EFFECTS OF PRECIPITATION AND AIR TEMPERATURE ON
NITROGEN DISCHARGES FROM RHODE RIVER WATERSHEDS
DAVID L. CORRELL, THOMAS E. JORDAN and DONALD E. WELLER
Smithsonian Environmental Research Center, Edgewater, MD 21037, U.S.A.
(e-mail: correll@serc.si.edu)
(Received 21 January 1998; accepted 17 November 1998)
Abstract. We studied discharges of total-N, nitrate, ammonium, and total organic-N from seven
contiguous small watersheds on the Atlantic Coastal Plain in Maryland for up to 25 yr. These water-
sheds have perched aquifers so all groundwater discharges as well as surface runoff were measured at
V-notch weirs which included volume-integrating flow-proportional samplers. Interannual variations
in annual and seasonal precipitation during this study spanned approximately the range of 160 yr
weather records in the region. Annual total-N area yields from the overall watershed varied nine-
fold, correlations of all N-parameter discharges with precipitation were highly significant, and power
function regressions of precipitation vs N-discharge explained from 36 to 59% of the variance.
Nitrogen fluxes from a cropland watershed were much higher and more variable with volume of
precipitation, while fluxes from a forested watershed were much lower and were primarily composed
of organic-N. Correlations of N-fluxes with precipitation were higher in the winter and spring. An-
nual and seasonal N-concentrations also often increased significantly with precipitation. Variations
in seasonal air temperature sometimes explained significant amounts of variance in N-discharges,
especially ammonium. A model composed of regressions was used to construct graphical and tabular
summaries.
Keywords: ammonium, nitrate nitrogen, organic-N, watershed, weather effects
1. Introduction
Excessive N-discharges to estuaries including Chesapeake Bay are believed to be
the key to over-enrichment or eutrophication of these valuable natural resources
and diffuse or non-point sources are believed to be the dominant sources of these
excessive N-inputs (Boynton et al., 1995); Seitzinger and Sanders, 1997).
A large number of studies have measured the discharge of nitrate, ammonium,
and organic-N from various watersheds. The usual focus has either been compara-
tive effects of land use in a small region (e.g. Cooper and Thomsen, 1988; Correll
and Dixon, 1980; Hill, 1978; Hirose and Kuramoto, 1981), differences in dis-
charges among widely separated regions (e.g. Beaulac and Reckhow, 1982; Frink,
1991; Malmer and Grip, 1994), or patterns of change in nitrogen concentrations
from a single watershed due to a storm event (e.g. Burt and Arkell, 1987; McDif-
fett et al., 1989; O?Brien et al., 1993). These studies have clearly established that
land use, particularly intensive agriculture, has a strong influence on N-discharges.
They have also shown that N-discharge fluxes from a given watershed are seasonal,
Water, Air, and Soil Pollution 115: 547?575, 1999.
? 1999 Kluwer Academic Publishers. Printed in the Netherlands.
548 D. L. CORRELL ET AL.
peaking in the winter and spring in temperate regions. The evidence seems fairly
good that there are also significant differences in N-discharges from a given land
use in different regions, perhaps due to geological and climatic differences. It is
also clear that during individual storm events concentrations of parameters such
as nitrate, ammonium and organic-N span a broad range and usually are not well
correlated with the hydrograph.
However, very few studies have analyzed the effects of interannual variations
in precipitation and temperature on discharges of N-fractions. Such an analysis re-
quires many years of data from the same watershed. A few studies (e.g. Alexander
et al., 1996; Lucey and Goolsby, 1993) have examined the relationship of interan-
nual variations in stream water discharge with nitrate fluxes, but these studies were
of larger rivers with more complex land use, waste water outfalls, reservoirs, and
water withdrawals. In contrast, our study provides a unique perspective because
the watersheds were first and second order streams with relatively simple land use
compositions.
In this study we have sampled for up to 25 yr the discharges of nitrate, total
ammonium, and total organic-N from seven contiguous small subwatersheds of
the Rhode River in Maryland, U.S.A., which differed in land use, but had sim-
ilar weather, soils, geology, and hydrology. The watersheds were continuously
monitored for discharge with V-notch weirs which included volume-integrating
flow-proportional samplers. It is our objective to define the effects of various in
mean annual, seasonal, and weekly precipitation and air temperature on discharges
of N-fractions from forested, cropped, grazed, and mixed land use watersheds on
the inner mid-Atlantic Coastal Plain of North America.
