TIAER
PR 98-01
LAKE WACO/BOSQUE RIVER WATERSHED INITIATIVE REPORT:
Determining Nutrient Contribution by Land Use
for the Upper North Bosque River Watershed
Anne McFarland and Larry Hauck
January 1998
Revised May 1998
Texas Institute for Applied Environmental Research
Tarleton State University •Box T0410 •Tarleton Station •Stephenville •Texas •76402
(254) 968-9567 •FAX (254) 968-9568
ABSTRACT
Over a 4-year period, flow and nutrients were monitored at 13 sites in the upper
North Bosque River watershed. Drainage areas differed in the percent of dairy
waste application fields, forage fields, wood/range and urban land represented.
A multiple regression approach was used to determine export coefficients from
these heterogeneous drainage areas to determine the relative contribution by
source of orthophosphate-phosphorus (PO4-P), total phosphorus (total-P) and
total nitrogen (total-N) into the River. The largest export coefficients were
associated with dairy waste application fields followed by urban, forage fields
and wood/range. Point source loadings from municipal wastewater treatment
were included separately as a nutrient source. While comprising about 7 percent
of the watershed, dairy waste application fields were associated with 65 percent
of PO4-P loadings and 48 percent of total-P loadings. Forage fields, comprising
about 20 percent of the watershed, were associated with 37 percent of total-N
loadings followed by dairy waste application fields with 33 percent.
Lake Waco/Bosque River Watershed Initiative Report
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Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
ACKNOWLEDGMENTS
Funding sources for this study include the United States Department of
Agriculture - Natural Resources Conservation Service, the Clean Rivers Program
of the Texas Natural Resource Conservation Commission, the United States
Environmental Protection Agency and the State of Texas. The authors wish to
thank Dr. Jerry Lemunyon of the USDA-NRCS in Arlington, Texas and Dr.
Tommy Daniel of the University of Arkansas Fayetteville, Arkansas for their
review comments on early drafts of this report. The authors would also like to
acknowledge the support of landowners who allowed access to their property for
in-stream monitoring. Without the willing cooperation of these individuals, this
study would not have been possible.
Mention of trade names or equipment manufacturers does not represent
endorsement of these products or manufacturers by TIAER.
Lake Waco/Bosque River Watershed Initiative Report
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Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
TABLE OF CONTENTS
ABSTRACT ...................................................................................................................... I
ACKNOWLEDGMENTS................................................................................................. III
LIST OF FIGURES.........................................................................................................VI
LIST OF TABLES...........................................................................................................VI
INTRODUCTION ............................................................................................................. 1
METHODS....................................................................................................................... 4
RESULTS........................................................................................................................ 5
Step 1. Determine Land Uses Above Stream Sampling Sites..............................................................5
Step 2. Develop Hydrographs and Calculate Flow History for Each Site .........................................6
Step 3. Calculate Time History of Nutrient Mass Loadings for Each Site ........................................7
Step 4. Determine Nutrient Export Coefficients for Urban Land Areas ...........................................8
Step 5. Determine Nutrient Export Coefficients for Agricultural Land Uses ...................................8
Step 6. Compare Calculated Export Coefficients to Literature and Field Plot Values ..................11
Step 7. Calculate Stephenville WWTP Loadings...............................................................................13
Step 8. Compare Estimated Nutrient Loadings with Monitored Loadings.....................................14
Step 9. Calculate Percent Contribution by Land-Use Sector ...........................................................16
Step 10. Perform Sensitivity Analysis on Percent Contributions .....................................................17
Step 11. Evaluate Relative Contributions by Source with Variations in Flow................................19
DISCUSSION AND CONCLUSIONS ............................................................................ 23
LITERATURE CITED .................................................................................................... 27
Lake Waco/Bosque River Watershed Initiative Report
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LIST OF FIGURES
Figure 1. Location of TIAER sampling sites used in mass loading calculations for the UNBR
watershed. .....................................................................................................................................2
Figure 2. Comparison of predicted with measured nutrient loadings at North Bosque River sites
BO040 and BO070 for the period of November 10, 1993 through January 31, 1995.
Numbers in parentheses represent the percent error between measured and predicted
values. .........................................................................................................................................15
Figure 3. Predicted loadings and percent contribution by source for North Bosque River sites
BO040 and BO070 for the period of November 10, 1993 through January 31, 1997. ................16
Figure 4. Average monthly flow at North Bosque River site BO070 for the period of November
1993 through January 1997.........................................................................................................19
Figure 5. Predicted loadings and percent contribution by source for North Bosque River site
BO040 for the periods of May and August 1994. ........................................................................21
Figure 6. Predicted loadings and percent contribution by source for North Bosque River site
BO070 for the periods of May and August 1994. ........................................................................22
Figure 7. Long-term monthly average (1955 - 1990) and average total rainfall for November 1993
through January 1997 across six National Weather Observer sites in Erath and
Hamilton Counties, Texas. ..........................................................................................................24
LIST OF TABLES
Table 1. Land uses associated with the drainage area above sampling sites in the upper North
Bosque River watershed. ..............................................................................................................6
Table 2. General storm sampling frequency for monitoring sites................................................................7
Table 3. Water volumes and nutrient masses estimated for 13 stream sampling sites in the
upper North Bosque River watershed for November 10, 1993 through January 31, 1997. ..........8
Table 4. Parameter estimates for land-use variables (x) versus nutrient loadings (y) using a zerointercept multiple regression model. Nutrient loadings were based on a 1,179-day
period from November 10, 1993 through January 31, 1997 and prorated to an annual
2
basis. All three nutrient models had an R = 0.95 with an associated p-value of 0.0001...........10
Table 5. Calculated nutrient export coefficients for dominant land uses in the upper North
Bosque River watershed for November 10, 1993 through January 31, 1997. These
export coeffients represent the values normalized to an annual basis for the study
period...........................................................................................................................................11
Table 6. Literature values for total-P and total-N mass loading coefficients as compiled by Frink
(1991) and Reckhow et al. (1980). 'n' equals the number of different studies evaluated............12
Table 7. Calculated mass loading coefficients for eight field plots used for dairy waste application
in the upper North Bosque River watershed (Flowers et al., 1996).............................................13
Table 8. Estimated standard deviation associated with the mean percent contribution for
November 10, 1993 through January 31, 1997 by sector from 10,000 simulations.
Simulations used a random generator to determine the export coefficient associated
with each land use based on the standard deviation of each coefficient. ...................................18
Table 9. Estimated export coefficients using a zero-intercept multiple regression model for a 31
day period from May 1, 1994 through May 31, 1994. Coefficient values are prorated to
an annual basis. ..........................................................................................................................19
Table 10. Estimated export coefficients using a zero-intercept multiple regression model for a 31
day period from August 1, 1994 through August 31, 1994. Coefficient values are
prorated to an annual basis. ........................................................................................................20
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Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
Lake Waco/Bosque River Watershed Initiative Report:
Determining Nutrient Contribution by Land Use
for the Upper North Bosque River Watershed
INTRODUCTION
In 1990, the upper North Bosque River watershed was identified as an impacted
watershed due to nonpoint source pollution (Texas Water Commission and
Texas State Soil and Water Conservation Board, 1991).
Noticeable
eutrophication of several small bodies of water within the watershed (McFarland
and Hauck, 1997a; BRA, 1994) and elevated nutrient concentrations in
tributaries to the North Bosque River (McFarland and Hauck, 1997b) support
the need for a reduction in nutrient loadings to the North Bosque River.
