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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