2. Methods
2.1. SITE DESCRIPTION
The watersheds studied are all subwatersheds of the Rhode River, a tidal tributary
to the Chesapeake Bay in Maryland, U.S.A. (38510N, 76320W). The Rhode River
is within the inner Atlantic Coastal Plain. This region characteristically has fre-
quent intense precipitation events, especially in the spring and summer and highly
erodible soils. The watershed has sedimentary soils from the Pleistocene Talbot
formation at low elevations on the eastern part of the watershed, Eocene Nanjemoy
formation soils at low elevations further west, Miocene Calvert formation soils at
intermediate elevations and Pleistocene Sunderland formation soils at the highest
elevations. The soils are fine sandy loams and the mineralogy of the soils is fairly
uniform, with a high level of montmorillonite and quartz, intermediate levels of
illite and kaolinite, and low levels of gibbsite, chlorite, potassium feldspar, and pla-
gioclase (Correll et al., 1984). Soils have high moisture capacity, but relatively low
infiltration rates, so that intense storms generate overland storm flows. However,
WEATHER EFFECTS ON NITROGEN DISCHARGES 549
TABLE I
Areas (ha) and land use composition (%) of Rhode River study subwatersheds in 1976
(Correll, 1977)
Watershed Area Forest Row crops Pasture and Residential Old fields
hay fields
101 226 38 10 27 6 19
102 192 47 18 22 6 7
103 253 63 2 16 5 14
108 150 39 24 20 3 14
109 16.3 36 64 0 0 0
110 6.3 100 0 0 0 0
111 6.1 27 0 73a 0 0
a Until 1989, when it was planted in pine seedlings.
about 65 to 75% of annual water discharge is via groundwater. bedrock is about
1000 m below the surface, but the Marlboro Clay layer forms an effective aquiclude
slightly above sea level throughout the watershed (Chirlin and Schaffner, 1977).
Each subwatershed has a perched aquifer so that overland storm flows, interflow,
and groundwater discharges all move to the channel draining each subwatershed.
The slopes of the watersheds average between five and nine percent. The study
watersheds ranged in size from 6.1 to 253 ha and differed in land use (Table I).
Three watersheds were drained by first order streams. One (# 110) was completely
forested, another (# 109) was primarily row-cropped, and one (# 111) was pri-
marily land rotationally grazed until the spring of 1989, when it was planted with
pine seedlings (Correll et al., 1995). The other watersheds were drained by second
order streams and had mixed land use. For more detailed descriptions of the site
see Correll (1981) and Corell and Dixon (1980).
2.2. SAMPLING
Discharges of water from each watershed were measured with sharp-crested V-
notch weirs, whose foundations were in contact with the Marlboro Clay aquiclude
(Correll, 1977). All weirs were 120 notches, except for watershed 111, which
was 150. Each weir had an instrument building and a stilling well. Depths were
measured to the nearest 0.3 mm with floats and counterweights and were recorded
every 5 min for watersheds 101, 109, 110, and 111; every 15 min for the other
watersheds. Prior to the summer of 1974, discharges from watershed 101 were
measured with a 90 V-notch weir and recorded on a strip chart (Pluhowski, 1981).
Water samples were composited and volume-integrated for one week intervals,
then promptly collected and returned to the laboratory. A Stevens, model 61R, flow
meter actuated the sampling of an aliquot once every 154 m3 of flow on the second
550 D. L. CORRELL ET AL.
order streams and once every 77 m3 of flow on the first order streams. The water
sample was drawn from the stream channel upstream of the weir and was deposited
into a plastic container pretreated with 20 mL of 18 N sulfuric acid to prevent
biological or enzymic activity during storage. After collection samples were either
analyzed immediately or stored at 4 C.
Rainfall volume and air temperature data were obtained from the Center?s weather
station, located on watershed 101 (Higman and Correll, 1982; and subsequent
data). Rainfall volumes were measured with standard manual rainfall gauges, backed
up wih a Belfort weighing gauge. Air temperatures were recorded with maxi-
mum/minimum mercury thermometers and a Belfort recording hygrothermograph.
2.3. SAMPLE ANALYSIS
Total Kjeldahl (TKN) was determined by digestion to ammonium with sulfuric
acid, Hengar granules, and hydrogen peroxide (Martin, 1972). The ammonium in
the digestate was steam distilled and analyzed by Nesslerization (APHA, 1989).
Ammonium was also determined in undigested aliquots by oxidation to nitrite with
alkaline hypochlorite (Strickland and Parsons, 1972) and analysis of the nitrite by
reaction with sulfanilamide (APHA, 1989). Ammonium was the sum of dissolved
and acid-extractable particulate ammonium. Total organic-N was calculated by
substracting ammonium from TKN. The sum of nitrate and nitrite was measured by
reducing nitrate to nitrite with cadmium amalgam, and analyzing nitrite by reaction
with sulfanilamide (APHA, 1989).
2.4. DATA PREPARATION
Weir discharges and rain volumes were summed and mean daily air temperatures
were averaged for watershed weeks, which normally began on Monday; and sea-
sons, which were winter (December, January, February), spring (March, April,
May), summer (June, July, August), and fall (September, October, November).
Watershed years began with December. Discharge volumes were multiplied by
volume-integrated concentrations to obtain weekly fluxes of various N-fractions.
Seasonal and annual mean concentrations of N-fractions were calculated by sum-
ming weekly fluxes, then dividing by summed water discharges. Mean Rhode River
watershed discharge data from the study watersheds (Table I) were weighted by
area and are, for convenience, referred to as Rhode River or overall watershed
discharges.
