Title page for ETD etd-18441213972900

Type of Document Dissertation
Author Yagow, Eugene R.
URN etd-18441213972900
Title Auxiliary Procedures for the AGNPS Model in Urban Fringe Watersheds
Degree PhD
Department Biological Systems Engineering
Advisory Committee
Advisor Name Title
Bohland, James R.
Dillaha, Theo A. III
Mostaghimi, Saied
Perumpral, John V.
Wolfe, Mary Leigh
Shanholtz, Vernon O. Committee Chair
  • geographic information systems
  • index
  • model interface
  • nonpoint source pollution
Date of Defense 1997-02-28
Availability unrestricted
The Agricultural Nonpoint Source model (AGNPS) is a

single-event grid-based model used for simulating runoff,

sediment and nutrients from agricultural areas. This study

involved using geographic information system (GIS) spatial

data and functionality to improve the spatial and temporal

assignment of parameter values for the AGNPS 5.0 model

and incorporated methods for representing urban fringe

land uses and their nonpoint source (NPS) pollution

contributions in model inputs. Auxiliary procedures for

modeling with AGNPS were developed both for enhancing

input into the model and for enhancing modeled output. On

an event basis, one procedure automated the creation of

complex-formatted AGNPS 5.0 model input files using GIS

as a spatial data manager. One pair of alternative

procedures were developed to automate the assignment of

parameter values on an event basis. One procedure used

typical average annual parameter values, and the second

assigned parameter values using adaptations of existing

time-dependent relationships. On a monthly basis, a

sequencing procedure was created to perform multiple runs

with the model for a list of storms while updating

parameters for each event and aggregating monthly

modeled spatial output. Another pair of alternative

procedures were developed to facilitate the simulation of

monthly output from AGNPS modeled events. The first of

these aggregated event output for all storms in each month,

while the second supplemented the aggregated output with

baseflow and septic system loads. The study area was the

6,500 ha urbanizing Bull Run watershed in northern

Virginia, which was modeled as 14,621 cells. Databases

were assembled and 109 selected storm events within a

16-year period were modeled using the above procedures.

Event data were added together, where necessary, to

correspond with observed data from composite-sampled

intervals. Output from the two event parameterization

procedures were compared with monitored loads calculated

for 89 composite periods, while output from the two

monthly simulation procedures were compared with

monthly monitored data for 23 complete months. The

monitored-modeled comparisons were considered

inconclusive. Evidence strongly suggested that the rainfall

records from a rain gauge outside the watershed did not

correspond well with monitored runoff. The average runoff

produced with the AGNPS model from the 109 selected

storms amounted to 40.7% of rainfall, consistent with the

calculated long-term average of 38% for the Bull Run

watershed. A nonpoint source pollution index was

developed to utilize monthly modeled total nitrogen, total

phosphorus, and suspended sediment. Individual rating

curves were developed to separately transform loads and

concentrations of each pollutant into sub-index values. The

maximum sub-index from each parameter was added

together and averaged for the index. The index was

calculated at the watershed outlet from monitored data, and

in a spatially-distributed fashion along all streams from

simulated output.

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