Phosphorus levels, in particular, appear to be the primary concern with
concentrations considerably above Texas Natural Resource Conservation
Commission (TNRCC) screening levels (0.1 mg/L for orthophosphatephosphorus and 0.2 mg/L for total phosphorus; TNRCC, 1996) at several stream
and reservoir sampling sites within the watershed (McFarland and Hauck, 1997a
& 1997b). Because phosphorus is generally the limiting nutrient to algae growth
in freshwater systems (e.g., Hecky and Kilham, 1988), continued phosphorus
loadings to the upper North Bosque River watershed are likely to be
problematic.
The upper North Bosque River watershed is largely located within the
boundaries of Erath County, Texas, and is defined as the drainage area above the
North Bosque River at U.S. Highway 281 in Hico, Texas. This location is
identified by U.S. Geological Survey (USGS) site 08094800 and by the Texas
Institute for Applied Environmental Research (TIAER) sampling site BO070
(Figure 1). The watershed covers a little over 93,250 ha (230,000 acres) and is
mainly comprised of rural land used for a variety of agricultural activities. The
watershed contains about 100 dairies with a combined milking herd size of about
34,000 cows. Other important agricultural enterprises in the watershed include
the production of peanuts, range cattle, pecans, peaches and forage hay. The city
of Stephenville (estimated population 16,000) and a small portion of the city of
Dublin (estimated population 4,000) are also located within the watershed. The
Stephenville wastewater treatment plant (WWTP) is the only permitted point
source discharge within the watershed.
To aid in determining the nutrient loadings associated with various sources
within the upper North Bosque River watershed, TIAER began monitoring
stream water quality in early 1991. A detailed discussion of TIAER’s
monitoring program is presented in McFarland and Hauck (1995) and
McFarland and Hauck (1997a & 1997b). While most early monitoring consisted
only of grab samples, 19 automatic samplers were installed from late 1992
through the fall of 1993. These automatic samplers collect stormwater samples
and measure water level continuously at five minute intervals. Site specific
stage-discharge relationships developed from manual measurements of flow are
used to develop streamflow from water level data. Routine grab sampling at
monthly or bi-weekly intervals complements the stormwater sampling program.
Lake Waco/Bosque River Watershed Initiative Report
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Of the 19 automatic sampling sites in the watershed, a sufficiently complete
period of record from November 10, 1993 through January 31, 1997 was
available at 13 sites for use in quantifying nutrient loadings and sources within
the upper North Bosque River watershed (Figure 1).
Figure 1. Location of TIAER sampling sites used in mass loading calculations for the UNBR watershed.
2
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
Quantifying nutrient loadings by source in a watershed is an important step in
managing streams or waterbodies where eutrophication is a potential problem.
Typically the most effective way to control eutrophication of lakes and streams
is by decreasing external nutrient loadings. Because money and other resources
are often limited, nutrient sources and the relative contribution of each source
needs to be identified to efficiently target reduction efforts. Frequently in
watershed management planning, nutrient loads for individual land uses are
developed indirectly from nutrient export coefficients (Loehr et al., 1989).
These coefficients represent the quantity of nutrients generated per unit area per
unit time. Export coefficients are often expressed in units of mass per area per
time, such as kilograms per hectare per year (kg/ha/yr), although units such as
kilograms per capita per year (kg/capita/yr) may be used when the loading rate is
considered a direct function of population density (Loehr et al., 1989). Export
coefficients are multiplied by the area of the watershed occupied by each land
use to estimate the mass loading contribution of various sources to a river
segment or other waterbody area.
Generalized export coefficients are often used in management planning due to
the high cost of directly monitoring loadings from individual land uses. Recently
released watershed loading models such as WATERSHEDSS (WATER, Soil,
and Hydro-Environmental Decision Support System) developed by the North
Carolina State University Water Quality Group (Osmond et al., 1997) and
BASINS (Better Assessment Science Integrating Point and Nonpoint Sources)
developed by the U.S. Environmental Protection Agency (USEPA, 1996) present
generalized export coefficients for many land uses, but allow the input of
regionally specific export coefficients by the user. Regionally specific export
coefficients are recommended because variations in precipitation, soils, and
management practices associated with specific land uses between regions often
limit the transferability of export coefficients (Clesceri, et al., 1986).
Knowledge of the conditions under which export coefficients were determined is
important, because export coefficients generally represent nonpoint source
loadings driven by precipitation.
The objectives of this paper are:
1.
to determine nutrient export coefficients for land uses specific to the
upper North Bosque River watershed for the period November 10,
1993 through January 31, 1997,
2.
to estimate the relative contribution of various point and nonpoint
sources to the loadings of soluble reactive phosphorus (as
orthophosphate-phosphorus; PO4-P), total phosphorus (total-P), and
total nitrogen (total-N) to the upper North Bosque River during this
period, and
3.
to evaluate the relative importance of various loading sources under
low and high flow conditions.
Lake Waco/Bosque River Watershed Initiative Report
3
METHODS
Land-use information, in conjunction with monitoring data, was used to estimate
and validate export coefficients for nonpoint sources and relative nutrient
loadings of PO4-P, total-P and total-N by sector for point and nonpoint sources
in the upper North Bosque River watershed for the period November 10, 1993
through January 31, 1997 following the steps outlined below:
1.
Determine land uses in the drainage area above each monitoring site.
2.
Develop hydrographs and calculate flow history for each site.
3.
Combine flow hydrograph with discrete measurements of nutrient
concentrations taken during storm events and baseflow to provide a
time history of mass loadings for each sampling site.
4.
Determine nutrient export coefficients for urban land areas using
monitoring data from the sole urban stream monitoring site (MB040).
5.
Apply multiple regression models to determine optimal estimates of
nutrient export coefficients for the major agricultural land-use
categories in the watershed.
6.
Compare estimated nutrient export coefficients for urban and
agricultural land uses to literature values and estimates from field plots
studies specific to the upper North Bosque River watershed.
7.
Determine mass loadings for the only permitted point source discharge
in the watershed, the Stephenville WWTP, using monitoring data and
effluent discharge information.
8.
Compare predicted nutrient loadings, based on export coefficient
values and WWTP loadings, with measured loadings at two sites
(BO040 and BO070), not used in Steps 4-7, to validate calculated
export coefficients.
9.
Calculate the contribution by land-use sector to nutrient loadings in the
North Bosque River below Stephenville (site BO040) and the North
Bosque River at Hico (site BO070).
10. Test the sensitivity of the calculated contributions to variations in the
agricultural export coefficients associated with the standard deviation
of each coefficient.
11. Evaluate the impact of low and high flow conditions on the relative
contribution of nutrient loadings by various sources.
4
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
RESULTS
Step 1.
Determine Land Uses Above Stream Sampling Sites
General land-use information was developed from Landsat Thematic Mapper
(TM) images obtained from an overflight taken on August 28, 1992. This landuse information was stored as a geographic information system (GIS) data layer.
The location of dairies and dairy waste application fields was obtained from
TNRCC dairy permits and dairy waste management plans and overlaid on the
general land-use data layer. Waste application fields represent areas permitted
for liquid and/or solid manure application. In the watershed, over 90 percent of
the permitted dairy waste application fields are described as coastal
bermudagrass (Cynodon spp.) fields (McFarland and Hauck, 1995), although
crop rotations of sorghum (Sorghum spp.) and winter wheat (Triticum spp.) are
not uncommon. Operating dairies and the location of dairy waste application
fields represent information as of January 1995. Although some variation in
land use is expected to occur over time, for the purposes of this study, land use
was assumed to be stable. Further discussion on the development of the land-use
information is described in McFarland and Hauck (1997b).
The land uses associated with the drainage area above each sampling site are
presented in Table 1. Most of the sampling sites represent predominately rural
or agricultural land uses with urban areas comprising less than two percent of the
total watershed area. Urban nonpoint source impacts are represented by site
MB040 located within the city of Stephenville. Site MB040 captures rainfall
runoff from portions of the residential and downtown areas of Stephenville.