When flow was loo low to obtain an integrated sample for analysis, spot sam-
ples were analyzed. From five to ten percent of the discharge data for any given
weir were missing due to equipment failures. When no significant precipitation
occurred during the data gap, these data were estimated by interpolation of data
from the same weir. When storm events occurred during the data gap, these data
were estimated by correlation with discharge data from the watershed with the most
similar nitrogen discharge behavior.
WEATHER EFFECTS ON NITROGEN DISCHARGES 551
The mean and range of annual and seasonal precipitation which were observed
during this study were put into perspective by comparison with a longer-term
record. Higman and Correll (1982) summarized data collected from 1967 to 1977
at our research center, from 1857 to 1967 at the U.S. Naval Academy in Annapolis,
MD; and from 1817 to 1856 at U.S. Army Fort Severn, on the Severn River near
Annapolis for a total record of 160 yr. Gaps in the Naval Academy data were
filled with data from Fort Severn. All three sites are on the upper western shore
of Chesapeake Bay within 20 km of each other and at the times of data collection
were fairly small towns, thus minimizing any ?heat island? effects.
3. Results
3.1. NITROGEN FLUXES
3.1.1. Annual
Annual fluxes of total-N, nitrate, total organic-N, and total ammonium from the
Rhode River watershed increased substantially with precipitation volume (Figure
1). Total-N area yields varied from about one to nine kg N ha?1 yr. Linear cor-
relations of N-fluxes with precipitation were all highly significant (P < 0.0001)
but power function regressions fit the data best (highest R2) and explained from
36 to 59% of the variance. Organic-N fluxes increased more rapidly and were
more correlated with precipitation than nitrate or ammonium fluxes. Annual fluxes
from a cropland watershed were much higher, varying with volume of precipitation
from 0.33 to 24.4 kg N ha?1 yr, also highly significant and volume of precipitation
explained from 42 to 64% of the variance. The fluxes of all forms of N from the
cropland watershed increased more rapidly with precipitation than was found for
the overall Rhode River watershed (Figure 1). In the case of a forested watershed,
fluxes of total-N were much lower and varied from 0.07 to 5 kg ha?1 yr. Most of
the total-N discharged from the forested watershed was organic-N and the organic-
N fluxes were similar to those from the overall Rhode River watershed, however
the fluxes of ammonium and nitrate were much lower from the forested watershed.
Total-N and organic-N fluxes and precipitation volume were highly significantly
correlated, but the significance for nitrate and ammonium were much lower (P =
0.05 and 0.15, resp.). For a grazed watershed the relationships between precipita-
tion and N-discharges were much weaker (R2 = 0.20 to 0.32, P > 0.05). Total-N
fluxes varied from 0.33 to over 5 kg N ha?1 yr and were composed of about equal
amounts of organic-N and and nitrate with a small amount of ammonium.
Annual temperature variations generally had little relationship with N-fluxes
from the watersheds. Usually the combination of precipitation and air temperature
explained only a few percent more of the variance in N-discharges than precipi-
tation alone. The exception was for nitrate flux from the grazed lands, where the
R2 for linear regressions improved from 0.19 to 0.29 when air temperature was
included.
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Figure 1. Variations in nitrogen parameter annual fluxes from area-weighted Rhode River watershed with volume of precipitation.
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Figure 2. Variations in spring nitrogen fluxes from areas-weighted Rhode River watershed with volume of precipitation.
554 D. L. CORRELL ET AL.
3.1.2. Winter and Spring
In the winter and spring when most N-discharge usually occurred, correlations
between total-N and nitrate discharges from the overall watershed and precipitation
were higher than for annual data and all were highly significant. For the overall
Rhode River watershed in the winter, the best regression fits were usually linear
and volume of precipitation explained 65% of the variance in nitrate and total-
N discharge. Nitrate accounted for well half of the total-N discharge in winter.
Cropland winter discharges were dominated by nitrate, while forest and grazing
land discharges were dominated by organic-N. Spring total-N discharge from the
overall watershed ranged from 0.28 to over 7.4 kg N ha?1 spring (Figure 2) and
over half of this was organic-N. Volume of spring precipitation explained from
60 to 86% of the variation in spring discharges of N-parameters from the overall
watershed. Spring cropland discharges were composed of about equal amounts of
nitrate and organic-N plus small amounts of ammonium. Variations in precipitation
explained 67 to 73% of the variance in total-N, organic-N, and ammonium spring
fluxes from cropland, but only 46% for nitrate. Spring forest total-N discharges
varied from 0.033 to 2.8 kg N ha?1 spring, mostly as organic-N. Precipitation
volume explained 78 to 79% of the variance in spring forest discharges of total-
N, nitrate, and organic-N, but only 57% for ammonium. Spring total-N discharges
from grazing lands were similar in magnitude to those from forest, but were com-
posed of about equal amounts of nitrate and organic-N and were less correlated
with precipitation. All correlations were significant (P < 0.01, except ammonium
flux, P = 0.05). The best regression fits for grazing land were exponential.
