1
This paper is a part of an organized joint effort from
within the ASCE, Environmental and Water Resources Institute (EWRI), Water
Quality and Drainage Committee under the Task Committee "Non-Point Source
Water Quality Models: Their Use and Application"; and the USDA-CSREES
Southern Region Research Project S-273 "Development and Application of
Comprehensive Agricultural Ecosystems Models". This paper was
originally presented at the 1998 International ASAE Meeting in Orlando, FL as
Paper Number 982199.
Theo
A. Dillaha, Mary Leigh Wolfe, Adel Shirmohammadi, F. Wes Byne2
Summary:
ANSWERS-2000
is a distributed parameter, physically-based, continuous simulation, farm or
watershed scale, upland planning model developed for evaluating the
effectiveness of agricultural and urban BMPs in reducing sediment and nutrient
delivery to streams in surface runoff and leaching of nitrogen through the
root zone. The model is intended for use by planners on ungaged watersheds
where data for model calibration is not available. The model divides the area
simulated into a uniform grid of square (1 hectare or smaller), within which
all properties (surface and subsurface soil properties, vegetation, surface
condition, crop management, and climate) are assumed homogeneous. The model
uses breakpoint precipitation data and simulates hydrologic processes with a
30-second time step during runoff events and with a daily time step between
runoff events. The model simulates interception; surface retention/detention;
infiltration; percolation; sediment detachment and transport of mixed particle
size classes; crop growth; plant uptake of nutrients; N and P dynamics in the
soil; nitrate leaching; and losses of nitrate, ammonium, total Kjeldahl
nitrogen, and P in surface runoff as affected by soil, nutrient, cover and
hydrologic conditions. The model has an ArcInfo based user interface that
facilitates data file creation and manipulation. The model is in the public
domain and is available via ftp.
Keywords:
Water quality, modeling, erosion, nutrients, watershed
Introduction
This
paper was developed as part of the "Project to Evaluate Use and
Application of Water Quality Models", which is a component of Southern
Regional Research Projects S249, "The Impact of Agricultural Systems on
Surface and Groundwater Quality", and S271, "Development and
Application of Comprehensive Agricultural Ecosystems Models". The
objective of this project is not to validate or compare the accuracy of the
various models, but rather to document the intended use/purpose of the models,
general model characteristics, verified applications, input/output
requirements, known limitations, etc. This paper describes the ANSWERS model
portion of this project.
ANSWERS
History
Beasley
and Huggins (1982) developed the original ANSWERS (Areal Nonpoint Source
Watershed Environment Response Simulation) model in the late 1970s. The model
was based on one of the first true distributed parameter hydrologic models
(Huggins and Monke, 1966). The original ANSWERS was a distributed parameter,
event-oriented, planning model developed to evaluate the effects of BMPs on
surface runoff and sediment loss from agricultural watersheds. ANSWERS
subdivides the watershed into a uniform grid of square cells (Figure 1). Land
use, slopes, soil properties, nutrients, crops, and management practices are
assumed uniform within each cell. Differences between cells allow the model to
consider the heterogeneous nature of watersheds and the site specific
effectiveness of individual BMPs. Typical cell sizes range from 0.4 to 1 ha
with smaller cells providing more accurate simulations. Ten to twelve
parameter values must be provided for each homogeneous cell. Within each cell,
the model simulates interception, surface retention/detention, infiltration
using Holtan's method (Holtan, 1961), surface runoff, percolation through the
infiltration control zone, sediment detachment and sediment transport. Flow
was from routed downslope to adjacent overland flow cells or in channel cells.
The model could simulate BMPs such as conservation tillage, ponds, grassed
waterways, tile drainage (Bottcher et al., 1981) and other practices whose
effects on the physically based model input parameters could be described. An
original weakness of the ANSWERS model was its erosion model, which was
largely empirical and simulated only gross sediment transport. The model was
modified in the early 1980s to simulate the particle size distribution of
eroded sediment (Dillaha and Beasley, 1983) using Yalin's method (Yalin, 1961)
to estimate sediment transport. Rewerts and Engel (1991) developed GIS
interfaces for this version of the model. In the late 1980s, phosphorus (Storm
et al., 1988) and nitrogen (Dillaha et. al., 1988) transport versions of the
event-oriented model were developed. They considered the transport of
dissolved and adsorbed orthophosphorus, nitrate and dissolved and adsorbed
ammonium and TKN.
