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Geographic Information Systems and Models
The latest hydrologic/water quality computer models developed by ARS
(USDA-Agricultural Research Service) and Texas Agricultural Experiment
Station (TAES) are used to address the overall ecosystem management of
an entire basin or watershed. This process involves both field or farm
scale models and watershed or basin scale models:
Whole farm/small watershed model – APEX (Agricultural Policy
Environmental Extender)
Basin scale model – SWAT (Soil and Water Assessment Tool)
The State Conservationist of NRCS in Texas recognized the benefit these
tools could have in addressing water quality/quantity issues in the
state. To facilitate the use of this new technology, he located a staff
of NRCS employees at the Blackland Research Center to work directly with
scientists and researchers. This group now known as the Water Resource
Assessment Team (WRATs) provides feedback for model improvement. These
improvements are related to input and output as the models are applied
in small watersheds over the State. The WRATs also assemble or develop
GIS (Geographic Information System) data required as input to the
models. Substantial amounts of reimbursable funding have been provided
from partnerships with others to carry out this work.
A multi-agency effort has been underway since 1992 to adapt these models
to assess current problems and the effects BMPs (Best Management
Practices) can have in abating the problems associated with watershed
management. TAES has assisted ARS with databases and GIS interfaces for
the models and USDA-NRCS has helped with technology transfer, support
and application of the models within NRCS and for many of their clients
and partners. The multi-agency effort allows pooling of technical
resources, funding, in-kind services and orderly transfer of technology
The WRATs use several GISs including the US Corps of Engineers' (USACE)
Geographic Resources Analysis Support System (GRASS), Blackland GRASS,
ESRI's Arcview & ArcGis, and ERDAS's Imagine. Simulations using SWAT are
being performed in UNIX on SUN workstations and in Windows on desktop
PCs. APEX simulations are performed in the Windows environment on
desktop PCs. Relational databases used by NRCS include INFORMIX and
Access.
Most of the developmental work involving GIS at the ARS/TAES laboratory
has been with a base scale of 1:250,000 which is readily available for
all of the United States. These GIS layers are the foundation for the
HUMUS (Hydrologic Unit Model for the United States) project , a
cooperative effort between NRCS, ARS, and TAES at the Temple, TX
laboratory. The purpose of the HUMUS project is to assist in the
Resource Conservation Act (RCA) assessment of the status and condition
of water resources of the nation under current and projected management
conditions. SWAT model technology was originally developed for the HUMUS
project.
The need for assessments of smaller areas with a high level of detail
requires that greater detail of GIS databases be available. The HUMUS
project as an example, uses the STATSGO soils geographic database
(1:250,000 scale base) as one of the GIS layers in simulating entire
river basins. STATSGO polygons represent soils associations that may
include 20-30 individual soil series. Soils information for many of the
WRAT projects is developed from CBMS (computer based mapping system
1:24,000 scale) and SSURGO data that provide much more detail in the GIS
layer and model input. A link from the spatial data to the relational
soils database provides soil properties for each soil to the SWAT model
input.
The integration of the water quality model and GIS reduces significantly
the time to prepare input data for models and simplifies model
operation. As GIS layers become readily available, the effort to
simulate current versus projected management will involve minimum time
frames and personnel.
Examples of SWAT output
Examples of GIS graphics
SWAT is a basin-scale, continuous time water quality model integrated
with a GIS to extract input data to simulate basin hydrology and
conditions. Development of SWAT involved combining a routing procedure
to the SWRRB simulation model. This allows loadings at sub-basin outlets
to be routed through the stream network on a real time basis to the
receiving reservoir or point of interest. Integration of GIS and SWAT
eased the task of providing input for hundreds of sub-basins and
multiple simulations.
SWAT has a unique feature that allows the output of other model runs to
be imported at stream routing nodes throughout the watershed simulation.
A simulation using very detailed data for a small subbasin of the
watershed can be integrated into a general assessment of the entire
watershed above a reservoir. This can indicate the targeted basin's
effects on loadings at a basin outlet or reservoir. SWAT can handle
other features such as point sources of water inflow/outflow and can
accommodate irrigation diversions, return flows, wastewater treatment
outfalls, and other municipal or industrial permitted uses. To be a
realistic simulation of the watershed, the model must handle both
nonpoint sources and all permitted point sources as well as water
transfers in or out of the basin. Thus predicted stream flow can be
compared to measured stream gage records.
Initially, SWAT was run on the small watersheds in much the same manner
as for the HUMUS Project. As more detailed GIS data was developed, the
subbasins were reduced in size to work toward more reasonable
representation of actual conditions.