As in the case of the annual data, including air temperature in multiple linear
regressions for winter data had little effect except for the grazed watershed 111. In
that case all correlations were improved considerably by including air temperature.
For total-N, R2 increased from 0.55 to 0.79; for nitrate, R2 increased from 0.43 to
0.69; for organic-N R2 increased from 0.63 to 0.76; and for total ammonium, R2
increased from 0.49 to 0.71. For watershed 111 in the winter a multiple regression
appears significantly improved over using only precipitation. For spring data, only
the linear regression for total ammonium from grazed lands was improved by
including air temperature in the regression (R2 = 0.23 and 0.49, resp.), but there
was a similar improvement for organic-N flux from cropland (R2 = 0.57 and 0.71,
resp.).
3.1.3. Summer and Fall
Much less N was usually discharged from the watershed in the summer and fall and
the correlations with precipitation were lower in the summer than in the spring. The
best regression fits (highest R2) were usually exponential in the summer, except for
grazing lands which were usually linear. In the fall the best regression fits were usu-
ally power functions, except for grazing lands, which were usually linear. For the
Rhode River watershed total-N fluxes varied from 0.008 to 3.7 kg N ha?1 summer
or fall and were dominated by organic-N. Precipitation volume explained 42?46%
WEATHER EFFECTS ON NITROGEN DISCHARGES 555
of the variance in summer N fluxes and 48?64% for fall fluxes and all of these
correlations were highly significant (P < 0.001). Fall total-N fluxes from cropland
ranged from 0.001 to 2.5 kg total-N ha?1 fall. Fall correlations of N fluxes from
cropland and forest with precipitation were fairly high and significant (P < 0.005),
but for grazed land were nearly zero.
Summer air temperatures only helped explain N fluxes from the grazed lands,
for which the linear correlations with precipitation were quite low. For example,
including air temperature improved the R2 for nitrate from 0.026 to 0.12 and for
organic-N from 0.066 to 0.14.
3.1.4. Statistical Models
Regression curves for the annual fluxes of N parameter from the watersheds ver-
sus precipitation volume (Figure 3) help visualize the patterns of flux for total-N,
nitrate, total organic-N, and total ammonium. For the overall watershed, organic-N
is dominant at all levels of precipitation, followed by nitrate. For cropland, nitrate
is dominant for dry years, but in wetter years organic-N becomes at least equally
important, while ammonium is always a minor component. For forest, nitrate and
ammonium are about equal and do not increase very rapidly with precipitation, but
organic-N increases very rapidly with precipitation and is always dominant. For
grazed land, there is an intermediate pattern, with nitrate only a little lower than
organic-N at all levels of precipitation. In the winter, in wet years nitrate becomes
more important than organic-N for the overall watershed (Figure 4), while in the
spring the pattern for the overall watershed (Figure 5) is similar to the annual
pattern. In the summer, the regressions curve upward much more steeply with
increasing precipitation, except for the grazed watershed (Figure 6) and at high
precipitation organic-N dominated the cropland fluxes. In the fall, N-fluxes from
cropland were about equally composed of nitrate and organic-N (Figure 7).
These regressions of N-fluxes versus precipitation and long-term rainfall records
were used to construct Tables II and III. The regression equations and their coef-
ficients of determination are listed in Table IV and constitute a statistical model
of N-fluxes from these watersheds as a function of precipitation. For the overall
Rhode River watershed, cropland, forest, and grazed land, annual and seasonal
N-discharge fluxes of total-N, nitrate, organic-N, and ammonium were calculated
for time periods of low, average, and high precipitation. For convenience we used
the mean precipitation,  one standard deviation (SD) from the mean, and  2
SD from the mean. Generally, the ranges of precipitation that occurred during this
study are bracketed by the  2 SD category.
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Figure 3. Variations in annual watershed nitrogen fluxes with volume of precipitation. Curves are from regression equations in Table IVA.
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Figure 4. Variations in winter watershed nitrogen fluxes with volume of precipitation. Curves are from regression equations in Table IVB.
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Figure 5. Variations in spring watershed nitrogen fluxes with volume of precipitation. Curves are from regression equations in Table IVC.
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Figure 6. Variations in summer watershed nitrogen fluxes with volume of precipitation. Curves are from regression equations in Table IVD.
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Figure 7. Variations in fall watershed nitrogen fluxes with volume of precipitation. Curves are from regression equations in Table IVE.