The
current version of the model, ANSWERS-2000, is a continuous simulation model
that was developed in the mid 1990s (Bouraoui and Dillaha, 1996). In this
version, the nutrient submodels were overhauled and improved infiltration
(Green and Ampt), soil moisture and plant growth components were added to
permit long-term continuous simulation. Bouraoui (1994) describes the current
version of the model in detail. ANSWERS-2000 simulates transformations and
interactions between four nitrogen pools including stable organic N, active
organic N, nitrate and ammonium. Transformations of nitrogen include
mineralization simulated as a combination of ammonification and nitrification,
denitrification, and plant uptake of ammonium and nitrate. The model maintains
a dynamic equilibrium between stable and active organic N pools. Four
phosphorus pools are simulated: stable mineral P, active mineral P, soil
organic P and labile P. Equilibrium is maintained between stable and active
mineral P and between active mineral P and labile P. Plant uptake of labile P
and mineralization of organic P are also simulated.
The
continuous version, ANSWERS-2000, was tested on two watersheds in
Watkinsville, Georgia, and performed well in predicting runoff, sediment,
nitrate, dissolved ammonium, sediment-bound TKN, and dissolved phosphorus
losses from both watersheds (Table 1 and Figures 2 to 8 for the P2 watershed).
The model did not predict sediment-bound ammonium losses from either watershed
well. The model was also tested on the 1153 ha Owl Run watershed in Virginia.
The model performed well for the largest storms, and cumulative predictions of
runoff volume, sediment yield, nitrate, ammonium, sediment-bound TKN, and
orthophosphorus were within 40% of the measured values (Table 2). In a
practical application of ANSWERS-2000, the model was run for eight years on a
225 ha subwatershed in the Nomini Creek watershed in Virginia. Potentially
critical source areas were selected, and BMPs (conservation tillage) were then
implemented on 10, 20, 30 and 100% of the cropland in the watershed. Targeting
was found to significantly increase the efficiency of BMP application with
respect to water quality (Table 3). For example, nutrient reductions were 2.4
and 2.1 times as great on a per hectare basis for nitrogen and phosphorus,
respectively, when BMPs were targeted to 10% of the cropland with the highest
sediment losses as compared to application of BMPs to all cropland. The model
also showed that fields that had very high sediment and nutrient yields did
not always have large impacts on yields at the watershed level.
To
facilitate use of the model, the ANSWERS-2000 NPS decision support system was
developed (Wolfe et al. 1995). This ArcInfo based decision support system has
a knowledge-based system to advise the user on parameter selection and
database creation. The user interface also keeps track of BMP scenarios
evaluated and controls model output. The user interface links components and
functions of the ANSWERS-2000 modeling system in a manner that is reasonably
transparent to the user. A set of menus, prompts, helps, and rule-based
functions guide the user, but does not eliminate interaction with the system.
Procedures are incorporated to flag problems associated with incorrect data
and report when system constraints have been violated and provide appropriate
corrective action. The user interface consists of a main menu and secondary
menus. The menus allow the user to provide site-specific information for input
parameters for the ANSWERS-2000 model. The modeling-related tasks facilitated
by the interface include the following: selection of the land area of interest
(watershed, farm, or field) from a GIS coverage; automatic access to a soils
parameter file with options for replacing general parameter values with
site-specific information; developing the grid overlay of the watershed;
calculation of topographic characteristics; generating synthetic weather data
if desired; assigning crop and tillage parameter values; creation of the
ANSWERS-2000 input data file; running the model; displaying model output;
applying alternative practices to user-specified land areas; and comparing
predicted output values for alternative scenarios. In addition, a help
function in the interface includes specific information for the user on how to
proceed with the different menu selections.
In a
recent development, a groundwater component was added to ANSWERS-2000. This
version of the model was validated at the local, field and watershed scales.
