Surface Water Assessment of Three Louisiana Watersheds

Watershed Update
Vol. 3, No. 2
March - April 2005
Surface Water Assessment of Three Louisiana Watersheds
Y. Jun Xu and Adrienne Viosca
School of Renewable Natural Resources
Louisiana State University AgCenter
Baton Rouge, LA 70803
Email: [email protected]
http://hydrology.lsu.edu/
Louisiana has acquired the title as Sportsman’s Paradise due to the available
hunting and fishing opportunities provided by the many rivers, lakes, and bayous. In
addition to these water resources, aquifers are also abundant and benefit agricultural and
forestry industries. While the state has more surface water available (84 percent) than
many
other
Louisiana’s 12 Basins
states, rapid
Atchafalaya
urbanization
Barataria
and intensive
Calcasieu
agricultural
Mermentau
and forestry
Mississippi
practices have
Ouachita
increased the
Pearl
potential for
Pontchartrain
deteriorating
Red
the quality of
Sabine
the
state’s
Terrebonne
surface waters.
Vermilian-Teche
Figure 1: Louisiana’s river basins and watersheds
Watersheds are increasingly becoming the primary planning unit for natural
resource management. Currently, Louisiana uses a set of 475 sub-segment watersheds in
12 river basins (Figure 1) as a spatial framework for its surface water quality assessment.
The Louisiana Department of Environmental Quality (LDEQ) maintains a statewide
water quality database that compiles discrete sample data collected from over 600
monitoring locations across the state. The state agency continues to collect water samples
from Louisiana’s bayous, streams, rivers, and lakes for chemical and physical property
analyses.
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In addition to the LDEQ’s surface water surveillance, the United States
Geological Survey (USGS) collects water quality samples at many locations across the
state. Most importantly, the federal agency maintains a statewide streamflow monitoring
network that ensures continuous measurements of river stage and discharge of hundreds
of bayous, streams, and rivers. Utilizing the large datasets provided by both agencies,
water quality assessments are being conducted in three river basins located across the
state’s northern upland and southern coastal plain regions. These basins are the Ouachita,
Pontchartrain, and Atchafalaya (Figure 1).
Forest BMP Effectiveness at the Watershed Scale
Forest Best Management Practices (BMPs) are designed to reduce the impacts of
forestry operations on water quality. Over the past two decades, many studies have been
conducted in assessing the beneficial effects of BMPs in various geographical locations
and under different forest types. Most of these studies have focused on the plot-scale
effects and little is known of the effects BMPs have within a larger scale, such as a
drainage network or a watershed. It is being increasingly recognized that knowledge of
forest BMP impacts on water quality at a spatial magnitude is crucial for refining and
improving the current management practices. A new project designed to address this
issue is being conducted in a medium-size watershed called Flat Creek, located in the
Ouachita Basin of northern Louisiana (Figure 2).
The research is specifically designed to quantify the initial impact and the extent
of BMP timber harvesting activities on water quality by monitoring twelve locations
throughout the watershed for two years (Figure 2). Three study plots have been selected
in the watershed, two sites will be harvested implementing forestry BMPs and one site
will remain undisturbed and serve as the control site. One pair of ISCO automatic water
samplers will be stationed in all three areas, one upstream of the site and one
downstream, for a total of six intensive monitoring sites. Water samples will be collected
during storm events for a period of two years and analyzed for nutrient and sediment
runoff.
In addition, six water quality monitoring sites have been established downstream
of the harvested areas to serve as the extensive monitoring locations. Water samples will
be manually collected on a monthly basis for the two year duration and analyzed for
nutrient and sediment concentrations. During the first year no harvesting or forest
management practices will occur at the sites and water collected will serve as a baseline
for the study.
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March - April 2005
Figure 2: Flat
Creek watershed
with monitoring
locations
It is
expected that a
change in water
quality at the
intensive sites
will occur, but
it is not clear if
a change will be
detected
downstream at
the extensive
sites. The goal
of the project is
to develop a
modeling
system that is
able to predict
the effects of
forest practices
on water quality
within an entire
watershed.
