sun
          • 1:1.1.2 Solar Declination
          • 🌄1:1.1.3 Solar Noon, Sunrise, Sunset, and Daylength
        • 1:1.2 Solar Radiation
          • 👽1:1.2.1 Extraterrestrial Radiation
          • 1:1.2.2 Solar Radiation under Cloudless Skies
          • 1:1.2.3 Daily Solar Radiation
          • 1:1.2.4 Hourly Solar Radiation
          • 1:1.2.5 Daily Net Radiation
        • 1:1.3 Temperature
          • 1:1.3.1 Daily Air Temperature
          • 1:1.3.2 Hourly Air Temperature
          • 1:1.3.3 Soil Temperature
          • 1:1.3.4 Water Temperature
        • 1:1:4 Wind Speed
      • Chapter 1:2 Atmospheric Water
        • 1:2.1 Precipitation
        • 1:2.2 Maximum Half-Hour Rainfall
        • 1:2.3 Water Vapor
        • 1:2.4 Snow Cover
        • 1:2.5 Snow Melt
          • 1:2.5.1 Snow Pack Temperature
            • Snow Melt Equation
      • Chapter 1:3 Weather Generator
        • 1:3.1 Precipitation
          • 1:3.1.1 Occurrence of Wet or Dry Day
          • 1:3.1.2 Amount of Precipitation
        • 1:3.2 Maximum Half-Hour Rainfall
          • 1:3.2.1 Monthly Maximum Half-Hour Rain
          • 1:3.2.2 Daily Maximum Half-Hour Rain Value
        • 1:3.3 Distribution of Rainfall Within Day
          • 1:3.3.1 Normalized Intensity Distribution
          • 1:3.3.2 Generated Time to Peak Intensity
          • 1:3.3.3 Total Rainfall and Duration
        • 1:3.4 Solar Radiation & Temperature
          • 1:3.4.1 Daily Residuals
          • 1:3.4.2 Generated Values
          • 1:3.4.3 Adjustment for Clear/Overcast Conditions
            • Maximum Temperature
            • Solar Radiation
        • 1:3.5 Relative Humidity
          • 1:3.5.1 Mean Monthly Relative Humidity
          • 1:3.5.2 Generated Daily Value
          • 1:3.5.3 Adjustment for Clear/Overcast Conditions
        • 1:3.6 Wind Speed
      • Chapter 1:4 Climate Customization
        • 1:4.1 Elevation Bands
        • 1:4.2 Climate Change
    • Section 2: Hydrology
      • Surface Runoff
        • Runoff Volume: SCS Curve Number Procedure
        • SCS Curve Number
        • Soil Hydrologic Groups
        • Antecedent Soil Moisture Condition
        • Retention Parameter
        • Slope Adjustments
        • Runoff Volume: Green & Ampt Infiltration Method
        • Peak Runoff Rate
        • Time of Concentration
          • Overland Flow Time of Concentration
          • Channel Flow Time of Concentration
        • Runoff Coefficient
        • Rainfall Intensity
        • Modified Rational Formula
        • Surface Runoff Lag
        • Transmission Losses
      • Chapter 2:2 Evapotranspiration
        • 2:2.1 Canopy Storage
        • 2:2.2 Potential Evapotranspiration
          • 2:2.2.1 Penman-Monteith Method
            • Soil Heat Flux
            • Aerodynamic Resistance
            • Canopy Resistance
            • Combined Term
          • 2:2.2.2 Priestley-Taylor Method
          • 2:2.2.3 Hargreaves Method
          • 2:2.2.4 Reading Measured or Estimated PET
        • 2:2.3 Actual Evapotranspiration
          • 2:2.3.1 Evaporation of Intercepted Rainfall
          • 2:2.3.2 Transpiration
          • 2:2.3.3 Sublimation and Evaporation from the Soil
            • Sublimation
            • Soil Water Evaporation
          • 2:2.3.4 Evaporation from Ponded Water
      • Soil Water
        • Soil Structure
        • Percolation
        • Bypass Flow
        • Perched Water Table
        • Lateral Flow
          • Lateral Flow Lag
      • Groundwater
        • Groundwater Systems
        • Shallow Aquifer
          • Recharge
          • Partitioning of Recharge Between Shallow and Deep Aquifer
          • Groundwater/Base Flow
          • Revap
          • Pumping
          • Groundwater Height
        • Deep Aquifer
    • Section 3: Nutrients/Pesticides
      • Nitrogen
        • Nitrogen Cycle in the Soil
          • Initialization of Soil Nitrogen Levels
        • Mineralization & Decomposition / Immobilization
          • Humus Mineralization
          • Residue Decomposition & Mineralization
        • Nitrification & Ammonia Volatilization
        • Denitrification
        • Atmospheric Deposition
          • Nitrogen in Rainfall
          • Nitrogen Dry Deposition
