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Nutrient Management -- Nitrogen

U.S. Department of Agriculture, Agricultural Research Service(ARS), Great Plains Systems Research, Fort Collins, Colorado.

Nitrate Leaching and Economic Analysis Package (NLEAP) is a field-scale computer model developed to provide a rapid and efficient method of determining potential nitrate leaching associated with agricultural practices. It combines basic information concerning on-farm management practices, soils, and climate and then translates the results into projected N budgets and nitrate leaching below the root zone and to ground water supplies, and estimates the potential off site effects of leaching.

NLEAP has three levels of analysis to determine leaching potential; an annual screening, a monthly screening and an event-by-event analysis. The annual screening analysis may be used to indicate a potential for nitrate leaching. The monthly and event-by-event analysis can be used to demonstrate the effect of alternative management strategies on reducing nitrate leaching potential.

The NLEAP model was designed to answer questions regardiung potential leaching of nitrate. The processes modeled include movement of water and nitrate, crop uptake, denitrification, ammonia volatilization, mineralization of soil organic matter, nitrification, and mineralization - immobilization associated with crop residue, manure, and other organic wastes.

The screening procedure uses a simplified annual water and nitrogen budget and is designed to give only a general estimate of potential leaching of nitrate. The monthly budget analysis calculates leaching with consideration for the seasonal and monthly effects of percipitation, temperature, evapotranspiration, and farm management. The event-by-event analysis provides the best estimate of nitrate leaching. Its water and nitrogen budgets track the impacts of each precipitation, irrigation, fertilization, and tillage event on potential nitrate leaching. The event-based procedure is recommended for analysis of potential nitrate leaching to domestic water supply.

  • Validation testing on 50+ sites across 10 states has shown the model can predict residual soil nitrates and nitrate leaching to within approximately 20-50 lb N/acre/year.
  • Thorough documentation is available. The publication, Managing Nitrogen for Groundwater Quality and Farm Profitability, edited by R.F. Follet, D.R. Keeney, and R.M. Cruse and published by the Soil Science Society of America, contains the documentation for processes modeled in NLEAP. A standardized training workbook for the USDA - Natural Resources Conservation Service was developed by model leaders and national coordinators. A detailed training handbook is available.
  • The program uses data entry screens with pop-up data selection and on-line help menus. Internal checking verifies data type and range; errors are identified.
  • Four regionalized crop, soils, and climate data bases have been developed for the contiguous United States. Soils and crop data bases are available for all four regions. Climate data bases are currently available for the upper midwest, northeast, south and part of the western United States. In states where a regional data base is not available, the user must obtain and manually input the required soils and climate information. Users in all regions are encouraged to develop local data bases, as the regional data bases may not accurately describe local conditions.
  • The user can access material from the internal data base files, enter their own farm-specific and field-specific information, or use a mixture of user specified and data base information.
  • User input may be saved to a data file for use in future runs.
  • The regional climate data sets (originating from the NRCS, National Water and Climate Center) contain monthly and daily precipitation, monthly average pan evaporation, and monthly average air pan evaporation, and monthly average air temperature for an average, a wet year, and a dry year.
  • The regional soils data sets are summarized from NRCS Soils-5 and Soils-6 data. They represent the majority of agricultural soils in each of the four regions. The soils and climate datasets are available with the book published through the Soil Science Society of America.
  • The analysis and discussion option provides a detailed written summary and analysis that can help identify shortcomings in water and nitrogen management strategies.
  • The graphics option allows the user to graph model inputs and /or outputs alone or in combination for individual consideration or simultaneous comparison.
  • Modeling soil nitrogen processes in organic soils is not currently available. Their inclusion will require additional research and modeling efforts.
  • Management problems associated with insects, disease, and weed pests; nutrient deficiencies with phosphorous, potassium, and micronutrients; and other cultural practices, such as planting populations, are not considered. But, the user should consider the effects of these problems when estimating crop yield.
  • Care should be taken when using the screening analysis or the monthly analysis to model nitrogen transformations and nitrate leaching in sandy or course grain soils in areas that have a relatively high level of precipitation or irrigation.
  • The model should not be used where rapid water infiltration, leaching, denitrification, and ammonium volatilization require time steps smaller than 1 day. Other situations that should not be modeled include those in which water and solute transport in an aquifer are important considerations, complex layering in the soil profile exists, or a shallow water table supplies crop needs.
  • Sequential year runs involving complex crop rotations are difficult to simulate.

NLEAP requires an IBM AT 186, 386, 486, or pentium compatible system that has at least 640K of low memory, DOS Version 2.1 or newer, and 4.5 megabytes of disk storage space. The use of a math coprocessor is not required, but is recommended. NLEAP may be run with either a monochrome or color monitor. Program execution is unpredictable if any memory resident programs, such as Sidekick, PC Tools, Ram disk emulators, and network programs, are loaded.

The capability to operate NLEAP in conjunction with GRASS and other GIS systems has been developed and is currently being validated.

  • A metric version of NLEAP is being developed for international use.
  • The model is being modified to simplify simulation of sequential year runs and expand file handling capabilities.
  • A routine is being added that will separate gaseous denitrification losses into N2 and N2O components.
  • Fertilizer and irrigation input routines are being improved.
  • Future enhancements will permit the user to directly access NRCS soil and climate data bases for NLEAP input.
  • A plan is underway to reprogram NLEAP using the C++ language, so that the model may be converted to the UNIX platform and incorporated into the NRCS Field Office Computing System (FOCS).

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