Soil Erosion Network: Model Metadata
APEX - Water and Wind Erosion

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1 Model identification Soil Erosion Network - Model
1.1 Model name APEX - Water and Wind Erosion
1.2 Most recent version APEX 4220

APEX 8190 
(information updated 12/2000)

1.3 Date of release August 1994
2 Water erosion and Wind Erosion
3 Contact person
3.1 Name J R Williams
3.2 Address USDA-ARS, 808 E Blackland Road, Temple, TX 76503
3.3 Tel +1 817 770 6508
3.4 Fax +1 817 770 6561
3.5 Email
4 Model Author(s) J Williams, J Arnold, J Kiniry, K Potter, K King
5 Model components
5.1 Water erosion: USLE/MUSLE (several variations)
5.2 Wind erosion: Modified Manhattan KS model (Bagnold energy component)
5.3 Hydrology: Runoff- modified SCS CN; Peak rate-modified rational or SCS TR-55; ET: 4 options options, Penman, Penman-Montieth, Hargreaves, Priestly-Taylor
5.4 Site/topography: Whole Farm/small watershed/landscape; watershed may be subdivided
5.5 Plant growth: General crop growth model simulates about 30 crops
5.6 Management: Irrigation, fertilizer, liming, furrow diking, crop rotation. Variety of tillage operations
5.7 Soil: Up to 30 soil layers. Core data: Bulk density, field capacity, wilting point, organic Co texture
5.8 Chemistry: N and P cycling; pesticide fate; water quality
5.9 Weather: Precipitation, temperature (max, min) solar radiation, relative humidity, wind (speed & direction). Input or generated
6 Model characteristics
6.1 Spatial
6.1.1 Class of area: Whole farm, small watershed, landscape
6.1.2 Flow routing system: Streamline - natural subareas
6.1.3 Minimum area: 1 ha
6.1.4 Maximum area: 100 km2
6.2. Temporal
6.2.1 Timestep: 1 day
6.2.2 Single or multiple event? Continuous
6.2.3 Maximum simulation duration: 1000 years
7 Model's representation of processes
7.1 Water erosion processes
7.1.1 Interrill: Combined
7.1.2 Rill: Combined
7.1.3 Gully: None now - development is high priority
7.1.4 Streambank: Yes - sediment routing degrades channel as a function of flow velocity
7.1.5 Deposition: Yes - governed by flow velocity and particle size
7.2 Wind erosion processes:
7.2.1 Creep/surface roll: Yes - Bagnolds eg
7.2.2 Saltation: Yes - Bagnolds eg
7.2.3 Suspension; Yes - Bagnolds eg
7.3 Hydrological processes:
7.3.1 Evaporation/transpiration Penman, Penman - Montieth, Hargreaves, Priestly- Taylor
7.3.2 Runoff: SCS CN modified for volume; modified rational of SCS TR55 for peak rate
7.3.3 Infiltration: SCS CN modified
7.3.4 Subsurface flow: Yes - lateral and vertical computed simultaneously when field capacity is exceeded. Vertical governed by saturated conductivity; lateral by land slope
7.3.5 Return flow: Yes - function of lateral flow and subsurface travel time from watershed centroid to outlet
7.4 Plant processes:
7.4.1 Light interception and photosynthesis: Yes - Beer's law; potential daily biomass growth limited by minimum stress factor - water, nutrients, temperature, aeration
7.4.2 Dry matter and residue: Yes - at harvest yield is removed (harvest index). Standing dead residue falls to the soil surface at a rate driven by accumulated rainfall, and surface & subsurface residue is decayed as a function of soil moisture and temperature.
7.4.3 Root growth: Depth driven by heat units; limited to max for crop or soil depth. Root mass a function of water use. Limited by bulk density, alumini saturation, or temperature
7.4.4 Pests: Insects, weeds, diseases. Generic model driven by moisture, temperature & biomass
7.4.5 CO2 sensitivity: Through canopy resistance in Penman-Montieth eqn and crop coefficient converting energy to biomass
7.5 Soil: Up to 30 layers permitted with variable thickness. Erosion removes soil from profile; layer 2 becomes thinner until it approaches O, then removed from system. Properties change with erosion, tillage mixing, mineralization of organic matter
7.5.1 Crust development: -
7.5.2 Aggregate properties: Large and small aggregates estimated from texture - and used in sediment routing
7.6 Chemistry:
7.6.1 Nutrients: N and P cycles: mineral, organic (NO3, NH3, active & stable organic N; labile P, active and stable mineral and organic P. Water and sediment transport, crop uptake, mineralization- immobilization nitrification, denitrification, N fixation, volatilization
7.6.2 Carbon: Tracks organic N
7.6.3 Pesticides: GLEAMS pesticide component. Wash off of plants, decay in soil & on foliage, leaching runoff and sediment transport
7.6.4 Other contaminants: None
7.7 Weather: Precipitation, temperature (max & min), solar radiation, wind speed, relative humidity. Input or generated
8 Input Data (* = Mandatory)
8.1 Weather: Precipitation*, temperature* (monthly means), solar radiation, wind speed, relative humidity
8.2 Soil: Bulk density*, texture*, Organic C*, field capacity, wilting point, saturated conductivity, % rock, pH, sum of bases, CEC, Al saturation, CaCO3, labile P, mineral P active & stable, NO3, Organic N active & stable.
8.3 Hydrology: SCS CN2, watershed area, channel length & steepness, Mannings N. Optional: channel depth
8.4 Plant cover: Crop parameter table; planting and harvest dates
8.5 Soil surface cover: Initial standing and flat residue
8.6 Management: Tillage table; operation schedule (complete crop rotation by dates)
8.7 Topography/site characteristics: Land slope & steepness, Mannings N
8.8 Micro-topography: Random roughness and ridge interval and height created by each tillage operation
9 Output data Weather, hydrology, erosion, nutrients, pesticides, crop production, soil properties. By sub-area, routing reach and total watershed. Option S: daily, monthly, annual; user selects outputs
10 Programming language Fortran
11 Computer requirements IBM PC compatible with at least 640K
12 Documentation
12.1 Scientific documentation: Sharpley & Williams, 1990. EPIC-Erosion/Productivity Impact Calculator: 1. Model Documentation, USDA Tech Bull # 1768.
12.2 User's guide: EPIC User's Guide; USDA ARS SCS, and TAES; Vanicek and Daneshil Editors.
12.3 Technical documentation: Sharpley & Williams, 1990. EPIC-Erosion/Productivity Impact Calculator: 2. User Manual.
13 Availability Upon request from developer

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