A Watershed Simulation Model

APEX has components for routing water, sediment, nutrients, and pesticides across complex landscapes and channel systems to the watershed outlet as well as groundwater and reservoir components.  A watershed can be subdivided as much as necessary to assure that each subarea is relatively homogeneous in terms of soil, land use, management, and weather.  APEX was constructed to evaluate various land management strategies considering sustainability, erosion (wind, sheet, and channel), economics, water supply and quality, soil quality, plant competition, weather, and pests.

The routing of water, sediment, nutrient, and pesticide capabilities are some of the most comprehensive available in current landscape-scale models and can be simulated between subareas and channel systems within the model

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Components

• Carbon Cycling Routine

  • estimates soil changes as a function of climatic conditions, soil properties, and management practices
  • simulates storage and transfer of carbon and nitrogen among pools

• Manure Management Inputs

  • manure production and ingredient inputs can be solid or liquid form for a variety of livestock, swine, and poultry

• Manure Erosion

  • measures organic nutrient and carbon losses from feedlots and manure application

•  Nitrogen (N) Cycling and Losses

  • simulates the complete nitrogen cycle: atmospheric N inputs, fertilizer/manure N applications, crop N uptake, mineralization, immobilization, nitrification, denitrification, ammonia volatilization, organic N transport on sediment, and nitrate-nitrogen losses in leaching, surface runoff, lateral subsurface flow, and tile flow

• Feedlot Dust

  • measures dust emissions and distribution

• Routing Component

  • routes water through channels or flood plains to simulate long-term water, sediment, nutrient, and pesticide yields from whole farms and small watersheds

• Reservoir Component

  • determines amount of flood storage

• Economics Component

  • simulates cost and income accounting

Capabilities

• Climate Change

  • CO₂ effects on Plants–Increases radiation use efficiency
  • Atmospheric CO₂ Change–Input or simulated as a function of time
  • Greenhouse Gas Emission–CO₂, N₂O

• Soil Water (FC – WP)

  • Input/Static–Field capacity and wilting point
  • Calculated, Rawls–A function of soil texture and organic C
  • Calculated, Nearest Neighbor–Uses data base of 38,000 soils to obtain nearest fit for texture and organic C

• PET Equations

  • Penman–Uses temperature, solar radiation, wind speed, humidity
  • Penman-Monteith–Same inputs as Penman but improved equations, good choice if inputs are available
  • Priestly-Taylor–Uses temperature and solar radiation
  • Hargreaves–Temperature input, computes extraterrestrial radiation, use when radiation data is limited
  • Baier-Robertson–Temperature input, developed in Canada for use in cold climates

•  Spatial Rainfall Generator

  • Distributes rainfall to subareas based on duration and distance from storm centroid

• Peak Runoff Rate

  • Modified Rational–Based on maximum short duration rainfall amount and duration
  • SCS TR55–NRCS method

• Runoff Volume Estimate

  • Five Variations of Curve Number–Directly linked to soil water or indexed to CN retention parameter
  • Green & Ampt–Infiltration equation

• Air Quality

  • Feedlot Dust Distribution–Dust emission and down wind deposition

• Irrigation

  • Manual–Scheduled by date and volume
  • Automatic–Triggered by water stress, applies volume to bring soil water to field capacity
    • can take water from stream, reservoir or aquifer
    • can transfer water from one stream or reservoir to another

• Water Table Dynamics

  • Driven by 30 day antecedent rainfall and ET

• Grazing/Animals

  • Automatic Rotation among Subareas–Start and stop grazing dates input or determined by available forage
  • Simple Stocking Rate Input–Same start and stop rules without rotation
  • Confined Feeding Areas–Animals may be totally confined or allowed in a pasture a fraction of the day
  • Preferential Grazing

• Furrow Dikes

  • Built and removed by date, auto rebuild when overtopped, conserve water, control erosion

• Pesticides

  • Pesticide Damage to Crops–Reduces crop yield, pest growth function of moisture, temperature, plant cover
  • Pest Control by Applying Pesticides–Reduces pest population according to kill effectiveness of pesticide
  • Pesticide Fate–Uses modified GLEAMS to estimate pesticide losses, routes through reaches and reservoirs
  • Computes Probability Distribution of Runoff Losses Duration from 1-90 Days

Applications

APEX can perform long-term continuous simulations for modeling the impacts of different nutrient management practices, tillage operations, conservation practices, alternative cropping systems, and other management practices on surface runoff and losses of sediment, nutrients, and other pollutant indicators.  Example applications include:

• evaluate effects of global climate and carbon dioxide changes on crop yields
• assess alternative manure applications and other management scenarios
• design biomass production for energy
• demonstrate hydrologic balance components and pollutant transport for different cropping and forestry production systems
• simulate intensive rotational grazing scenarios depicting movement of livestock between paddocks
• evaluate the effects of different tree harvesting treatments on forested watersheds
• analyze the potential benefits of “woody draws” (relatively small, natural drainage areas covered by trees or shrubs in agricultural landscapes)
• determine cropping and conservation practice effects on lake systems
• simulate landscape management decisions such as harvest unit size, total area harvested, and rotation length
• assess environmental impact of pollutants into lakes
• evaluate effects of buffer strips
• simulate nonpoint-source pollution impacts from cultivated cropland
• design economical and environmentally safe landfill sites

Model Downloads