SWAT and GIS integrated as a modeling tool can guide management
decisions regarding runoff, sediment, nutrient and pesticide loadings
for small watersheds. This tool allows assessment or evaluation of
effects from a watershed based on hydrologic and hydraulic boundaries
consistent with basic principles and standards for planning treatment
alternatives in water resource projects.
The Agricultural Policy/Environmental eXtender (APEX) model was
developed for use in whole farm/small watershed management
(Williams et al., 2000). The model 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.
Management capabilities include irrigation, drainage, furrow
diking, buffer strips, terraces, waterways, fertilization,
manure management, lagoons, reservoirs, crop rotation and
selection, pesticide application, grazing, and tillage. Besides
the farm management functions, APEX can be used in evaluating
the effects of global climate/CO2 changes; designing
environmentally safe, and economical landfill sites; designing
biomass production systems for energy; and other spin-off
applications. The model operates on a daily time step and is
capable of simulating hundreds of years if necessary. Farms may
be subdivided into fields, soil types, landscape positions, or
any other desirable configuration. APEX is not currently
directly integrated with a GIS.
The individual field simulation component of APEX is taken from
the Environmental Policy Integrated Climate (EPIC) model. The
drainage area considered by EPIC is generally a field-sized
area, up to 100 ha (247 acres), where weather, soils, and
management systems are assumed to be homogeneous. The major
components in EPIC are weather simulation, hydrology,
erosion-sedimentation, nutrient cycling, pesticide fate, plant
growth, soil temperature, tillage, economics, and plant
environment control. Although EPIC operates on a daily time
step, the optional Green and Ampt infiltration equation
simulates rainfall excess rates at shorter time intervals (0.1
h). The model offers options for simulating several other
processes including five PET equations, six erosion/sediment
yield equations, and two peak runoff rate equations. EPIC can be
used to compare management systems and their effects on
nitrogen, phosphorus, pesticides and sediment. The management
components that can be changed are crop rotations, tillage
operations, irrigation scheduling, drainage, furrow diking,
liming, grazing, tree pruning, thinning, and harvest, manure
handling, and nutrient and pesticide application rates and
timing.
The APEX model was developed to extend the EPIC model
capabilities to whole farms and small watersheds. In addition to
the EPIC functions, APEX has components for routing water,
sediment, nutrients, and pesticides across complex landscapes
and channel systems to the watershed outlet. APEX also has
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,
etc. The routing mechanisms provide for evaluation of
interactions between subareas involving surface runoff, return
flow, sediment deposition and degradation, nutrient transport,
and groundwater flow. Water quality in terms of nitrogen
(ammonium, nitrate, and organic), phosphorus (soluble and
adsorbed/mineral and organic), and pesticides concentrations may
be estimated for each subarea and at the watershed outlet.
Commercial fertilizer or manure may be applied at any rate and
depth on specified dates or automatically. The GLEAMS pesticide
model is used to estimate pesticide fate considering runoff,
leaching, sediment transport and decay. Because of routing and
subdividing there is no limit on watershed size. However, a
practical limit may be about 2500 km2 (965 mi2) because of the
detailed crop/management system of APEX. The major uses of APEX
have been dairy manure management to maintain water quality in
Erath and Hopkins Counties, TX, and a national study to assess
the effectiveness of filter strips in controlling sediment and
other pollutants. APEX has its own data bases for weather
simulation, soils, crops, tillage, fertilizer, and pesticides.
The potential for use of APEX and SWAT computer models for
watershed management and nonpoint source pollution assessment is
in an upward trend and toward an unlimited use of the
technology.
The WRAT staff has provided support to several staffs in setting up
projects for modeling with APEX and/or SWAT on both UNIX workstations
and PC computers. This support has ranged from simply loading all
software and databases on the hardware to the other extreme of
developing much of the GIS data and step by step assistance in executing
the model runs. In most cases the support has been provided by the
staffs coming to the Temple laboratory and working along side the WRAT
staff for periods of time of a few days up to intermittent visits over
several months.
Detects of very small concentrations of pesticides in several drinking
water sources in the State have accentuated the need for computer
modeling tools. These issues require a fast but scientific analysis with
realistic alternative solutions to maintain public water quality. It is
also essential that the technology used for these analyses be uniformly
applied across the vast ranges of ecosystems, climate and needs of the
landscape of the State. These issues exist far beyond the boundaries of
Texas – they can and should be applied nationwide and even worldwide.
The primary functions that can be provided by the WRAT staff to other
staffs within NRCS or other agencies can be summarized as:
Training and application support in GIS (GRASS, Arcview and ArcInfo)
Training and application support in APEX and SWAT.
Application support in classification of LandSat imagery to define
categories of land cover within GIS.
Turnkey analysis of water quality/water quantity problems on a watershed
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