BO040, located on the North Bosque River below Stephenville, contains only a
relatively small portion of urban land in its drainage basin (less than 4 percent)
but is impacted by urban influences, particularly at baseflow, due to its location
about 400 meters (1/4 mile) below the discharge of the Stephenville WWTP.
Site BO070, located on the North Bosque River at Hico, Texas, defines the
mouth of the upper North Bosque River watershed. Sites BO040 and BO070
were selected for the validation of the export coefficient presented in Step 8
because these sites represent the variety of point and nonpoint source nutrient
loadings within the watershed.
Lake Waco/Bosque River Watershed Initiative Report
5
Table 1. Land uses associated with the drainage area above sampling sites in the upper North Bosque River
watershed.
Site
Wood
(%)
Range
(%)
Forage
Fields
(%)
Dairy Waste
Appl. Fields
(%)
Peanut
(%)
Orchard
(%)
Water
(%)
Urban
(%)
Barren
(%)
Total
Drainage
Area (ha)
19.2
44.8
21.5
10.1
2.4
1.0
0.2
0.0
0.7
5,436
AL040
BO040
23.7
27.4
30.2
11.7
1.6
0.3
0.7
3.8
0.7
25,717
BO070
23.2
45.0
20.1
7.2
1.4
0.4
0.5
1.7
0.5
93,248
GC100
22.2
49.0
18.0
6.9
2.2
0.3
0.5
0.7
0.2
26,170
IC020
16.0
49.3
16.6
17.3
0.4
0.0
0.0
0.0
0.4
1,820
MB040
0.0
0.0
0.0
0.0
0.0
0.0
0.0
100.0
0.0
171
NF005
10.6
33.3
13.8
41.7
0.0
0.0
0.2
0.0
0.3
448
NF010
17.7
40.6
38.0
3.4
0.0
0.0
0.0
0.0
0.3
518
12.9
25.9
15.3
45.4
0.0
0.0
0.3
0.0
0.2
791
†
NF020
NF050
20.3
29.5
38.7
9.8
0.5
0.2
0.7
0.0
0.4
8,285
SF020
35.6
60.5
3.6
0.0
0.0
0.0
0.2
0.0
0.1
849
SF075
28.0
28.5
26.3
14.6
1.4
0.1
0.8
0.0
0.3
12,272
SP020
30.6
53.6
15.4
0.0
0.0
0.3
0.1
0.0
0.1
1,589
† An 8 ha field permitted for land application of septage waste is located immediately above site NF020. This land area comprises about two
percent of the drainage area and is included with dairy waste application fields in the above land-use categories.
Step 2.
Develop Hydrographs and Calculate Flow History
for Each Site
Water level was monitored at each sampling site at five-minute intervals
throughout the study period. Stage-discharge relationships were developed for
most sites based on manual measurements of flow. Where applicable, specific
hydraulic relationships for structures such as culverts were used for the stagedischarge relationship. Water volume was calculated by multiplying the
instantaneous discharge for a given water level value by the five-minute time
interval for level readings, assuming a constant flow between each five-minute
reading. Daily discharges were calculated by summing the five-minute volumes
for each day.
Daily records for each site were evaluated for completeness and the extent of
available records across sites. A consistent period of record occurred between
November 10, 1993 and January 31, 1997 for the 13 sampling sites. When
missing water level data occurred due to inoperative equipment, missing daily
discharge volumes were estimated using values from an operating site exhibiting
the most similar hydrologic response to the site with missing data. Linear
regression relationships were developed from daily discharge values for common
periods of record between sites as the basis for estimating missing daily volumes.
6
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
Step 3.
Calculate Time History of Nutrient Mass Loadings
for Each Site
Nutrient masses were calculated by combining nutrient concentrations with flow
information based on the collection time of each water quality sample. A
midpoint rectangular integration method was used to calculate loadings by
dividing the flow hydrograph into intervals based on the collection date and time
of each water quality sample (Stein, 1977). For storm samples, the first water
quality sample in a storm event was associated with flow beginning an hour
before the sample time to associate the initial rise in the flow hydrograph with
this first sample. The last water quality sample taken during a given storm event
was associated with the falling limb of the hydrograph and succeeding baseflow
levels until the next baseflow or storm event sample occurred. Stormwater
samples were generally collected using a set sampling frequency depending on
the size of the drainage area above the sampling site (Table 2). The variable
time interval between sequential samples, as shown in Table 2, provides more
frequent sampling on the typically fast rising portion of the hydrograph and less
frequent sampling on the typically slow falling portion of the hydrograph. Grab
samples to characterize baseflow were collected monthly until November 1994
when a bi-weekly sampling schedule was initiated (McFarland and Hauck,
1997b).
Table 2. General storm sampling frequency for monitoring sites.
Smaller Tributary Sites
(Drainage Area < 3,000 ha)
Larger Tributary and Mainstem Sites
(Drainage Area > 3,000 ha)
Sample #
Time Interval
Sample #
Time Interval
1
Initial
1
Initial
2-4
1 hour
2-4
1 hour
5-8
2 hours
5-6
2 hours
9-24†
6 hours
7-24†
8 hours
† For large storm events, more than 24 samples may be collected.
Routine laboratory analyses included total Kjeldahl nitrogen (TKN), nitritenitrogen (NO2-N), nitrate-nitrogen (NO3-N), PO4-P and total-P. Total-N was
derived as the sum of TKN, NO2-N and NO3-N for mass loading calculations.
Daily masses were summed for the entire period between November 10, 1993
and January 31, 1997 to give a total loading. Dividing the total loading by the
drainage area above each sampling site provided an area-weighted loading for
each site (Table 3).
All samples were collected and analyzed based on United States Environmental
Protection Agency (USEPA) guidelines (Kopp and McKee, 1983) following an
approved Quality Assurance Project Plan (QAPP). For the period November
1993 through August 1996, monitoring efforts were conducted under an
USEPA-approved QAPP for the USEPA funded project Livestock and the
Environment: A National Pilot Project (TIAER, 1993). Since September 1996,
all monitoring efforts have occurred under a TNRCC-approved QAPP for the
United States Department of Agriculture funded Lake Waco/Bosque Rivers
Initiative (TIAER, 1996).
Lake Waco/Bosque River Watershed Initiative Report
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Table 3. Water volumes and nutrient masses estimated for 13 stream sampling sites in the upper North
Bosque River watershed for November 10, 1993 through January 31, 1997.
Flow Volume
PO4-P
Total-N
Site
(m3)
AL040
16,200,000
2,970
6,360
1.17
10,100
1.86
33,700
6.20
BO040
88,600,000
3,450
62,400
2.43
98,500
3.83
415,000
16.15
BO070
331,000,000
3,540
74,200
0.80
186,000
1.99
796,000
8.54
GC100
91,100,000
3,480
13,600
0.52
43,800
1.67
214,000
8.16
IC020
4,620,000
2,540
2,810
1.54
4,790
2.63
17,000
9.37
MB040
1,990,000
11,600
447
2.61
1,230
7.19
5,500
32.16
NF005
1,510,000
3,370
2,810
6.28
4,120
9.20
10,600
23.61
NF010
2,030,000
3,930
563
1.09
1,800
3.31
7,800
14.16
NF020
2,730,000
3,450
4,040
5.11
6,420
8.11
16,000
20.18
NF050
28,000,000
3,370
12,700
1.53
25,200
3.04
92,000
11.11
SF020
3,170,000
3,740
112
0.13
663
0.78
4,310
5.08
SF075
27,000,000
2,200
10,400
0.85
21,500
1.76
90,400
7.36
SP020
6,400,000
4,020
252
0.16
813
0.51
4,300
2.71
Step 4.