        • Fixation
        • Upward Movement of Nitrate in Water
        • Leaching
        • Nitrate in the Shallow Aquifer
      • Phosphorus
        • Phosphorus Cycle
          • Initialization of Soil Phosphorus Levels
        • Mineralization & Decomposition / Immobilization
          • Humus Mineralization
          • Residue Decomposition & Mineralization
        • Sorption of Inorganic P
        • Leaching
        • Phosphorus in the Shallow Aquifer
      • Pesticides
        • Wash-off
        • Degradation
        • Leaching
      • Bacteria
        • Wash-off
        • Bacteria Die-off/Re-growth
        • Leaching
      • Carbon
        • Sub-model Description
        • Changes from previous version
        • Analytical Solutions
    • Section 4: Erosion
      • Sediment
        • MUSLE
          • Soil Erodibility Factor
          • Cover and Management Factor
          • Support Practice Factor
          • Topographic Factor
          • Coarse Fragment Factor
        • USLE
          • Rainfall Erodibility Index
        • Snow Cover Effects
        • Sediment Lag in Surface Runoff
        • Sediment in Lateral & Groundwater Flow
      • Nutrient Transport
        • Nitrate Movement
        • Organic N in Surface Runoff
          • Enrichment Ratio
        • Soluble Phosphorus Movement
        • Organic & Mineral P Attached to Sediment in Surface Runoff
          • Enrichment Ratio
        • Nutrient Lag in Surface Runoff and Lateral Flow
      • Pesticide Transport
        • Phase Distribution of Pesticide
        • Movement of Soluble Pesticide
        • Transport of Sorbed Pesticide
          • Enrichment Ratio
        • Pesticide Lag in Surface Runoff and Lateral Flow
      • Bacteria Transport
        • Bacteria in Surface Runoff
        • Bacteria Attached to Sediment in Surface Runoff
          • Enrichment Ratio
        • Bacteria Lag in Surface Runoff
      • Water Quality Parameters
        • Algae
        • Carbonaceous Biological Oxygen Demand
          • Enrichment Ratio
        • Dissolved Oxygen
          • Oxygen Saturation Concentration
    • Section 5: Land Cover/Plant
      • Growth Cycle
        • Heat Units
          • Heat Unit Scheduling
        • Dormancy
        • Plant Types
      • Optimal Growth
        • Potential Growth
          • Biomass Production
            • Impact of Climate on Radiation-Use Efficiency
            • Modification of Biomass Calculation for Trees
          • Canopy Cover and Height
          • Root Development
          • Maturity
        • Water Uptake by Plants
          • Impact of Low Soil Water Content
          • Actual Water Uptake
        • Nutrient Uptake by Plants
          • Nitrogen Uptake
            • Nitrogen Fixation
          • Phosphorus Uptake
        • Crop Yield
      • Actual Growth
        • Growth Constraints
          • Water Stress
          • Temperature Stress
          • Nitrogen Stress
          • Phosphorus Stress
        • Actual Growth
          • Biomass Override
        • Actual Yield
          • Harvest Index Override
          • Harvest Efficiency
    • Section 6: Management Practices
      • General Management
        • Planting/Beginning of Growing Season
        • Harvest Operation
        • Grazing Operation
        • Harvest & Kill Operation
        • Kill/End of Growing Season
        • Tillage
          • Biological Mixing
        • Fertilizer Application
        • Auto-Application of Fertilizer
        • Continuous Application of Fertilizer
        • Pesticide Application
      • Water Management
        • Irrigation
          • Manual Application of Irrigation
          • Auto-Application of Irrigation
        • Tile Drainage
        • Impounded/Depressional Areas
        • Water Transfer
        • Consumptive Water Use
        • Point Source Loadings
      • Urban Areas
        • Characteristics of Urban Areas
        • Surface Runoff from Urban Areas
        • USGS Regression Equations
        • Build Up/Wash Off
          • Street Cleaning
      • Septic Systems
        • Biozone Algorithm
          • Buildup of Live Bacterial Biomass
          • Fate and Transport of Bacterial Biomass
          • Field Capacity