Freshwater Resources in the Lake Pontchartrain Basin
Lake Pontchartrain is a large oligohaline estuary along the northern coast of the Gulf
of Mexico. The entire lake drainage basin is a 12,170 square kilometer watershed
encompassing 16 parishes in southeast Louisiana and four counties in Mississippi. Nearly
1.5 million people directly live around the 1,619 square kilometer lake that supports
various species of fish, birds, mammals, and plants. As with many estuary ecosystems in
the world, Lake Pontchartrain has been subjected to numerous anthropogenic impacts
over the past half century including urban and agricultural runoff, shell dredging,
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March - April 2005
artificial saltwater input, shoreline alteration, and industrial discharge. There is a concern
that the combination of climate and land use changes may have dramatic impacts on the
freshwater input and that the changes may pose a threat to the stability of ecosystems in
southeast Louisiana. Freshwater inflow and sediment from upland tributaries are critical
to the development of the diverse wetland, marsh, and aquatic ecosystems in the Lake
Pontchartrain Basin. A study is currently being conducted in three watersheds, the Amite,
Tickfaw, and Tangipahoa river watersheds (Figure 3), on the seasonality and interannual
variability of freshwater inflow to Lake Pontchartrain. The goal of this research is to
determine the extent a potential change in temperature and precipitation will affect water
and sediment yields from the tributaries to Lake Pontchartrain and how these changes
will be modified by urbanization and land use changes in the basin areas.
Tickfaw
Amite
Tangipahoa
#
#
#
7378500
7376000
7375500
0
#
#
0
#
0
#
#
#
S
#
Baton Rouge
pas
aure
eM
Lak
rt rain
cha
ont
P
e
Lak
S
#
New Orleans
Figure 3: The Amite, Tickfaw and Tangipahoa River watersheds (from left to right) and Lake
Maurepas and Lake Pontchartrain in Southeastern Louisiana
The Amite, Tichfaw, and Tangipahoa river watersheds contribute 70% of the
freshwater inflow to Lake Pontchartrain. Sixty years of river discharge and climatic data
are being analyzed for trends in seasonal, annual and decadal changes. The preliminary
results (Wu and Xu, 2005) show that, on average, the three watersheds delivered 5 km3 of
fresh water each year to Lake Pontchartrain, with a large variation from 1 to 9 km3 yr-1.
The discharge in these watersheds was highest from January to May and lowest from July
to October (Figure 4), indicating a potential threat of high nutrient runoff into Lake
Pontchartrain during the springtime. Additionally, the monthly inflow during the wet
months was positively correlated with the monthly precipitation, while the monthly
inflow during the dry months was subject to evapotranspiration.
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March - April 2005
0.30
Amite River
Tickfaw River
Tangipahoa River
Monthly discharge [km3]
0.25
0.20
0.15
0.10
0.05
0.00
Jan
Feb
Mar
Apr
May
Jun Jul
Aug
Sep
Oct
Nov
Dec
Figure 4: Monthly freshwater discharge from the Amite, Tickfaw and Tangipahoa Rivers
The results (Xu and Wu, 2005) also show that the discharge from the Amite river
watershed increased significantly over a 60 year period. This is mainly due to increased
peakflow caused by land use change. A notable finding in this study is that a 20-year low
flow period from 1954-1973 (0.88 km3 yr-1) and a 24-year high flow period from 19751998 (1.45 km3 yr-1), coincided with both the climate variation and population growth in
the watersheds.
Nitrogen Retention of the Atchafalaya River Swamp Basin
A dead zone of water with dissolved oxygen less than 2 mg L-1 has developed off
the shore of Louisiana in the northern Gulf of Mexico. Studies on the hypoxia have
shown that an average midsummer hypoxic zone of 8,000-9,000 km2 during 1985-1992
increased to 16,000-20,700 km2 during 1993-2001 on the Louisiana/Texas continental
shelf. This two-fold increase of hypoxic zone over a relatively short period of time has
been attributed to the increase of riverborne nutrients that can exacerbate coastal water
eutrophication, favor harmful algal blooms, aggravate oxygen depletion and alter marine
food webs. Nitrogen, especially nitrate-nitrogen, is the most probable cause of hypoxia.
Ninety percent of the nitrogen input originates from the 3-million-km2 Mississippi River
basin, which comprises 41% of the conterminous United States. The control of this
hypoxia is both ecologically and economically important because the continental shelf
fishery in the Gulf of Mexico is approximately 25% of the U.S. total.