(m3/ha)
Total-P
(kg)
(kg/ha)
(kg)
(kg/ha)
(kg)
(kg/ha)
Determine Nutrient Export Coefficients for Urban
Land Areas
The nutrient export coefficients for urban land were calculated based on the
area-weighted mass loadings for site MB040. Site MB040 was the only site
representing 100 percent urban land (Table 1). All other sites contained less
than four percent urban land. Prorated to an annual basis the area-weighted
nutrient loadings for MB040 produced urban export coefficients of 0.8 kg PO4P/ha/yr, 2.2 kg total-P/ha/yr, and 10.0 kg total-N/ha/yr (Table 3).
Step 5.
Determine Nutrient Export Coefficients for
Agricultural Land Uses
Nutrient export coefficients for the agricultural land-use areas within the
watershed were not as simple to derive as for the urban areas. Each agricultural
drainage basin represents a different mix of land uses rather than a single land
use (Table 1). Typically export coefficients are determined by monitoring land
uses, such as forest, row crops or urban, using field plots to isolate individual
land uses (Reckhow et al., 1980). While monitoring single land-use watersheds
may be ideal, most watersheds, even small ones, are generally comprised of a
variety of different land uses. The extensive in-stream monitoring network in the
upper North Bosque River watershed was designed specifically to include a
range of different land uses to monitor the variety of contributing nutrient
sources (McFarland and Hauck, 1997b).
8
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
To isolate the loading contribution from these heterogeneous drainage areas,
multiple regression techniques were used to develop the nutrient export
coefficients for the major agricultural land uses in the watershed based on
procedures described by Hodge and Armstrong (1993). The dependent variable
was the nutrient loading at each site (Step 3), while the independent variables
were the fraction of the drainage area above each site represented by each landuse category (Step 1). The parameters of the resulting multiple regression model
for each nutrient define optimized export coefficients across all sites for each
land-use category for the time period evaluated. All multiple regression models
were developed using a forced zero intercept, thus, giving a loading of zero
when all independent variables equal zero.
Further, the assumption was made that in-stream losses and transformations of
nutrients were insignificant to the point of each monitoring site. The assumption
of negligible in-stream nutrient losses seemed supportable, particularly for totalP and total-N, because the monitored streams used in the regressions are
characterized by intermittent flow and the majority of nutrient transport occurs
during a few large rainfall-runoff events. This assumption is more problematic
for PO4-P, which may undergo rapid transformation processes in-stream, but is
necessary to remain consistent with the definition of an export coefficient, i.e., a
coefficient that represents the nutrients exported from a given land area to a
stream and, thus, inherently not including in-stream processes.
Of the 13 sites considered, 10 sites were used in estimating nutrient export
coefficients for agricultural land uses. Sites BO040 and BO070 were reserved to
validate the calculated nutrient export coefficients, and site MB040 was used to
calculated the urban nutrient export coefficients as described in Step 4. The sites
used in determining the nutrient export coefficients for agricultural land uses
were:
AL040, GC100, IC020, NF005, NF010,
NF020, NF050, SF020, SF075 and SP020.
The land in each drainage basin was classified into three major land-use
categories:
1.
Dairy Waste Application Fields
2.
Forage Fields
3.
Wood/Range
Dairy waste application fields, as described in Step 1, are primarily coastal
bermudagrass fields used for solid and liquid waste application. Solid manure is
surface applied without incorporation on coastal bermudagrass fields, while a
variety of irrigation systems are used to apply the liquid effluent. Forage fields
are primarily coastal bermudagrass fields not used for dairy waste application.
About 17 percent of the forage fields in the upper North Bosque River watershed
are in a sorghum/wheat rotation rather than coastal bermudagrass (McFarland
and Hauck, 1995). The wood/range land area of the upper North Bosque River
watershed is part of the Cross Timbers vegetation region of Texas and is
comprised primarily of scrub live oak (Quercus virginiana) and juniper
(Juniperus spp.) in the woodland areas with tallgrass species such as little
bluestem (Schizachyruim scoparium), indiangrass (Sorghastrum nutans) and
switchgrass (Panicum virgatum) in the native rangeland areas (Schuster and
Hatch, 1990).
Lake Waco/Bosque River Watershed Initiative Report
9
These three land-use categories (dairy waste application fields, forage fields and
wood/range) were defined to minimize the effects of multicollinearity and to
obtain reasonable coefficients for the land uses evaluated. The land-use
categories of peanuts, orchard, water and barren were not included in the
multiple regression models because each represented a very small portion of any
one drainage area (less than three percent) making a reasonable estimation of
parameter values for these categories statistically unrealistic (Table 1). The
loadings from the land areas associated with peanuts, orchard, water and barren
were assumed to be minor and part of the error term in the model calculations.
Range and wood were initially evaluated as separate land uses and then
combined into a single land-use category of wood/range based on
multicollinearity effects between these two categories. Wood and range were
considered interrelated land areas in the watershed in that livestock generally
graze these lands as a single unit rather than as separately fenced land units, thus,
supporting the use of a wood/range category.
The results of the multiple regression analysis are summarized in Table 4. While
the multiple regression models for each constituent were highly significant
(α=0.05), significant parameter values for the land-use variables were indicated
only for dairy waste application fields in all three models. This indicates that the
land-use categories used as independent variables in the multiple regression
model explain a large proportion of the variability in PO4-P, total-P and total-N
loadings, but that the specific contribution from forage fields and wood/range
could not be statistically defined as significantly different from zero at α=0.05.
The large relative magnitude of the parameter values for dairy waste application
fields compared to forage fields and wood/range appears to mask a clear
definition of parameter values for forage fields and wood/range. Although the
positive parameter values for forage fields and wood/range were not statistically
different from zero, they still represented optimized values from the current data
set and appeared to be reasonable estimates of the nutrient export coefficients for
these land uses.
Table 4. Parameter estimates for land-use variables (x) versus nutrient loadings (y) using a zero-intercept
multiple regression model. Nutrient loadings were based on a 1,179-day period from November 10, 1993
through January 31, 1997 and prorated to an annual basis. All three nutrient models had an R2 = 0.95 with
an associated p-value of 0.0001.
Multiple Regression Parameter Estimates
Land Use
(independent variables)
Model 1
PO4-P (kg/ha/yr)
Parameter Estimate
Model 2
Total-P (kg/ha/yr)
Model 3
Total-N (kg/ha/yr)
p-value
Parameter Estimate
p-value
Parameter Estimate
p-value
12.28 + 1.88 *
0.0003
Dairy Waste Appl. Fields
3.84 + 0.39 *
0.0001
5.46 + .63 *
0.0001
Forage Fields
0.16 + 0.52 ns
0.7459
1.04 + 0.83 ns
0.2480
5.40 + 2.48 ns
0.0660
Wood/Range
-0.07 + 0.18 ns
0.7194
-0.02 + 0.29 ns
0.9402
0.63 + 0.86 ns
0.4935
* indicates that the parameter value is significantly different from 0.0 at α= 0.05; ns indicates the parameter value is not significantly different
from 0.0 at α=0.05.