          • Clogging Effect on Hydraulic Conductivity
          • Soil Moisture and Percolation
          • Nitrogen, BOD, Fecal Coliform
          • Phosphorus Removal
          • Model Assumptions
        • Integration of Biozone Algorithm
          • Simulating Active and Failing Systems
      • Filter Strips and Grassed Waterways
        • Filter Strips
          • Empirical Model Development
          • Sediment Reduction Model
          • Nutrient Reduction Models
            • Total Nitrogen
            • Nitrate Nitrogen
            • Total Phosphorus
            • Soluble Phosphorus
          • VFS SWAT+ Model Structure
        • Grassed Waterways
    • Section 7: Main Channel Processes
      • Water Routing
        • Channel Characteristics
        • Flow Rate and Velocity
        • Variable Storage Routing Method
        • Muskingum Routing Method
        • Transmission Losses
        • Evaporation Losses
        • Bank Storage
        • Channel Water Balance
      • Sediment Routing
        • Landscape Contribution to Subbasin Routing Reach
        • Sediment Routing In Stream Channels
          • Simplified Bagnold Equation (Default method)
          • Physics Based Approach for Channel Erosion
        • Channel Erodibility Factor
        • Channel Cover Factor
        • Channel Downcutting and Widening
      • In-Stream Nutrient Processes
        • Algae
          • Chlorophyll a
          • Algal Growth
            • Local Specific Growth Rate of Algae
            • Local Respiration Rate of Algae
            • Local Settling Rate of Algae
        • Nitrogen Cycle
          • Organic Nitrogen
          • Ammonium
          • Nitrite
          • Nitrate
        • Phosphorus Cycle
          • Organic Phosphorus
          • Inorganic/Soluble Phosphorus
        • Carbonaceous Biological Oxygen Demand
        • Oxygen
          • Oxygen Saturation Concentration
          • Reaeration
            • Reaeration By Fickian Diffusion
            • Reaeration By Turbulent Flow Over A Dam
      • In-Stream Pesticide Transformations
        • Pesticide In The Water
          • Solid-Liquid Partitioning
          • Degradation
          • Volatilization
          • Settling
          • Outflow
        • Pesticide In The Sediment
          • Solid-Liquid Partitioning
          • Degradation
          • Resuspension
          • Diffusion
          • Burial
        • Mass Balance
      • Bacteria Routing
        • Bacteria Decay
        • Bacteria Sediment
      • Heavy Metal Routing
    • Section 8: Water Bodies
      • Impoundment Water Routing
        • Reservoirs
          • Surface Area
          • Precipitation
          • Evaporation
          • Seepage
          • Outflow
            • Measured Daily Outflow
            • Measured Monthly Outflow
            • Average Annual Release Rate For Uncontrolled Reservoir
            • Target Release For Controlled Reservoir
        • Ponds/Wetlands
          • Surface Area
          • Precipitation
          • Inflow
          • Evaporation
          • Seepage
          • Outflow
            • Pond Outflow
            • Wetland Outflow
        • Depressions/Potholes
          • Surface Area
          • Precipitation
          • Inflow
          • Evaporation
          • Seepage
          • Outflow
            • Overflow
            • Release Operation
            • Tile Flow
      • Sediment In Water Bodies
        • Mass Balance
        • Settling
        • Sediment Outflow
      • Nutrients In Water Bodies
        • Nutrient Transformations
        • Total Balance
        • Eutrophication
          • Phosphorus/Chlorophyll a Correlations
      • Pesticides In Water Bodies
        • Pesticide In The Water
          • Solid-Liquid Partitioning
          • Degradation
          • Volatilization
          • Settling
          • Outflow
        • Pesticide In The Sediment
          • Solid-Liquid Partitioning
          • Degradation
          • Resuspension
          • Diffusion
          • Burial
        • Mass Balance
      • Bacteria In Water Bodies
        • Bacteria Decay
    • References
Powered by GitBook
On this page
Export as PDF
  1. Theoretical Documentation
  2. Section 3: Nutrients/Pesticides
  3. Carbon