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TABLE II
Annual nitrogen discharge fluxes (g N ha?1 yr) from Rhode River watersheds as a function of precipitation (cm yr?1). Flux values calculated from
regressions of nitrogen discharge versus precipitation
Precipitation Area-Weighted Watershed Watershed 109 (crops) Watershed 110 (forest) Watershed 111 (grazed)
TN NO3 TON TNH4 TN NO3 TON TNH4 TN NO3 TON TNH4 TN NO3 TON TNH4
?2 SD (64.4 cm) 706 258 375 85.9 935 653 194 83.5 83.3 11.7 57.8 22.9 241 92.9 110 51.6
?1 SD (86.2 cm) 1530 522 865 159 2590 1540 723 208 339 35.0 244 53.6 553 213 259 82.0
mean (108.0 cm) 2790 900 1650 256 5680 3000 2000 421 1000 81.5 795 103 1050 406 502 117
+1 SD (129.8 cm) 4560 1400 2800 377 10800 5160 4600 749 2430 162 2020 176 1770 685 862 157
+2 SD (151.6 cm) 6890 2050 4370 523 18600 8160 9280 1220 5120 291 4440 277 2740 1070 1360 201
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TABLE III
Seasonal nitrogen discharge fluxes (g N ha?1 season) from Rhode River watersheds as a function of precipitation (cm/season). Flux values calculated from
regressions of nitrogen discharge versus precipitation
Precipitation Area-Weighted Watershed Watershed 109 (crops) Watershed 110 (forest) Watershed 111 (grazed)
TN NO3 TON TNH4 TN NO3 TON TNH4 TN NO3 TON TNH4 TN NO3 TON TNH4
A. Winter
?2 SD (10.22 cm) 206 82.7 169 52.0 842 523 226 53.6 40.9 3.67 29.7 6.55 55.9 29.5 16.6 7.79
?1 SD (17.41 cm) 601 277 316 70.8 1340 874 367 75.6 96.0 9.53 70.4 12.2 197 97.8 65.3 20.40
mean (24.6 cm) 996 395 464 89.7 1840 12220 509 94.4 167 17.7 123 18.3 445 213 159 38.2
+1 SD (31.79 cm) 1390 623 611 108 2350 1570 650 111 252 28.0 187 24.7 815 379 307 60.8
+2 SD (38.98 cm) 1790 896 759 127 2840 1920 792 127 349 40.3 260 31.4 1320 600 519 87.9
B. Spring
?2 SD (10.94 cm) 192 54.1 109 20.2 337 288 47.3 28.4 21.5 1.33 16.3 4.60 123 48.2 45.9 18.3
?1 SD (19.47 cm) 572 174 325 49.3 1060 734 220 78.0 135 10.9 106 19.2 250 100 100 28.8
mean (28.0 cm) 1140 365 647 86.6 2180 1320 581 147 430 40.7 342 47.3 507 209 220 45.3
+1 SD (36.53 cm) 1880 626 1070 131 3710 2030 1180 234 1000 107 963 91.6 1030 435 480 71.3
+2 SD (45.06 cm) 2790 958 1590 181 5630 2860 2070 338 1960 230 1600 154 2090 906 1050 112
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TABLE III
Continued.
Precipitation Area-Weighted Watershed Watershed 109 (crops) Watershed 110 (forest) Watershed 111 (grazed)
TN NO3 TON TNH4 TN NO3 TON TNH4 TN NO3 TON TNH4 TN NO3 TON TNH4
C. Summer
?2 SD (8.40 cm) 66.1 13.2 39.3 10.6 66.6 34.2 13.6 8.93 7.09 0.498 7.64 1.54 96.7 23.6 25.1 12.7
?1 SD (19.9 cm) 206 39.5 132 26.8 245 104 71.6 28.2 43.3 1.76 43.1 6.38 139 28.8 55.8 20.6
mean (31.4 cm) 644 118 444 67.8 902 314 378 88.9 265 6.23 243 26.5 182 33.9 85.1 28.4
+1 SD (42.9 cm) 2010 352 1490 171 3320 951 1990 280 1620 22.0 1370 110 225 39.0 114 36.2
+2 SD (54.4 cm) 6270 1050 5020 432 12200 2880 10500 884 9900 77.9 7750 458 267 44.1 141 44.0
D. Fall
?2 SD (6.64 cm) 1.29 0.13 0.92 0.22 0.15 0.030 0.062 0.016 0.052 0.023 0.030 0.016 77.9 5.62 52.7 4.77
?1 SD (15.57 cm) 24.4 3.80 16.4 3.07 8.52 2.47 3.65 0.882 2.54 0.298 1.79 0.366 84.7 6.88 58.7 9.85
mean (24.5 cm) 117 23.2 75.9 12.3 73.7 25.9 32.0 6.11 19.9 1.17 15.7 1.95 91.5 7.66 64.7 11.7
+1 SD (33.43 cm) 341 80.1 217 32.0 324 130 142 23.0 82.0 2.98 69.5 6.14 98.3 8.25 70.7 13.6
+2 SD (42.36 cm) 771 206 483 66.3 999 441 441 63.3 241 6.07 216 14.7 105 8.73 76.7 15.5