At the local and field, it accurately predicted drainage below the root zone
and evaporation for different vegetative covers. At the watershed scale, it
accurately reproduced piezometric levels and trends across the watershed
(Bouraoui et al., 1997). It is not known if this version of the model
simulates nutrient transport.
Current
Developments
Additional
work on ANSWERS-2000 is currently being sponsored by the USDA (USDA-NRI
project), the Virginia Chesapeake Bay Local Assistance Department, and the
Virginia Water Resources Research Center. The principal objectives of these
projects are to:
1.
Replace the existing empirical sediment detachment submodel with a more
reliable and robust physically-based sediment detachment submodel.
2.
Add a channel erosion and scour submodel.
3.
Improve the existing nitrogen cycle/ammonium submodel to improve predictions
of dissolved and adsorbed ammonium transport.
4.
Develop new submodels to simulate the effects of buffers, detention
facilities, and wetlands on sediment and nutrient loss.
5.
Develop new procedures to better simulate nutrient loss from turf areas in
urban areas.
6.
Develop new procedures or submodels to simulate the effects of urbanization on
sediment and nutrient yields and channel stability in rural watersheds.
7.
Improve the user interface.
Model
Evaluation Criteria for ANSWERS-2000
1
Model Use Characteristics
A·
Intended Use of the Model
1)
Water quality and hydrologic characteristics addressed
Surface
runoff (continuous hydrograph at watershed outlets and other designated cells)
Erosion
and sediment transport (up to 10 particle size classes). Sediment yield from
and or deposition in each cells. Time varying sedigraph for different particle
size classes at watershed outlets or designated cells.
Nitrate
and adsorbed and dissolved ammonium, TKN, orthophosphorus yields in surface
runoff from each cell. Nitrate leaching below the root zone from each cell.
Nutrient concentrations versus time at watershed outlets or designated cells.
Processes
simulated: spatially varying breakpoint rainfall, interception, surface
retention/detention, infiltration, percolation, surface runoff (overland and
channel flow), crop growth, evapotranspiration, surface cover, sediment
detachment and transport for up to 10 particle size classes, soil nitrogen and
phosphorus cycles (organic and inorganic, dissolved and adsorbed nutrient
pools, nitrate leaching, nutrient losses in surface runoff
2)
Modeling Scale:
Spatial:
field, farm (multiple watersheds or parts of multiple watersheds), watershed.
Uniform grid of square cells 1 hectare or less in size is required for
computational accuracy. Intended for use in medium sized watersheds (500 to
3000 hectares where upland processes dominate the hydrologic cycle.
Temporal:
Continuous simulation. 30 second time step during runoff events, daily time
step otherwise. Recommended simulation period of 20 or more years.
3)
Economics: Not currently simulated.
D·
Target Audience
Intended
for planners but is currently impractical for use by anyone except experienced
modelers and researchers because of lack of documentation and a more
user-friendly interface. Currently used primarily by researchers with a strong
knowledge of hydrologic, soil and crop processes.
E·
Verified Applications
6)
Extent of Model Tests
ANSWERS-2000
validation studies have been conducted on the USEPA/USDA Watkinsville, GA
small watersheds/fields (Piedmont) and the Owl Run (Piedmont) and Nomini Creek
(Coastal Plain) watersheds in Virginia for 14 to 36 month periods. Model
predictions of average annual yields with little or no calibration compared
favorably (within a factor of two) with observed data except for ammonium.
Additional tests are needed with longer-term records and in other regions. See
Bouraoui (1995) and Bouraoui and Dillaha (1996).
7)
Sensitivity Analysis
Sensitivity
analysis was conducted on the data sets developed for the Watkinsville, GA
validation runs and are discussed in detail by Bouraoui (1995). The parameters
having the biggest impact on model output were the soil clay and silt content
because of the impact on infiltration and surface runoff. The most sensitive
parameters for the major outputs include:
Runoff
volume: silt and clay content, solar radiation.
Sediment
yield: clay content
Nitrate
in runoff: clay content
Dissolved:
sensitive runoff volume parameters
Sediment-bound
TKN: clay content, soil N level, active organic N
Sediment-bound
P: initial labile P, clay and silt content
Dissolved
P: initial labile P, clay and silt content
H·
Input/Output
9)
Ease of developing input data sets:
Relatively
straight forward with the ArcInfo based user interface. Very difficult and
time consuming without.