Sub-model Description

Last updated 3 years ago

The sub-model described here pertains to soil processes at the hydrologic unit response level. The new sub-model has one pool for soil organic C, N, and P (SC,SN,SPS_C,S_N,S_PSC​,SN​,SP​ respectively, kg m−2^{-2}−2), and separate pools for residue and manure C, N, and P(RC,RN,RPR_C,R_N,R_PRC​,RN​,RP​ and MC,MN,MPM_C,M_N,M_PMC​,MN​,MP​, respectively, kg m−2^{-2}−2). The pools are not separated in active and stable pools. Microbial activity on SOM, manure, and residues decomposes simultaneously the organic C, N, and P. Decomposition of residues may add to (mineralization) or subtract from (immobilization) the inorganic N and P pools, depending on the C:N and C:P ratio of the decomposing pools (RCN,RCP,MCNR_{CN},R_{CP},M_{CN}RCN​,RCP​,MCN​, and MCPM_{CP}MCP​, respectively) and those of the SOM or destiny pool (SCN,SCPS_{CN},S_{CP}SCN​,SCP​), and on the humification rate (h). The humification rate represents the fraction of C in the decomposing residues that are effectively incorporated in the SCS_CSC​ pool, the reminder being respired as CO2CO_2CO2​. The balance equations for each soil layer for the organic residue, manure, and soil pools are as follows.

Residue and manure pools (represented as R and M) gain mass through additions (IRI_RIR​ and IMI_MIM​ for residue and manure, respectively) and lose mass from decomposition following first order kinetics:

dRcdt=IRC−fEkRRC,\frac{dR_c}{dt}=I_{RC}-f_Ek_RR_C,dtdRc​​=IRC​−fE​kR​RC​, 3:5.1.1a

dMCdt=IMC−fEkMMC,\frac{dM_C}{dt}=I_{MC}-f_Ek_MM_C,dtdMC​​=IMC​−fE​kM​MC​, 3:5.1.1b

fE=(fTfWf0)fp,f_E=(f_Tf_Wf_0)^{fp},fE​=(fT​fW​f0​)fp, 3:5.1.1c

where fEf_EfE​ is the combined effect of the soil factors temperature (fTf_TfT​), moisture (fWf_WfW​), and aeration (f0f_0f0​), fpfpfp (0.67) is a power regulating the multiplicative effect of the three environmental factors, and kRk_RkR​ (0.05 day−1^{-1}−1) and kMk_MkM​ (0.025 day−1^{-1}−1) are the optimum decomposition rate (day−1^{-1}−1) for residues and manure, respectively. The addition of residues (or manures) from different sources are bulked with the existing pool and not tracked separately, with rates shown above assumed to apply to all sources. If these decomposition rates are made residue- or manure-specific, every time there is an addition of residues or manure the effective decomposition rates would have to be calculated as weighted averages of the rate of the already decomposing pools and that attributed to the newly added materials, with a weighting function that should change over time, or each pool tracked separately, both rather impractical propositions.

For the soil pools, the differential equations are as follows:

dSCdt=hRfEkRRC+hMfEkMMC−kSSC,\frac{dS_C}{dt}=h_Rf_Ek_RR_C+h_Mf_Ek_MM_C-k_SS_C,dtdSC​​=hR​fE​kR​RC​+hM​fE​kM​MC​−kS​SC​, 3:5.1.2a

dSNdt=hRfEkRRC+hMfEkMMCSCN−kSSN,\frac{dS_N}{dt}=\frac{h_R f_E k_R R_C+h_Mf_Ek_MM_C}{S_{CN}}-k_SS_N,dtdSN​​=SCN​hR​fE​kR​RC​+hM​fE​kM​MC​​−kS​SN​, 3:5.1.2b

where hRh_RhR​ and hMh_MhM​ are the residue and manure humification rates (kg kg−1^{-1}−1) and kSk_SkS​ is the apparent organic matter decomposition rate (day−1^{-1}−1). The humification rates depend on the current SCS_CSC​ and a reference SCS_CSC​ (SCCS_{CC}SCC​):