564 D. L. CORRELL ET AL.
TABLE IV
Regressions of nitrogen fluxes (g N ha?1 time period) from Rhode River watersheds with
precipitation volumes (X = cm time?1 period)
A. Annual
1. Area-weighted mean watershed 2. Watershed 109 (crops)
TN = 0.0109X2:66 R2 = 0.59 TN = 0.000455X3:49 R2 = 0.61
NO3 = 0.0108X2:42 R2 = 0.36 NO3 = 0.00301X2:95 R2 = 0.42
TON = 0.0241X2:87 R2 = 0.58 TON = 0.00000129X4:52 R2 = 0.64
TNH4 = 0.0131X2:11 R2 = 0.44 TNH4 = 0.000182X3:13 R2 = 0.61
3. Watershed 110 (forest) 4. Watershed 111 (grazed)
TN = 0.000000166X4:81 R2 = 0.66 TN = 0.00176X2:84 R2 = 0.32
NO3 = 0.00000193X3:75 R2 = 0.40 NO3 = 0.000650X2:85 R2 = 0.21
TON = 0.0000000396X5:07 R2 = 0.69 TON = 0.000528X2:94 R2 = 0.32
TNH4 = 0.000125X2:91 R2 = 0.31 TNH4 = 0.0686X1:59 R2 = 0.20
B. Winter
1. Area-weighted mean watershed 2. Watershed 109 (crops)
TN = 54.9X ? 355 R2 = 0.65 TN = 69.7X +130 R2 = 0.48
NO3 = 1.32X1:78 R2 = 0.60 NO3 = 48.7X +25.7 R2 = 0.40
TON = 20.5X ? 40.5 R2 = 0.48 TON = 19.7X + 24.2 R2 = 0.36
TNH4 = 2.62X + 25.2 R2 = 0.24 TNH4 = 12.0X0:644 R2 = 0.11
3. Watershed 110 (forest) 4. Watershed 111 (grazed)
TN = 0.993X1:60 R2 = 0.30 TN = 0.232X2:36 R2 = 0.56
NO3 = 0.0573X1:79 R2 = 0.28 NO3 = 0.158X2:25 R2 = 0.40
TON = 0.688X1:62 R2 = 0.31 TON = 0.0423X2:57 R2 = 0.67
TNH4 = 0.432X1:17 R2 = 0.14 TNH4 = 0.116X1:81 R2 = 0.45
C. Spring season
1. Area-weighted mean watershed 2. Watershed 109 (crops)
TN = 2.09X1:89 R2 = 0.81 TN = 2.88X1:99 R2 = 0.67
NO3 = 0.421X2:03 R2 = 0.60 NO3 = 5.98X1:62 R2 = 0.46
TON = 1.19X1:89 R2 = 0.86 TON = 0.0795X2:57 R2 = 0.73
TNH4 = 0.495X1:55 R2 = 0.62 TNH4 = 0.432X1:75 R2 = 0.68
3. Watershed 110 (forest) 4. Watershed 111 (grazed)
TN = 0.0104X3:19 R2 = 0.79 TN = 49.5e0:0718X R2 = 0.49
NO3 = 0.000220X3:64 R2 = 0.78 NO3 = 18.8e0:0743X R2 = 0.39
TON = 0.00701X3:24 R2 = 0.79 TON = 16.8e0:0793X R2 = 0.54
TNH4 = 0.0122X2:48 R2 = 0.57 TNH4 = 10.2e0:0460X R2 = 0.28
WEATHER EFFECTS ON NITROGEN DISCHARGES 565
TABLE IV
Continued.
D. Summer season
1. Area-weighted mean watershed 2. Watershed 109 (crops)
TN = 28.8e0:0855X R2 = 0.43 TN = 25.7e0:0979X R2 = 0.17
NO3 = 0.421X2:03 R2 = 0.60 NO3 = 5.98X1:62 R2 = 0.46
TON = 16.2e0:0911X R2 = 0.43 TON = 4.02e0:125X R2 = 0.17
TNH4 = 5.40e0:0696X R2 = 0.40 TNH4 = 3.86e0:0863X R2 = 0.15
3. Watershed 110 (crops) 4. Watershed 111 (grazed)
TN = 1.89e0:136X R2 = 0.42 TN = 3.71X + 65.5 R2 = 0.10
NO3 = 0.198e0:0949X R2 = 0.19 NO3 = 08445X + 19.9 R2 = 0.026
TON = 2.16e0:130X R2 = 0.31 TON = 3.51X0:925 R2 = 0.11
TNH4 = 0.543e0:107X R2 = 0.41 TNH4 = 0.679X + 7.04 R2 = 0.26
E. Fall season
1. Area-weighted mean watershed 2. Watershed 109 (crops)
TN = 0.00188X3:45 R2 = 0.60 TN = 0.0000180X4:76 R2 = 0.65
NO3 = 0.0000664X3:99 R2 = 0.64 NO3 = 0.00000160X5:19 R2 = 0.62
TON = 0.00153X3:38 R2 = 0.55 TON = 0.00000710X4:79 R2 = 0.66
TNH4 = 0.000671X3:07 R2 = 0.48 TNH4 = 0.00000715X4:27 R2 = 0.52
3. Watershed 110 (forest) 4. Watershed 111 (grazed)
TN = 0.00000953X4:55 R2 = 0.47 TN = 0.764X + 72.8 R2 = 0.0050
NO3 = 0.0000769X3:01 R2 = 0.29 NO3 = 3.58X0:238 R2 = 0.0034
TON = 0.00000348X4:79 R2 = 0.48 TON = 0.670X + 48.3 R2 = 0.0072
TNH4 = 0.0000146X3:69 R2 = 0.32 TNH4 = 0.212X + 6.55 R2 = 0.017
3.2. NITROGEN CONCENTRATIONS
3.2.1. Total Nitrogen
Mean annual total-N concentration in Rhode River discharges correlated positively
with the volume discharged (R2 = 0.30, P = 0.01) and with the volume of pre-
cipitation (R2 = 0.17, P = 0.06). When examined on a seasonal basis this positive
correlation of total-N concentration with watershed discharge was strongest in the
spring and winter (P < 0.01), but was essentially absent in the summer and fall.