10)
Requirements for field measured inputs:
Model
was developed as a planning tool and is intended for use on ungaged
watersheds. Calibration is beneficial but not required. Most input parameters
can be obtained from soil surveys, topographic and landuse maps and the user
interface, which aids in the selection of parameters and provides default
values.
11)
Richness of model output summaries:
Raw
output is overwhelming, but the user interface condenses, summarizes and
provides graphical output, that is relatively easy to understand and use.
12.
Model Characteristics
M·
Source and Availability of Model:
ANSWERS-2000
source code (FORTRAN 77) is available via FTP at: ftp://dillaha.ageng.vt.edu/pub/models/answers
via anonymous login. The existing ArcInfo interface (Sun Workstation) is still
undergoing testing and is being converted to Windows 98/NT ArcInfo, but should
be available by January 1999. An ANSWERS home page for model information and
distribution is under development and will be available in January 1999.
N·
Availability of Continuing Education/Training Opportunities for Model Users:
No
training programs have been conducted to date. None are anticipated unless a
public or private organization assumes responsibility for model maintenance
and distribution.
O·
Versions
16)
Event Based
a.
Documentation: A good users manual is out of print, but available from Theo
Dillaha (dillaha@vt.edu)
b.
Source Code Availability:
Via
Anonymous FTP at: ftp://dillaha.ageng.vt.edu/pub/models/answers
a.
Platforms:
Anything
with a FORTRAN compiler
2)
Continuous Simulation Version
a.
Documentation: Very poor. No formal users manual. Best guide is the user
interface.
b.
Source Code Availability:
Via
Anonymous FTP at: ftp://dillaha.ageng.vt.edu/pub/models/answers
b.
Platforms: Anything with a FORTRAN compiler. Currently running on a Sun
workstation, but other have it running on PCs. Needs a high end PC or
workstation with a FORTRAN compiler and ArcInfo
C·
Interfaces
4)
GIS
GRASS
for the event version (Rwerts and Engle, 1991)
ArcInfo
for the continuous version (Wolfe et al., 1995)
5)
General User Interface
ArcInfo
(AML) based user interface called FARMSCALE guides user through data file
creation, model execution and interpretation of model output.
F·
Input/Output Options
7)
Input data sources: soil survey, topographic maps, DEM, landuse maps, user
interface
8)
Output options: the user interface provides a variety of output options -
storm by storm, annual, average annual values, etc.
9)
Options for analysis of outputs: user interface can display outputs spatially
and temporally
J·
Sample Data Sets
Sample
input and output sets are at the FTP site.
K·
Calibration
Parameters
are generally physically-based and calibration is not required. Since the
model is intended for use on ungaged upland watersheds, availability of data
for calibration is unlikely. If observed data is available, calibration can be
used to improve estimates of parameters and model accuracy.
12.
Known Limitations and Applicability of the Model
M·
Sediment detachment submodel is empirical and out of date. Needs to be
replaced with a more physically-based submodel.
N·
Predictions of ammonium loss in surface runoff have been poor. This submodel
(or submodels influencing it) needs to be updated.
O·
Current procedures for simulating fertilizer placement are cumbersome and need
to be automated.
P·
The currently distributed version of the model does not simulate interflow and
groundwater contributions to baseflow. The model is therefore inappropriate
for use in watersheds where baseflow is significant. The groundwater version
of the model recently developed by Bouraoui et al. (1997) may overcome this
limitation.
Q·
There is no user's manual for the continuous simulation version of the model.
R·
There is very limited user support.
S·
The model does not currently simulate nutrient cycles and fate in receiving
waters. This limits the use of the model to small upland watersheds.
T·
The model does not simulate snow pack and melt and is thus unsuitable for use
in areas with significant winter snow accumulation and snowmelt.
21.
Publications/Reference List
Event-based
Version
Baun,
K., M. Bohn, R. Bannerman and J. Konrad. 1986. Application of the ANSWERS
model in a nonpoint source program. Final Report, EPA Grant No. R005750-01,
Wisconsin Dept. of Natural Resources, Madison, WI.