hR=hx(1−(SCSCC)α),h_R=h_x(1-(\frac{S_C}{S_{CC}})^{\alpha}),hR​=hx​(1−(SCC​SC​​)α), 3:5.1.3a

hx=0.09(2−e−5.5clay),h_x=0.09(2-e^{-5.5clay}),hx​=0.09(2−e−5.5clay),

hM=1.6hR,h_M=1.6h_R,hM​=1.6hR​, 3:5.1.3b

SCC=SBDZl(0.021+0.38clay)S_{CC}=S_{BD}Z_l(0.021+0.38clay)SCC​=SBD​Zl​(0.021+0.38clay) 3:5.1.4

Clay is the soil layer clay fraction (kg clay kg−1^{-1}−1 dry soil), SBDS_{BD}SBD​ is the soil layer bulk density (kg m−3^{-3}−3), and ZlZ_lZl​ is the soil layer thickness (m). The constant α\alphaα (default α=6\alpha=6α=6) modulates the response of the humification the current SCS_CSC​. The maximum attainable residue humification is approximately 0.18 kg kg−1^{-1}−1. The same humification is used for below and aboveground residues. The humification of C from manure was assumed to be 60% higher than that of fresh residues (approximately 0.29 kg kg−1^{-1}−1). The reference SCS_CSC​ or SCCS_{CC}SCC​ depends linearly on the soil layer clay fraction (Eq. 3:5.1.4) as proposed by Hassink and Whitmore (1997). When SC=SCCS_C=S_{CC}SC​=SCC​ then the humification is 0 and no accumulation of SCS_CSC​ above SCCS_{CC}SCC​ can occur (Kemanian and Stockle, 2010). In addition, this approach assumes a dependence of humification on the clay fraction, a dependence represented in many different ways in other models (e.g. Jenkinson, 1990; Bradbury et al., 1993). The control of hRh_RhR​ and hMh_MhM​ in Eq. [3.5.1.3] with α=6\alpha=6α=6 implies a non-linear response of hRh_RhR​ and hMh_MhM​ to SCS_CSC​. It is likely that a better formulation is needed for these functional equations to apply in organic horizons, tropical soils, or soils with a high proportion of volcanic ashes.

The C:N ratio of the newly formed organic matter or SCNS_{CN}SCN​ ranges from 8.5 to 14. The RCNR_{CN}RCN​ and MCNM_{CN}MCN​ determine how high SCNS_{CN}SCN​ can be when there is no mineral N available that can be an N source for the soil microbes. This estimate of SCNS_{CN}SCN​ is reduced depending on the mineral N availability, as follows:

SCN=8.5+2.7(1−11+(RCN110)3)(1+11+(Nmin8)3)S_{CN}=8.5+2.7(1-\frac{1}{1+(\frac{R_{CN}}{110})^3})(1+\frac{1}{1+(\frac{N_{min}}{8})^3})SCN​=8.5+2.7(1−1+(110RCN​​)31​)(1+1+(8Nmin​​)31​) 3:5.1.5

where NminN_{min}Nmin​ is the mineral N in the layer (mg N kg−1^{-1}−1 soil in this equation). The first term within brackets represents the control of the residues and the second term represents the control of mineral N on SCNS_{CN}SCN​ so that the higher the ratio of RCNR_{CN}RCN​ the higher the resulting SCNS_{CN}SCN​, and the higher the amount of NminN_{min}Nmin​ the lower SCNS_{CN}SCN​, with SCNS_{CN}SCN​ ranging from 8.5 to 14 kg C kg−1^{-1}−1 N in SOM. For manures, MCNM_{CN}MCN​ substitutes for RCNR_{CN}RCN​ and 55 substitutes for the constant 110. The ratio SCPS_{CP}SCP​ is derived from SCNS_{CN}SCN​ by assuming that the newly formed SOM will conserve the N:P ratio of the decomposing residue and manure. Thus, the C:N and C:P ratios of SOM are not constant but fluctuates according to these equations during the simulation.