10
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
The negative parameter values determined from the multiple regression output
for wood/range were unacceptable as nutrient export coefficients as some
positive loading was expected from all land uses. To estimate positive export
coefficients for PO4-P and total-P for wood/range, the loadings for the two “least
impacted” sites, SF020 and SP020, were averaged. Both SF020 and SP020
contain over 80 percent wood/range in their drainage areas and no dairy waste
application fields (Table 1). The forage fields above SF020 and SP020 are
primarily improved pasture of coastal bermudagrass (McFarland and Hauck,
1997b). Standardized to an annual basis the nutrient export coefficients
wood/range based on data from sites SF020 and SP020 are 0.05 kg PO4-P/ha/yr
and 0.20 kg total-P/ha/yr. While representing the best approximation given the
current data set, these coefficient values for PO4-P and total-P for the
wood/range category probably overestimate the contribution from wood/range,
because some intensive agricultural practices occur in both watersheds.
Step 6.
Compare Calculated Export Coefficients to
Literature and Field Plot Values
The reasonableness of coefficient values was evaluated by comparing calculated
nutrient export coefficients (Steps 4 and 5) with literature values and values from
field plot studies specific to the upper North Bosque River watershed. The
calculated nutrient export coefficients for the upper North Bosque River
watershed are presented in Table 5 for urban, dairy waste application fields,
forage fields and wood/range.
Table 5. Calculated nutrient export coefficients for dominant land uses in
the upper North Bosque River watershed for November 10, 1993 through
January 31, 1997. These export coefficients represent the values
normalized to an annual basis for the study period.
Nutrient Export Coefficients
PO4-P
(kg/ha/yr)
Total-P
(kg/ha/yr)
Total-N
(kg/ha/yr)
Dairy Waste Appl. Fields
3.84
5.46
12.28
Forage Fields
0.16
1.04
5.40
Wood/Range
0.05
0.20
0.63
Urban
0.80
2.20
10.00
Land Use
Calculated nutrient export coefficients for the land-use categories of forage
fields, wood/range and urban were compared to literature values provided in
reviews by Frink (1991) and Reckhow et al. (1980). Mean and median
coefficient values for the categories of forested, pasture/grazed, non-row crop
and urban watersheds are presented in Table 6 for total-P and total-N from the
studies reviewed. As indicated earlier, PO4-P is generally not considered when
calculating nutrient export coefficients due to the influence of in-stream
transformations on this constituent, but was included in this study due to the
importance of PO4-P as the most biologically available source of phosphorus in
most stream systems.
Lake Waco/Bosque River Watershed Initiative Report
11
Table 6. Literature values for total-P and total-N mass loading coefficients as compiled by Frink (1991) and
Reckhow et al. (1980). 'n' equals the number of different studies evaluated.
Total-P (kg/ha/yr)
Land Use
Mean Median
Total-N (kg/ha/yr)
Range
n
Mean Median
Range
n
Source
Forested
0.14
0.12
(0.01 - 0.28)
10
2.46
2.50
(0.10 -
7.60)
12
Frink (1991)
Watersheds
0.24
0.21
(0.02 - 0.83)
10
2.86
2.46
(1.38 -
6.26)
26
Reckhow et al. (1980)
Pasture/Grazed
0.52
0.50
(0.32 - 0.82)
7
3.51
3.25
(0.30 -
6.00)
8
Watersheds
1.50
0.81
(0.14 - 3.09)
14
9.27
5.66
(1.48 - 30.85)
13
Non-Row
1.36
0.39
(0.26 - 5.00)
5
5.68
5.00
(3.20 -
9.60)
5
Crop Watersheds
1.08
0.76
(0.10 - 2.90)
13
5.19
6.08
(0.97 -
7.82)
10
Reckhow et al. (1980)
Urban
1.37
1.44
(0.30 - 2.45)
13
10.28
7.90
(5.00 - 28.00)
13
Frink (1991)
Watersheds
1.91
1.10
(0.19 - 6.23)
23
9.97
5.50
(1.48 - 38.47)
19
Reckhow et al. (1980)
Frink (1991)
Reckhow et al. (1980)
Frink (1991)
The calculated nutrient export coefficients for agricultural land uses of the upper
North Bosque River watershed are comparable to literature values for forage
fields and wood/range (Tables 5 and 6). This study’s category of forage fields
can best be compared to the categories of pasture/grazed land and non-row crops
as presented in Table 6. The study’s wood/range category fits within the
categories of forested and pasture/grazed land in Table 6. The urban values
calculated for the upper North Bosque River watershed (2.2 kg total-P/ha/yr and
10.0 kg total-N/ha/yr) fit well within the bounds of the literature values (Table
6). The wide variation in literature values reflects the land uses as well as the
environmental conditions, such as rainfall, soils, slopes and management
practices, from which individual coefficients were produced. This emphasizes
the advantage of using regional or site specific export coefficients, when
available, in watershed management planning.
To best evaluate the reasonableness of the calculated nutrient export coefficients
for dairy waste application fields, coefficient values were compared to
measurements from field plot trials specific to the upper North Bosque River
watershed. Table 7 presents the results from monitoring small (0.03 to 0.52 ha)
dairy waste application field plots on farms in the upper North Bosque River
watershed. Runoff volume and water quality were monitored for at least a year
at each site. Plots 1, 2 and 6 represent effluent irrigation fields typical for this
watershed. Plot 3 represents a new manure application field, while Plot 4
represents the impact of a filter strip below Plot 3. Plot 5 has a past history of
manure application but did not received manure for at least two years prior to the
study and did not receive manure or commercial fertilizer during the study
period. Plots 7 and 8 represent manure application fields that did not receive
manure during the study period. Details of the field plot study are presented in
Flowers et al. (1996).
12
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
Table 7. Calculated mass loading coefficients for eight field plots used for dairy waste application in the
upper North Bosque River watershed (Flowers et al., 1996).
Plot Number
1
2
3
4
5
6
7
Sorghum
Coastal
Coastal
Coastal
Sorghum
Coastal
Hay/Winter Bermuda-grass Bermuda-grass BermudaHay/Winter
BermudaWheat
receiving
receiving
grass with
Wheat
grass with no
Irrigated with
Manure
Manure with past history of Irrigated with
manure
Effluent
Filter Strip‡
manure
Effluent
applied
application¥
during study
period
8
Basic Plot
Description
Sorghum
Hay/Winter
Wheat
Irrigated with
Effluent†
Monitoring
Period
21Dec9324May95
21Dec9324May95
05Jan9431Aug95
05Jan9431Aug95
11Jan9415Jun95
23Mar9431Aug95
08Jun9414Jun95
18Jan9401Apr95
# of days
520
520
604
604
521
527
372
439
PO4-P
(kg/ha/yr)
3.11
3.02
1.78
1.09
0.44
1.90
1.74
0.19
Total-P
(kg/ha/yr)
7.24
6.79
2.22
1.69
0.53
4.81
2.16
0.76
Total-N
(kg/ha/yr)
19.29
18.38
3.01
2.72
0.99
14.49
4.95
3.40
Sorghum
Hay/Winter
Wheat with
no manure
applied
during study
period
†
Effluent and manure application rates were based on the nitrogen needs of the crops on each field.
Plot 4 was located directly below Plot 3.
¥
No manure or commercial fertilizer was applied to Plot 5 during the study.
‡
The calculated nutrient export coefficients for dairy waste application fields
compare favorably, (i.e., are within the range of measured data) to field plot
measurements, although the field plot data presents a large range of coefficient
values, particularly for total-N. The differing time-frames and, thus, rainfall
conditions will directly influence coefficient values, and a wide range of
practices and application rates explain a large portion of the variance between
values.
Step 7.