Beasley,
D. B., Huggins, L.F., and Monke, E.J. 1980. ANSWERS: A model for watershed
planning. Trans. of the ASAE 23(4):938-944.
Bottcher,
A. B., E. J. Monke, and L. F. Huggins. 1981. Nutrient and sediment loadings
from a subsurface drainage system. Trans. Of the ASAE 24(5):1221-1226.
Dillaha,
T. A. and D. B. Beasley. 1983. Sediment transport from disturbed upland
watersheds. Trans. of the ASAE 26(6):1766-1772,1777.
Dillaha,
T. A., D. B. Beasley, and L. F. Huggins, 1982. Using the ANSWERS model to
estimate sediment yields on construction sites. J. Soil and Water Conservation
37(2):117120.
Holtan,
H. N. 1961. A concept for infiltration estimates in watershed engineering.
USDA-ARS Bulletin 41-51, Washington, DC. 25 p.
Huggins,
L. F. and E.J. Monke. 1966. The mathematical solution of the hydrology of
small watersheds. Technical Report No. 1, Water Resources Research Center,
Purdue University, West Lafayette, IN. 130 p.
Montas,
H.J., C.A. Madramootoo. Using ANSWERS Model to Predict Runoff and Soil Loss in
Southwestern Quebec. Transactions of the ASAE 34 4 1991 1752-1762.
Rewerts,
C. C., and B. A. Engel, 1991. ANSWERS on GRASS: Integrating a watershed
simulation with a GIS. ASAE Paper No. 91-2621, ASAE, St. Joseph, MI.
Storm,
D. E., T. A. Dillaha, S. Mostaghimi, and V. O. Shanholtz. 1988. Modeling
phosphorus transport in surface runoff. Transactions of the ASAE 31(1):117127.
Wilson,
J. P. 1996. GIS-based Land Surface/Subsurface Modeling: New Potential for New
Models? In Proceedings, Third International Conference/Workshop on Integrating
GIS and Environmental Modeling, Santa Fe, NM, January 21-26, 1996. Santa
Barbara, CA: National Center for Geographic Information and Analysis. CD.
Continuous
Simulation Version
Bouraoui,
F. 1995. Development of a continuous, physically-based, distributed parameter,
nonpoint source model. Ph.D. Dissertation. Virginia Polytechnic Institute and
State University, Blacksburg, VA. 330 p.
Bouraoui,
F. and T. A. Dillaha. 1996. ANSWERS-2000: Runoff and sediment transport model.
Journal of Environmental Engineering, ASCE 122(6):493-502.
Bouraoui,
F., G. Vachaud, R. Haverkamp and B. Normand. 1997. A distributed physical
approach for surface-subsurface water transport modeling in agricultural
watersheds. J. of Hydrology 203(1997):79-92.
Paz,
J. O., M. L. Wolfe, S. Mostaghimi, and T. A. Dillaha. 1996. Impact of Channel
Erosion on Sediment Yield Prediction for Agricultural. ASAE Paper No. 962036,
ASAE, St. Joseph, MI.
Wolfe,
M. L., T. A. Dillaha, S. Mostaghimi, C.D. Heatwole and W. D. Batchelor. 1995.
A farm scale water quality planning system for evaluating best management
practices. Final report submitted to the Chesapeake Research Consortium, Inc.,
under cooperative agreement NPS#1-A. Dept. Of Biological Systems Engineering,
Virginia Tech, Blacksburg, VA. 15 p.
Wolfe,
M. L., W. D. Batchelor, T. A. Dillaha, C.D. Heatwole and S. Mostaghimi. 1995.
Modeling the effects of farm management practices on off-site water quality.
Proceedings of the International Symposium on Water Quality Modeling, April,
1995. Kissimmee, Florida.
Yoon,
K. S., A. Shirmohammadi, W. J. Rawls. 1995. Application of Continuous,
Distributed Watershed/water Quality Model (ANSWERS) on a Mixed Landuse
Watershed ASAE Paper No. 952402, ASAE, St. Joseph, MI.