The correlations of seasonal total-N concentrations with volume of precipitation
were similar but somewhat lower. Total-N concentrations discharged generally
had very low correlations with mean seasonal air temperature and multiple cor-
relations with precipitation volume and air temperature were only slightly higher
than with precipitation alone. For example, Rhode River watershed spring total-
566 D. L. CORRELL ET AL.
N concentrations had correlations with precipitation alone and with precipitation
and air temperature of R = 0.736 and 0.750, resp. Multiple correlation between
weekly total-N concentrations discharged from the overall Rhode River watershed
and mean weekly precipitation and air temperature were fairly low. For example, a
statistical model based on multiple correlations, [TN conc. (g N L?1) = 272 + 19.4
(mean air temp over the prior 4 weeks) + 17.6 (Prcp0) + 8.8 (Prcp?1) +4.4 (Prcp?2)
+ 2.2 (Prcp
?3)], had a multiple correlation of 0.280. The subscripts indicate the
number of weeks that the precipitation data were lagged. The air temperature
variables accounted for most of the overall correlation.
3.2.2. Nitrate
Mean annual nitrate concentration in discharges from Rhode River watershed in-
creased with volume of discharge (R = 0.24, P = 0.28) and precipitation (R = 0.38,
P = 0.008). The correlation of nitrate concentration from Rhode River watershed
with volume of precipitation was highest in the winter and spring (P < 0.001) and
almost zero in the summer and fall (Figure 8). The correlation of nitrate concen-
tration with volume of discharge was highest for the forested watershed in the
spring (R2 = 0.50, P = 0.002). Only in the fall for the grazed watershed was the
correlation between nitrate concentration and air temperature notable (R = 0.416
vs ?0.110 for precipitation; multiple R = 0.419). Correlations of weekly time series
of nitrate concentrations discharged from the overall Rhode River watershed with
precipitation were low and positive, while those with air temperature were higher
and negative. A statistical model, [Nitrate conc. (g N L?1) = 185 ? 3.88 (mean of
prior 4 weeks of air temperature) + 4.66 (Prcp0) + 2.33 (Prcp?1) + 1.17 (Prcp?2)
+ 0.58 (Prcp
?3)], where subscripts are weeks of lag, had a multiple correlation of
0.333.
3.2.3. Total Organic Nitrogen
Mean annual organic-N concentration in Rhode River discharges was positively
correlated with watershed discharge (R = 0.449, P = 0.04). On a seasonal basis it is
clear that the highest correlation with watershed discharge was in the spring (R =
0.496, P = 0.02). When one examines the three first-order watersheds, which have
the most divergent land use compositions, in the spring organic-N concentrations
increased more rapidly with volume of precipitation and had higher correlations
with volume of precipitation in the row-cropped watershed (# 109; R2 = 0.73, P =
0.00001) than in the grazed or forested watersheds. Spring Rhode River watershed
linear correlations of organic-N concentration with precipitation (R = 0.619) were
improved somewhat by adding air temperature (multiple R = 0.683).
3.2.4. Total Ammonium
Total ammonium concentration in discharges from Rhode River watershed was
usually negatively correlated with volume of discharge and this relationship was
most evident in the winter (slope of linear regression = ?0.38, R2 = 0.19, P = 0.03).
W
EATH
ER
EFFECTS
O
N
N
ITRO
G
EN
D
ISCH
A
RG
ES
567
Figure 8. Variations in concentration of nitrate discharged from area-weighted Rhode River watersheds with volume of precipitation.
568 D. L. CORRELL ET AL.
In the summer and fall, only the grazed watershed ammonium concentrations were
correlated with discharge volume (R = ?0.61, P = 0.007; and R = ?0.48, P = 0.04;
resp.). Spring Rhode River ammonium concentration correlations with precipita-
tion (R = 0.316) were increased by including air temperature (multiple R = 0.377).
The correlations between summer air temperature and ammonium concentrations
were higher than with precipitation for the Rhode River watershed (R = 0.399 vs
?0.150), cropped watershed (# 109; R = 0.351 vs 0.024), and grazed watershed (#
111; R = ?0.366 vs ?0.314).