Calculate Stephenville WWTP Loadings
Loadings of PO4-P, total-P and total-N for the Stephenville WWTP were
estimated from monthly discharge records provided by the city of Stephenville
and water quality grab samples collected of the WWTP effluent. Because
sampling at the WWTP was not initiated until December 1993, values used to
calculate mass loadings for November 1993 were based on geometric mean
values for data collected between December 1993 and August 1995 (McFarland
and Hauck, 1997b). For certain months, values for of PO4-P, total-P and/or
total-N were unavailable. When this occurred, values were estimated based on
regression relationships between PO4-P and total-P and between NO3-N and/or
TKN with total-N. Monthly average concentrations were used with monthly
effluent values to calculate loadings by month. Monthly loadings were summed
to obtain a total loading of nutrients from the WWTP for the entire 1,179-day
period. Estimated mass loadings from the Stephenville WWTP into the North
Bosque River totaled 15,400 kg of PO4-P, 18,400 kg of total-P, and 62,800 kg of
total-N for the period of November 10, 1993 through January 31, 1997.
Lake Waco/Bosque River Watershed Initiative Report
13
Step 8.
Compare Estimated Nutrient Loadings with
Monitored Loadings
North Bosque River sites BO040 and BO070 provide an additional level of
verification beyond that afforded through the comparison of export coefficients
to literature values (Step 6). Sites BO040 and BO070 are associated with all
loading sources in the watershed, i.e., urban nonpoint, Stephenville WWTP,
dairy waste application fields, forage fields and wood/range, and, thus, can be
used to compare predicted nutrient loadings with measured loadings from all
sources.
A comparison of predicted loadings with measured loadings at sites BO040 and
BO070 is presented in Figure 2 for the period of November 10, 1993 through
January 31, 1997. Nonpoint source loadings were predicted by multiplying the
appropriate land-use export coefficient by the area above sites BO040 or BO070
associated with each land use. Point source loadings from the Stephenville
WWTP were calculated as discussed in Step 7 and added to nonpoint source
loadings as a prediction of total loadings for sites BO040 and BO070. All
loadings were standardized to the 1,179-day time period. The measured nutrient
loadings for BO040 and BO070 are as calculated in Step 3 (Table 4).
Predicted nutrient loadings were greater than measured loadings in all cases
except for PO4-P and total-N values at BO040 (Figure 2). At BO040, the
predicted PO4-P loading was about 9 percent less than the measured value, while
the total-N loading was about 14 percent less than measured. The largest percent
error occurred at BO070 for PO4-P where the predicted loading was 66 percent
greater than the measured loading. The over prediction of soluble reactive
phosphorus loadings at BO070 is probably the result of in-stream biochemical
transformations of PO4-P and sorption to suspended sediments. Adequate time
for these physicochemical and biochemical processes is afforded by the over 40
km reach of the river from the major sources of nutrients at the headwaters,
including many of the dairy waste application fields and the Stephenville WWTP
effluent, to the North Bosque River monitoring site at BO070.
While the impact of in-stream nutrient transformations are expected to be much
greater on PO4-P loadings than total-P or total-N loadings, PO4-P was included
in the estimation of nutrient export coefficients due to its importance in
eutrophication. PO4-P is often used to represent soluble P that is readily
available for algae uptake, as generally only a small portion of particulate P is
readily bioavailable (Sharpley et al., 1995). Earlier research in the watershed
indicated an increase in the percent of total-P represented by PO4-P as the
percent of dairy waste application fields in the drainage area above sampling
sites increased (McFarland and Hauck, 1997b), thus, emphasizing the need to
evaluate PO4-P sources within the watershed.
14
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
Site BO040
Nutrient Loading (kg in thousands)
500
400
300
Predicted
Measured
200
100
0
PO4-P
% error
(-9%)
Total-P
(8%)
Total-N
(-14%)
Site BO070
Nutrient Loading (kg in thousands)
1,000
800
600
Predicted
Measured
400
200
0
PO4-P
% error
(66%)
Total-P
(36%)
Total-N
(5%)
Figure 2. Comparison of predicted with measured nutrient loadings at North Bosque River sites BO040 and
BO070 for the period of November 10, 1993 through January 31, 1995. Numbers in parentheses represent
the percent error between measured and predicted values.
The overall agreement of predicted and measured nutrient loadings for all three
nutrients at the two North Bosque sites is very encouraging. The percent errors
in predictions were within reasonable expectations from the application of an
export coefficient approach. Even for direct measurements of in-stream
loadings, an error of +25 percent in nutrient loadings is not uncommon (Loehr et
al., 1989).
Lake Waco/Bosque River Watershed Initiative Report
15
Step 9.
Calculate Percent Contribution by Land-Use Sector
Based on the estimated loadings for point and nonpoint sources calculated in
Step 8, the percent allocation by source was estimated for sites BO040 and
BO070 (Figure 3). These contributions are specific to the 1,179-day period
evaluated (November 10, 1993 - January 31, 1997). Discussion of the sensitivity
of these results to variations in the agricultural land use export coefficients and
variability of results to variations in flow conditions are discussed in Steps 10
and 11, respectively.
Site BO040
Site BO070
PO4-P Loadings = 65,400 kg
Forage Fields
11%
Wood/Range
3%
Urban
4%
WWTP
24%
PO4-P Loadings = 132,000 kg
Forage Fields
12%
Wood/Range
8%
Urban
3%
Dairy Waste
Application
Fields
65%
Dairy Waste
Application
Fields
58%
Total-P Loadings = 116,000 kg
Forage Fields
22%
Urban
6%
WWTP
17%
Wood/Range
8%
Dairy Waste
Application
Fields
47%
Total-N Loadings = 384,000 kg
Total-P Loadings = 260,000 kg
Forage Fields
24%
Wood/Range
8%
Urban
WWTP
5%
7%
Wood/Range
16%
Dairy Waste
Application
Fields
48%
Total-N Loadings = 865,000 kg
Urban
6%
Urban
9%
Forage Fields
34%
WWTP
12%
WWTP
17%
Dairy Waste
Application
Fields
32%
Forage Fields
37%
WWTP
8%
Dairy Waste
Application
Fields
33%
Wood/Range
16%
Figure 3. Predicted loadings and percent contribution by source for North Bosque River sites BO040 and
BO070 for the period of November 10, 1993 through January 31, 1997.
16
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
At BO040, the largest percentage of phosphorus loadings were attributed to
dairy waste application fields representing 58 percent of the PO4-P loadings and
47 percent of the total-P loadings. Forage fields and dairy waste application
fields comprised similar loadings of total-N, representing 34 and 32 percent
respectively. Similar findings were indicated at BO070. While representing
about 7 percent of the total watershed area at BO070 (Table 1), dairy waste
application fields were found to contribute 65 percent of the PO4-P, 48 percent
of the total-P and 33 percent of the total-N loadings at BO070. Forage fields,
representing about 20 percent of the watershed area at BO070, was the largest
contributing sector of total-N loadings with 37 percent of the predicted mass.
Forage fields also contributed 12 percent of the PO4-P loadings and 24 percent
of the total-P loadings. Commercial fertilizer is probably the source of most of
the nitrogen and phosphorus loadings from forage fields, while organic fertilizer,
i.e., manure, represents the dominate nutrient source from dairy waste
application fields.
Step 10. Perform Sensitivity Analysis on Percent
Contributions
Because relatively large standard deviations were associated with the calculated
nutrient export coefficients for the agricultural land-use sectors, values presented
in Figure 3 are not absolute values but represent a range of potential values for
each contributing sector. To evaluate the sensitivity of the percent contribution
by contributing sector to the variability associated with the nutrient export
coefficients, the export coefficients calculated using the multiple regression
method were allowed to vary randomly within the normal distribution defined
for each coefficient by its standard deviation (Table 4). For those coefficient
values estimated using other methods, i.e., wood/range for PO4-P and total-P and
urban for all three constituents, the standard deviation for the normal distribution
was set equal to the coefficient value. Ten thousand simulations were made for
each nutrient applying a random number generator to calculate loadings for sites
BO040 and BO070. For each simulation, the export coefficient for each land
use was determined as the export coefficient plus or minus the quantity of a
value from a normal-distribution random-number generator multiplied by the
standard deviation associated with the export coefficient. The percent
contribution of each source to total predicted loadings was also re-calculated
during each simulation. The variability in WWTP loadings was not explored
because these loadings were directly monitored.