Residues and manure may not supply sufficient N and P to satisfy the SCNS_{CN}SCN​ (Eq. 3:5.1.2b) and SCPS_{CP}SCP​ (Eq. 3:5.1.2c), in which case N and P will be mined from the inorganic soluble pools. If the inorganic pools cannot supply N and P for decomposition to proceed, then the decomposition rate of residue and manure is reduced. Therefore, lack of mineral N and P in solution may slow down decomposition. The net mineralization is obtained from:

MINRN=dRCdt(1RCN−hRSCN),MIN_{RN}=\frac{dR_C}{dt}(\frac{1}{R_{CN}}-\frac{h_R}{S_{CN}}),MINRN​=dtdRC​​(RCN​1​−SCN​hR​​), 3:5.1.6a

MINMN=dMCdt(1MCN−hMSCN),MIN_{MN}=\frac{dM_C}{dt}(\frac{1}{M_{CN}}-\frac{h_M}{S_{CN}}),MINMN​=dtdMC​​(MCN​1​−SCN​hM​​), 3:5.1.6b

where MINRNMIN_{RN}MINRN​ and MINMNMIN_{MN}MINMN​ are the net mineralization rates (kg m−2^{-2}−2 day−1^{-1}−1) from decomposing residues (Eq. 3:5.1.6a) and manure (Eq. 3:5.1.6b), respectively, with negative values indicating immobilization and positive values net mineralization. The same equation applies for P with appropriate C:P ratios. The SOM decomposition rate (kSk_SkS​) is calculated from:

kS=kxftoolfE(SCSCC)β.k_S=k_xf_{tool}f_E(\frac{S_C}{S_{CC}})^{\beta}.kS​=kx​ftool​fE​(SCC​SC​​)β. 3:5.1.7

The apparent SCS_CSC​ turnover rate (kSk_SkS​) is scaled down from an optimum of 4.5% yr−1^{-1}−1 for undisturbed soils (kxk_xkx​ = 0.000123 day−1^{-1}−1 so that kxk_xkx​ × 365 = 0.045) based on environmental conditions and SCS_CSC​. The power β\betaβ modulates kSk_SkS​ (default β\betaβ= 0.5) so that when SCS_CSC​ is low, the turnover rate slows sharply. Substituting Eq. 3:5.1.7 for kSk_SkS​ in Eq. 3:5.1.2a with β\betaβ= 0.5 renders a kinetics of order 3/2 for SCS_CSC​ decomposition, as opposed to the typical first order kinetics commonly use in SOM decomposition models (see Kemanian and Stockle, 2009). Tillage can enhance kSk_SkS​ through the factor ftoolf_{tool}ftool​. This factor is calculated independently for each soil layer and depends on the tillage tool mixing factor (fmixf_{mix}fmix​, range 0 to 1) and the soil texture. The mixing factor and the tillage depth determine the fraction of the soil layer that is mixed by a tillage operation. The ftoolf_{tool}ftool​ basal value is 1 and it is enhanced immediately after a tillage event based on the estimated cumulative fmixf_{mix}fmix​ (or fcmf_{cm}fcm​):

ftool=1+(3+5e−5.5clay)(fcmfcm+e1−2fcm)f_{tool}=1+(3+5e^{-5.5clay})(\frac{f_{cm}}{f_{cm}+e^{1-2f_{cm}}})ftool​=1+(3+5e−5.5clay)(fcm​+e1−2fcm​fcm​​) 3:5.1.8

The factor (ftoolf_{tool}ftool​) is reduced on a daily basis based on soil moisture to simulate soil settling. If ftoolf_{tool}ftool​ > 1 and a tillage operation is executed, the corresponding fmixf_{mix}fmix​ has to be added to the current fcmf_{cm}fcm​. This requires solving for fcmf_{cm}fcm​ by inverting Eq 3:5.1.6 before recalculating ftoolf_{tool}ftool​. Since Eq 3:5.1.6 is non-linear on fcmf_{cm}fcm​ the solution has to be obtained iteratively. In the SWAT+ code the solution for fcmf_{cm}fcm​ has been approximated by a functional equation to prevent recurrent iterations that can be computationally expensive. The SOM decomposition always causes net mineralization to be positive because humification is assumed to be zero. In actuality, the humification is not zero but since no explicit microbial pool is considered, the decomposition rate is an apparentapparentapparent decomposition rate that represents the net loss of C from the SC and underestimates the true turnover rate of organic C, N, and P (Jenkinson and Parry, 1989).