Agriculture Reference
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the same as that reaching the major waterways
or watershed outlet (Sharpley et al ., 2009).
Some work has been done to link farm and
watershed models (Ghebremichael et al ; 2013),
but more of this work is needed to ultimately
define the impact of farms on water quality at
the watershed outlet and beyond.
Non-point-source pollution from agricul-
ture has continued to be one of the major causes
of water quality degradation in streams and
lakes of the USA. Agricultural non-point source
pollution controls are commonly addressed
through various federal and state conservation
programmes providing financial and technical
assistance and through voluntary use of cost-
share best management practices by landown-
ers. The successes of non-point source pollution
control efforts depend upon proper identifica-
tion, targeting and remediation of critical source
areas (CSAs) of pollution. CSAs within a water-
shed contribute proportionally more pollutants
to the outlet than other areas. CSAs for surface
runoff represent an overlap of high pollution
source areas with areas prone to generate high
volumes of runoff and erosion. Their identifica-
tion through ground-truthing, geographic
information systems (GIS) and watershed-scale
modelling have helped guide P index develop-
ment and the placement of best management
practices on farms. CSAs for groundwater can
play a significant role in aquifer pollution when
the region is highly fractured, shrink-swell, tiled
or karst; however, these areas are difficult to
define and delineate.
in the eastern US (Paerl et al ., 2002), CSAs for
air pollution are very difficult to delineate as
transport factors are continually changing.
Locating a particular contributor, or source, of
non-point source emission within the watershed
can be difficult. Confirming that the majority of
emissions from that source redeposit directly to
the water body, the majority of the time, relies
on complex models. Models are being developed
and used to predict the transport, transforma-
tion and deposition of N and other constituents
(e.g. Wang and Chen, 2012), but little has yet
been done to link farm scale prediction of emis-
sions to airshed scale transport and deposition
models. As one example, Del Grosso et al . (2010)
used a biogeochemical model called DAYCENT
to simulate nitrous oxide emissions from crop-
land soils across the USA. They found spatial
variability of the emissions to depend mainly on
differences in N inputs via fertilizer and manure
application, whereas temporal variability was
driven by N mineralization caused by the weather.
To understand the ultimate impact of farm emis-
sions such as ammonia, better integration of
farm and airshed scale models is needed.
Future needs in the integration
of air and water quality issues
Addressing air and water quality in an integrated
manner is becoming increasingly vital to address
the needs of a growing population along with
pollution weakening the natural resiliency of ter-
restrial ecosystems, oceans and the atmosphere.
Increases in human population and their require-
ments for food, fibre and fuel are increasing
nutrient cycle imbalances (Paerl et al ., 2002).
Corresponding increases in water use and
changes in land use are likewise impacting the
hydrological cycle, decreasing streamflow and
precipitation (Wang et al ., 2009). Atmospheric
carbon dioxide is projected to increase through
the 21st century (WMO, 2010). Likewise, ozone
depletion and climatic warming will continue,
particularly if methane and nitrous oxide are not
strictly controlled (WMO, 2010). Carbon dioxide
deposition into the oceans is causing an increase
in acidification that is detrimental to marine life
(Hardt and Safina, 2010). EPA is working to bet-
ter integrate local ecosystem and watershed
Airshed evaluation
An airshed is the atmospheric area where the
majority of a given emission is deposited back to
a land region. Airsheds are often many times
larger than the watershed of the same region
(Fig. 10.2). For example, nitrous oxide emissions
from an airshed 6.5 times larger than the
Chesapeake Bay watershed, contribute 76% of
the oxidized-N deposition reaching that water-
shed (Paerl et al ., 2002). These emissions repre-
sent one-third of the nitrous oxide emissions in
eastern North America.
Although size variations among airsheds
are smaller than variations among watersheds
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