The largest variance in percent contribution was generally associated with forage
fields (Table 8) reflecting the relatively large standard deviations associated with
export coefficients for this land-use category. The smallest variance was
associated with the loadings from WWTPs, which was expected as WWTP
loadings were input as a constant value in all simulations. As anticipated, the
overall contribution of wood/range and urban remained relatively small even
though relatively large standard deviations were associated with the export
coefficients for these land uses.
Lake Waco/Bosque River Watershed Initiative Report
17
Table 8. Estimated standard deviation associated with the mean percent contribution for November 10, 1993
through January 31, 1997 by sector from 10,000 simulations. Simulations used a random generator to
determine the export coefficient associated with each land use based on the standard deviation of each
coefficient.
Site BO040
PO4-P (%)
Total-P (%)
Standard
Deviation
Mean
Contribution
Total-N (%)
Contributing Sector
Mean
Contribution
Standard
Deviation
Mean
Contribution
Standard
Deviation
Waste Appl. Fields
58
8
47
9
32
7
Forage Fields
11
11
22
13
34
12
Wood/Range
3
3
8
6
8
7
Urban
4
3
6
5
9
7
WWTP
24
4
17
3
17
4
Site BO070
PO4-P (%)
Total-P (%)
Standard
Deviation
Mean
Contribution
Total-N (%)
Contributing Sector
Mean
Contribution
Standard
Deviation
Mean
Contribution
Standard
Deviation
Waste Appl. Fields
64
11
48
12
33
9
Forage Fields
12
13
24
14
37
13
Wood/Range
8
6
16
12
16
14
Urban
3
3
5
4
6
5
WWTP
12
2
8
2
8
2
The agricultural export coefficients, while based on an extremely large database
from a monitoring perspective, represent a relatively small database (10 sites)
from a statistical perspective. Ideally, 30 or more sites would be used in such an
analysis to give adequate power to the regression analysis approach. The
relatively large standard deviations associated with the nutrient export
coefficients for dairy waste application fields and forage fields partially reflect
the size of the data set from which these coefficients were calculated as well as
the inherent variability in environmental characteristics, e.g., slope and soils, and
land management practices associated with each land use as aggregated and
compared between drainage areas. While additional sites, representing a broader
range of each land-use category, both urban and agricultural, might help refine
these estimates, the sensitivity analysis helps take into account the variability in
the export coefficients within land uses without the expense and time of
collecting additional data.
18
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
Step 11. Evaluate Relative Contributions by Source with
Variations in Flow
To evaluate the reliance of export coefficient values (and, thus, the percent
contribution by source) on stream flow conditions, nonpoint source land-use
coefficients were re-calculated using nutrient loadings from time periods
representing extreme low and high flow conditions. Flow conditions at BO070
were used to determine the time periods evaluated (Figure 4). During the study
period, the highest monthly flow conditions occurred in May 1994 at 20.3
m3/sec, while the lowest monthly flow conditions occurred in August 1994 at
0.07 m3/sec. Corresponding flow rates at BO040 were 4.0 m3/sec in May 1994
and 0.16 m3/sec in August 1994. The nutrient export coefficients estimated for
these two months prorated to an annual basis are presented in Tables 9 and 10.
Site BO070
Average Monthly Flow (m3/sec)
100.00
10.00
1.00
0.10
Jan-97
Nov-96
Sep-96
Jul-96
May-96
Mar-96
Jan-96
Nov-95
Sep-95
Jul-95
May-95
Mar-95
Jan-95
Nov-94
Sep-94
Jul-94
May-94
Mar-94
Jan-94
Nov-93
0.01
Figure 4. Average monthly flow at North Bosque River site BO070 for the period of November
1993 through January 1997.
Table 9. Estimated export coefficients using a zero-intercept multiple regression model for a 31-day
period from May 1, 1994 through May 31, 1994. Coefficient values are prorated to an annual basis.
Nutrient Export Coefficients
PO4-P (kg/ha/yr)
2
Land Use
Dairy Waste Appl. Fields
(Model R =0.85
p-value=0.0027)
Total-P (kg/ha/yr)
2
(Model R =0.87
p-value=0.0018)
Total-N (kg/ha/yr)
(Model R2=0.79
p-value=0.0088)
19.07
+
4.19
25.61
+
5.97
44.21
+
22.47
Forage Fields
1.94
+
5.18
4.77
+
7.37
34.27
+
27.79
Wood/Range
0.35
+
1.95
1.45
+
2.78
6.93
+
10.46
Lake Waco/Bosque River Watershed Initiative Report
19
Table 10. Estimated export coefficients using a zero-intercept multiple regression model for a 31-day period
from August 1, 1994 through August 31, 1994. Coefficient values are prorated to an annual basis.
Nutrient Export Coefficients
Land Use
PO4-P (kg/ha/yr)
Total-P (kg/ha/yr)
Total-N (kg/ha/yr)
(Model R2=0.39
p-value=0.3050)
(Model R2=0.75
p-value=0.0164)
(Model R2=0.79
p-value=0.0092)
Dairy Waste Appl. Fields
0.01
+
0.01
0.05
+
0.02
1.71
+
0.05
Forage Fields
0.00
+
0.01
0.00
+
0.01
0.00
+
0.06
Wood/Range
0.00
+
0.00
0.00
+
0.01
0.01
+
0.02
Rainfall within the upper North Bosque River watershed often exhibits spatial
variability that may cause an unknown distortion on sector contributions,
especially when evaluated for a relatively short period, such as a month. Spatial
variability of rainfall has the potential to be compounded by the spatial
variability of land uses across the watershed, e.g., most urban and dairy land uses
are in the northern and western portion of the watershed. Therefore, the extreme
high flow period should be considered representative of high flow conditions,
but not necessarily indicative of “average” watershed response to high stream
flow conditions. Similar reasoning pertains to the extreme low flow month,
though, as will be shown, the dominance of the Stephenville WWTP under low
flow conditions greatly reduces the importance of spatial variability of land use
and rainfall on total nutrient contribution to the North Bosque River. Within the
limitations imposed by spatial variability, evaluating relative contribution by
sector under the two extreme scenarios provided herein— high and low
streamflow— does provide an enhanced understanding of variability of point and
nonpoint source contributions in response to streamflow variation.
The variability in export coefficients is immediately noticeable in comparing the
values between Tables 9 and 10, as well as with the values calculated for the
longer time period in Table 5. As flow decreases, the nonpoint source export
coefficients decrease as a function of decreasing rainfall runoff. The wide
variability in coefficient values between time periods represents a shift in
loadings from primarily stormflow contributions in May 1994 to very low
baseflow contributions in August 1994. The longer 1,179-day period used in the
original estimation of the nutrient export coefficients for the upper North Bosque
River watershed was chosen not only to produce “average” coefficients for
“average” conditions within the watershed, but also to smooth the variability in
rainfall runoff conditions over time and between sites. These shorter time
periods (31 days) emphasize the impact of specific flow conditions on
coefficient and loading estimates.
The percent contribution by source for May 1994 and August 1994 are presented
in Figure 5 for site BO040 and in Figure 6 for site BO070. These two figures
clearly indicate the shift in relative contributions from predominately nonpoint
sources during high streamflow conditions to point sources during low
streamflow conditions. While nonpoint sources contribute greater overall
loadings to the upper North Bosque River, the relative contribution of the single
point source (Stephenville WWTP) should not be ignored, particularly if low
flow conditions are of concern.