Table
1: Runoff and Pollutant Yields for the Watkinsville, GA P2 and P4
Watersheds
|
Parameter |
P2
Watershed |
P4
Watershed |
|
Predicted |
Measured |
Percent
Error1 |
Predicted |
Measured |
Percent
Error |
|
Runoff,
mm |
28.8 |
27.8 |
-3.6 |
33.8 |
21.9 |
-54.3 |
|
Sediment,
kg |
4420 |
7420 |
-40.4 |
2170 |
1920 |
-13.0 |
|
NO3-N,
g |
1400 |
1920 |
+27.1 |
1750 |
1500 |
-16.7 |
|
NH4-N
(Adsorbed), g |
138 |
2680 |
+94.9 |
57.3 |
2190 |
+97.4 |
|
NH4-N
(Dissolved), g |
1980 |
2640 |
+25.0 |
1500 |
1290 |
-16.3 |
|
TKN,
g |
9770 |
11900 |
-17.9 |
6490 |
4780 |
-35.8 |
|
PO4-P
(Dissolved), g |
452 |
413 |
-9.4 |
257 |
477 |
+46.1 |
|
1
100*(Measured-Predicted)/Measured
|
Table
2: Measured and Predicted Runoff and Pollutant Yields for the Owl Run
Watershed
|
Date |
Rainfall |
Runoff |
Sediment |
NO3-N |
NH4-N |
PO4-P |
TKN |
|
|
(mm) |
(mm) |
(kg) |
(kg) |
(kg) |
(kg) |
(kg) |
|
July
26, 1991 |
|
|
|
|
|
|
|
|
Measured |
59 |
1.2 |
4610 |
127 |
1 |
33 |
65 |
|
Predicted |
|
0.9 |
1835 |
24 |
2 |
3 |
28 |
|
%
error1 |
|
25 |
60 |
81 |
250 |
91 |
57 |
|
Aug.
9, 1991 |
|
|
|
|
|
|
|
|
Measured |
103 |
18.6 |
44671 |
683 |
16 |
160 |
565 |
|
Predicted |
|
18.4 |
73637 |
566 |
35 |
180 |
998 |
|
%
error |
|
1 |
65 |
17 |
118 |
13 |
77 |
|
Sept.
4, 1991 |
|
|
|
|
|
|
|
|
Measured |
80 |
3.8 |
17346 |
109 |
58 |
50 |
350 |
|
Predicted |
|
2.9 |
14108 |
46 |
32 |
7 |
172 |
|
%
error |
|
24 |
19 |
58 |
45 |
86 |
51 |
|
Sept.
18, 1991 |
|
|
|
|
|
|
|
|
Measured |
61 |
3.8 |
17346 |
97 |
22 |
53 |
217 |
|
Predicted |
|
5.9 |
14108 |
35 |
43 |
17 |
205 |
|
%
error |
|
55 |
19 |
64 |
95 |
68 |
6 |
|
Sept.
24, 1991 |
|
|
|
|
|
|
|
|
Measured |
41 |
3.6 |
6798 |
86 |
4 |
43 |
146 |
|
Predicted |
|
0.6 |
1835 |
11 |
1 |
1 |
27 |
|
%
error |
|
83 |
73 |
87 |
75 |
98 |
82 |
|
Cumulative |
|
|
|
|
|
|
|
|
Measured |
344 |
31.0 |
90159 |
1102 |
101 |
339 |
1343 |
|
Predicted |
|
28.7 |
101049 |
682 |
113 |
208 |
1430 |
|
%
error |
|
7 |
12 |
30 |
12 |
39 |
6 |
|
1
100*(Measured-Predicted)/Measured
|
Table
3: Reductions in Pollutant Yields due to Targeting Conservation Tillage to
Critical Fields
|
Parameter |
Scenario
Number |
|
(1)
|
1
(2) |
2
(3) |
3
(4) |
4
(5) |
5
(6) |
6
(7) |
|
%
or cropland changed to conservation tillage |
0
|
8.5
|
21.5
|
32.7
|
32.7
|
100
|
|
Cells
or hectares changed |
0 |
6 |
15 |
23 |
23 |
70 |
|
Runoff |
|
|
|
|
|
|
|
Yield,
mm/yr |
69 |
68 |
68 |
67 |
68 |
66 |
|
Reduction,
% |
|
0.42 |
1.10 |
1.70 |
1.30 |
4.00 |
|
Sediment |