In January 2001, an action plan with the major goal of reducing nitrogen
discharge into the Gulf was cleared by state, tribal, and federal agencies and delivered to
Congress. The action plan called for a 30% nitrogen load reduction that is required to
ensure a reduction of 5-year running average of the Gulf hypoxia zone to less than 5,000
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km2 by 2015. A number of options are being considered for controlling nitrogen flow into
the Gulf. Fresh water diversion from the lower Mississippi River into the region’s
wetlands has been considered as one of the options. However, it is largely uncertain how
much nitrogen can actually be retained from overflowing waters in these natural
wetlands. Overall, there is a knowledge gap in what tools are available for accurate
assessment of nitrogen inflow and outflow, and removal potential for the complex and
diverse coastal floodplain systems.
An ongoing study is being conducted in the Atchafalaya River Swamp, a large
river swamp basin, which conveys 30% of the Mississippi River’s water into the Gulf of
Mexico (Figure 6). The research seeks answers to three critical questions:
(1) Does a natural swamp in this region remove a significant amount of nitrogen
from the overflowing water or does it release more nitrogen into the Gulf rather than
removing it?
(2) How seasonally and annually do the nitrogen removal or release rates
fluctuate?
(3) What are the relationships between the nitrogen removal capacity and the
basin’s hydrologic conditions such as river stage and discharge?
Figure 5: The Old River Control Structure where 30% of the Mississippi River’s water is
diverted into the Atchafalaya
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Monthly and annual nitrogen fluxes were quantified by utilizing the river’s longterm discharge and water quality data and their relationships with the basin’s hydrologic
conditions were investigated. Long-term nitrogen inflow at Simmesport and outflows at
Wax Lake Outlet and Morgan City (Figure 6) were analyzed. Nitrogen input – output
budgets were established to assess total nitrogen mass removal rates of the Atchafalaya
River Basin under different hydrologic regimes. Discharge data were obtained from the
New Orleans District Office of the U.S. Army Corps of Engineers and the Louisiana
District Office of the U.S. Geologic Survey in Baton Rouge. Water quality data was
obtained from the Louisiana Department of Environmental Quality. The water quality
data included monthly measurements on a series of chemical and physical parameters, of
which monthly Total Kjeldahl Nitrogen (TKN) and nitrate plus nitrite nitrogen
concentrations were used in this study.
Simmesport
Morgan
City
Wax Lake
Outlet
Figure 6: Atchafalya River Basin with inflow and outflow locations
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March - April 2005
The preliminary results (Xu, 2005) show that on average, TKN input into the
Atchafalaya was 200,323 tons/year and TKN output from the basin was 145,917
tons/year, resulting in a 27% removal rate of organic nitrogen. Monthly TKN input and
output in the basin were highest from March to June (input vs. output: 25,000 vs. 18,000
tons/month) and lowest from August to November (8,000 vs. 6,000 tons/month) (Figure
7). There was a large variation in both annual and inter-annual organic nitrogen removals.
The variability was positively correlated with the amount of inflow water at Simmesport,
suggesting that regulating the river’s inflow may help reduce nitrogen loading of the
Mississippi River to the Gulf of Mexico. Furthermore, the in-stream loss of organic
nitrogen indicates that previous studies may have overestimated nitrogen discharge from
the Mississippi-Atchafalaya River system.
Flux (tons)
30000
Removal (tons)
20000
Input
Output
15000
20000
Removal Rate
10000
10000
5000
0
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure 7: Seasonal TKN mass input, output, and removal of the Atchafalaya River Swamp
Basin
References
Wu, K. and Y.J. Xu. 2005. Long-term freshwater inflow and sediment loading to Lake
Pontchartrain. Hydrological Sciences Journal. (in review)
Xu, Y.J., and K. Wu. 2005. Seasonality and interannual variability of freshwater inflow to a large
oligohaline estuary in the Northern Gulf of Mexico. Estuarine, Coastal, and Shelf Science. (in
review)
Xu, Y.J. 2005. Nitrogen Retention of the Largest River Swamp in North America. In
Proceedings: The 3rd Conference on Watershed Management to Meet Water Quality Standards
and Emerging TMDL, March 5-9, 2005, Atlanta, Georgia, American Society of Agricultural
Engineers: 14-23.
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