20
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
High Streamflow (May 1994) - Site BO040
Low Streamflow (August 1994) - Site BO040
PO4-P Loadings = 7,300 kg
PO4-P Loadings = 420 kg
Forage Fields
19%
Urban
WWTP
2%
5%
Wood/Range
6%
Urban
8%
Dairy Waste
Application
Fields
68%
WWTP
92%
Total-P Loadings = 12,400 kg
Forage Fields
28%
Total-P Loadings = 630 kg
Urban WWTP
3%
4%
Urban
23%
Dairy Waste
Application
Fields
2%
Dairy Waste
Application
Fields
53%
Wood/Range
12%
WWTP
75%
Total-N Loadings = 46,600 kg
Total-N Loadings = 2,800 kg
Urban WWTP
2%
4%
Forage Fields
53%
Dairy Waste
Application
Fields
25%
Wood/Range
16%
Urban
33%
WWTP
65%
Dairy Waste
Application
Fields
2%
Figure 5. Predicted loadings and percent contribution by source for North Bosque River site BO040 for the
periods of May and August 1994.
Lake Waco/Bosque River Watershed Initiative Report
21
High Streamflow (May 1994) - Site BO070
Low Streamflow (August 1994) - Site BO070
PO4-P Loadings = 17,200 kg
PO4-P Loadings = 450 kg
Forage Fields
12%
Urban WWTP
2%
2%
Urban
12%
Dairy Waste
Application
Fields
64%
Wood/Range
20%
Total-P Loadings = 31,900 kg
Forage Fields
27%
WWTP
86%
Dairy Waste
Application
Fields
1%
Wood/Range
1%
Total-P Loadings = 740 kg
Urban WWTP
2%
1%
Dairy Waste
Application
Fields
46%
Urban
32%
WWTP
64%
Dairy Waste
Application
Fields
4%
Wood/Range
24%
Total-N Loadings = 126,900 kg
Urban
1%
WWTP
1% Dairy Waste
Application
Fields
20%
Forage Fields
50%
Wood/Range
28%
Total-N Loadings = 3,500 kg
WWTP
53%
Urban
43%
Wood/Range
1%
Dairy Waste
Application
Fields
3%
Figure 6. Predicted loadings and percent contribution by source for North Bosque River site BO070 for the
periods of May and August 1994.
22
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
DISCUSSION AND CONCLUSIONS
The multiple regression approach taken herein for determining nutrient export
coefficients maximizes the use of mass loading information contained within the
upper North Bosque River watershed monitoring database. The multiple
regression method allows use of in-stream flow and water quality data from
steam sites with heterogeneous land-use drainage areas without the need for
isolating individual land uses. Further, the multiple regression method provides
export coefficients representing the average of conditions and practices (e.g.,
soils, planting and harvest dates, fertilization timing and amounts, slopes, tillage
practices, and proximity to streams) of each land use across the upper North
Bosque River watershed, as opposed to export coefficients determined for the
more limited practices and conditions of single land-use drainage areas.
Whether determined by regression techniques from in-stream monitoring sites, as
in this study, or from monitoring of individual land uses, the export coefficients
are indicative of the climatic conditions under which the monitoring data were
collected. The longer the duration of the monitoring data set, the more likely the
export coefficients include a range of weather conditions (e.g., high and low
rainfall periods), typify average nutrient contributions, and do not include
potentially undesirable biases from over representation of meteorologic
extremes. This underscores the importance of understanding the weather
conditions under which export coefficients are determined before using them in
watershed planning efforts.
During a large portion of the study period, rainfall conditions were well above
average, thus indicating a potential overestimation of coefficient values
compared to “average” conditions (Figure 7). The total rainfall for the study
period was 305 cm (120 inches) as compared to average conditions for the same
months of 241 cm (95 inches); nearly 26 percent higher than average rainfall.
Similarly for flow, the longterm average annual discharge (1974-1995) from the
USGS gauging station at site BO070 was 1.9 m3/sec. During the study period,
the flow at BO070 averaged 3.2 m3/sec. These above average rainfall and flow
conditions should be considered in using the calculated export coefficient values
beyond the time period evaluated and in evaluating the relative nutrient
contribution by sector. The amount and intensity of rainfall over a period will
impact the potential contribution of various nonpoint source contributors as
different land uses will have different runoff responses.
The relative
contribution between point and nonpoint sources also will vary from high rainfall
to low rainfall periods. As shown in Figures 5 and 6, a reasonable expectation is
that point source loadings (i.e., the Stephenville WWTP) will increase in relative
important as a contributor with decreasing rainfall and streamflow.
Lake Waco/Bosque River Watershed Initiative Report
23
30
Precipitation (cm)
25
20
15
10
5
Long-Term Avg.
Jan-97
Nov-96
Sep-96
Jul-96
May-96
Mar-96
Jan-96
Nov-95
Sep-95
Jul-95
May-95
Mar-95
Jan-95
Nov-94
Sep-94
Jul-94
May-94
Mar-94
Jan-94
Nov-93
0
Study Period
Figure 7. Long-term monthly average (1955 - 1990) and average total rainfall for November 1993 through
January 1997 across six National Weather Observer sites in Erath and Hamilton Counties, Texas.
The calculated agricultural and urban nutrient export coefficients (Table 5)
provide a good indication of the nutrient contribution from land uses within the
upper North Bosque River watershed for total-P and total-N. Coefficient values
were within the range for similar land uses from other studies. Predicted
loadings of total-P and total-N using coefficient values also compared favorably
with measured loadings for two North Bosque River sites (BO040 and BO070;
Figure 2).
Based on the comparison of predicted and measured PO4-P loadings at the two
North Bosque River sites, the export coefficients for PO4-P appear to give
reasonable estimates of the contribution by sector for soluble reactive
phosphorus. Unlike total-P and total-N, which are less influenced by in-stream
transformations and losses, soluble reactive P or PO4-P is expected to experience
transformations and losses during downstream transport which would increase
with drainage basin size or stream travel time. However, the intermittent nature
of most of the stream sites used to determine the PO4-P export coefficients and
the relatively short travel time to these monitoring sites given their relatively
small drainage areas, provide limited opportunity for significant, in-stream PO4P transformations. In contrast, in-stream travel time in the North Bosque River,
in particular to site BO070 at the mouth of the study area, is sufficient to allow
some transformations and losses to occur. Therefore, the PO4-P export
coefficients provide reasonable estimates of nutrient loadings by contributing
sector into the North Bosque River, but do not provide as good an estimate of
actual loadings at downstream points in the river due to the importance of instream transformations and losses.
The calculated nutrient export coefficients, along with the point source
contributions from the Stephenville WWTP provide good estimates for
identifying and quantifying the sources of nutrients into the upper North Bosque
River. Over the entire period (Figure 2) and under the highest monthly flow
24
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
conditions of the study period (Figures 5 and 6), dairy waste application fields
were the dominant source of PO4-P loadings and forage fields and dairy waste
application fields were dominant sources of total-P and total-N loadings. At low
flow, the Stephenville WWTP represented the primary source of phosphorus and
nitrogen to the upper reaches of the North Bosque River. Although point source
nutrient loadings represent a much smaller portion of the overall loadings as
compared to nonpoint source loadings, the importance of point source loadings
should be considered if in-stream concentrations during low flow conditions are
a concern.
Lake Waco/Bosque River Watershed Initiative Report
25
26
Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed
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Determining Nutrient Contribution by Land Use for the Upper North Bosque River Watershed