|
|
|
|
|
|
|
Yield,
t/yr |
1556 |
1367 |
1257 |
1144 |
1187 |
700 |
|
Reduction,
% |
|
12.18 |
19.20 |
26.50 |
23.70 |
55.00 |
|
Reduction
cost, $/t-yr |
|
1.58 |
2.51 |
2.79 |
3.12 |
4.09 |
|
Nitrate-N |
|
|
|
|
|
|
|
Yield,
kg/yr |
22 |
22 |
22 |
21 |
22 |
21 |
|
Reduction,
% |
|
0.60 |
1.70 |
2.90 |
1.70 |
3.50 |
|
Reduction
cost, $/kg-yr |
|
2272.73 |
2005.35 |
1802.51 |
3074.87 |
4545.45 |
|
Dissolved
ammonium-N |
|
|
|
|
|
|
|
Yield,
kg/yr |
556 |
556 |
552 |
548 |
551 |
544 |
|
Reduction,
% |
|
0.07 |
0.65 |
1.50 |
0.90 |
2.20 |
|
Reduction
cost, $/kg-yr |
|
770.81 |
207.53 |
137.89 |
229.82 |
286.13 |
|
Adsorbed
ammonium-N |
|
|
|
|
|
|
|
Yield,
kg/yr |
74 |
73 |
67 |
61 |
66 |
57 |
|
Reduction,
% |
|
1.14 |
9.75 |
17.00 |
11.00 |
23.00 |
|
Reduction
cost, $/kg-yr |
|
355.62 |
103.95 |
91.41 |
141.28 |
205.64 |
|
TKN |
|
|
|
|
|
|
|
Yield,
kg/yr |
12025 |
10652 |
9488 |
8562 |
8947 |
5219 |
|
Reduction,
% |
|
11.42 |
21.10 |
28.80 |
25.60 |
56.60 |
|
Reduction
cost, $/kg-yr |
|
0.22 |
0.30 |
0.33 |
0.37 |
0.51 |
|
Total
phosphorus |
|
|
|
|
|
|
|
Yield,
kg/yr |
1714 |
1629 |
1525 |
1443 |
1488 |
1195 |
|
Reduction,
% |
|
4.95 |
11.05 |
15.80 |
13.20 |
30.30 |
|
Reduction
cost, $/kg-yr |
|
3.54 |
3.96 |
4.25 |
5.08 |
6.74 |
|
|
|
Figure
1: Representation of Processes Simulated by ANSWERS-2000
|
|
|
|
Figure
2: Measured and Predicted Runoff Volume for P2 Watershed
|
|
|
|
Figure
3: Measured and Predicted Sediment Yield for P2 Watershed
|
|
|
|
Figure
4: Measured and Predicted Nitrate Losses for P2 Watershed
|
|
|
|
Figure
5: Measured and Predicted Dissolved Ammonium Losses for P2 Watershed
|
|
|
|
Figure
6: Measured and Predicted Adsorbed Ammonium Losses for P2 Watershed
|
|
|
|
Figure
7: Measured and Predicted Adsorbed TKN Losses for P2 Watershed
|
|
|
|
Figure
8: Measured and Predicted Dissolved Orthophosphorus Losses for P2
Watershed
|
2
Professor, Biological Systems Engineering Department,
Virginia Tech, Blacksburg, VA 24061-0303, Phone: 540-231-6813, Fax:
540-231-3199, E-mail: dillaha@vt.edu
; Associate Professor, Biological Systems Engineering Department,
Virginia Tech, Blacksburg, VA 24061-0303, Phone: 540-231-6092, Fax:
540-231-3199, E-mail: mlwolfe@vt.edu;
Professor, Biological Resources Engineering Department, University of
Maryland, College Park, MD 20742, Phone: 301-405-1185; FAX: 301-314-9023,
E-mail: as31@umail.umd.edu;
M. S. Student, Biological Systems Engineering Department; Virginia
Tech, Blacksburg, VA 24061-0303