4. Discussion
4.1. ANNUAL FLUXES
Annual discharge fluxes of total-N, nitrate, total organic-N, and total ammonium
clearly were strongly influenced by the volume of precipitation as well as by land
use (Figure 1?7). Annual total-N fluxes from the overall Rhode River watershed
varied by almost an order of magnitude, but the combination of variation in pre-
cipitation and land use gave a range of 0.07 to 24.3 kg N ha?1 yr. (350-fold
range).
Annual precipitation varied almost two-fold during the period of study, exceed-
ing one standard deviation below the mean and almost two standard deviations
above the mean for the long-term records. The fact that the range of observed low
annual precipitation does not extend to minus two SD below the mean needs to be
kept in mind. The predictions of N-fluxes from regressions for years with very low
precipitation are extrapolated beyond the measured discharge data (Table II, Figure
3).
There were distinctly different patterns of variation in N-flux with precipita-
tion for different land uses. Total-N annual flux from forested watershed 110 was
highly correlated with precipitation and increased very rapidly, almost entirely
due to organic-N (Figure 3C). The forested watershed was completely vegetated
with deciduous hardwood forest, mostly old-growth, and had very little human
disturbance in the last 50 yr other than atmospheric deposition (Vaithiyanathan
and Correll, 1992). Thus, forested watershed # 110 provides a good measure of
how effectively natural vegetation in this region can retain N over a broad range
of weather conditions. The fairly high fluxes of organic-N in years of high pre-
cipitation probably reflect the steep slopes and highly erodible soils characteristic
of these inner Coastal Plain landscapes. Nitrogen discharges from this forested
watershed, on average, were similar to other forested watersheds (Table V).
Total-N annual flux from grazed land (watershed 111) was much less correlated
and increased less rapidly with precipitation, and was composed of roughly equal
amounts of nitrate and organic-N (Figure 3D). Nitrate flux from this watershed also
had some temperature dependence. Were these characteristics typical of grazed
W
EATH
ER
EFFECTS
O
N
N
ITRO
G
EN
D
ISCH
A
RG
ES
569
TABLE V
Comparison of Rhode River watershed nitrogen discharge fluxes with other temperature watersheds. All fluxes are in kg N ha?1 yr.
Watershed and Location Yr Total-N Nitrate-N Ammonium-N Organic-N References
A. Forested
Puruwai, New Zealand 2 3.7 2.8 0.06 0.77 Cooper and Thomsen (1988)
(native podacarp/mixed hardwoods)
Coastal Plain of MD and 1 1.3 0.06 0.23 1.0 Jordan et al. (1997a)
DEL, U.S.A. #s 301, 307 (native hardwood/pine)
Piedmont of MD, U.S.A. 1 4.8 3.5 0.20 1.1 Jordan et al. (1997b)
# 401 (native hardwoods)
Rhode River, MD, U.S.A. 19 1.8 0.14 0.16 1.5 this paper
# 110 (old growth deciduous hardwoods)
B. Cropland
Coastal Plain of GA, 3 4.2 1.5 0.11 2.6 Lowrance and Leonard (1988)
U.S.A., #s F, J. K, I, M (34.3% crops)
Coastal Plain of MD, 1 11.2 8.0 0.69 2.6 Jordan et al. (1997a)
DEL, U.S.A., #s 304, 305, 306, 308, 309, 310 (57.8% crops)
Piedmont of MD, U.S.A., 1 16.2 14.1 0.30 1.8 Jordan et al. (1997b)
#s 403, 404, 405, 408, 409, 410 (62.4% crops)
Rhode River, MD. U.S.A. 19 7.3 3.8 0.51 3.0 this paper
# 109 (64% crops)
570
D
.L.CO
R
R
ELL
ET
A
L.
TABLE VI
Continued.
Watershed and Location Yr Total-N Nitrate-N Ammonium-N Organic-N References
C. pasture (no mineral N fertilizer)
Purutaka, New Zealand 1 12.0 1.2 0.48 10.3 Cooper and Thomsen (1988)
(continuously grazed)
Oklahoma, U.S.A. # R-6, 2 1.9 0.32 0.20 1.4 Olness et al. (1975, 1980)
(rotationally grazed)
Oklahoma, U.S.A., # R-8 2 7.4 1.1 0.30 6.0 Olness et al. (1975, 1980)
(continuously grazed)
Rhode River, MD, U.S.A. 16 1.6 0.67 0.14 0.74 this paper
#111 (rotationally grazed)
WEATHER EFFECTS ON NITROGEN DISCHARGES 571
lands or merely idiosyncrasies of this one first order watershed? The fact that most
of this watershed was pasture combined with its southern aspect may make it more
sensitive to solar insolation effects such as warming and drying of surface soils
and this may not be typical of grazed lands in this region. Watershed 110 also
had a southern aspect, but the forest vegetation may have made it less sensitive to
solar insolation effects. Watershed 111 was subjected to low intensity rotational
grazing, rather than the high intensity management sometimes practiced in the
humid eastern United States, in which mineral fertilizers and feed supplements are
imported from outside the watershed in order to sustain high livestock populations.
Nitrogen parameter fluxes from watershed 111 were in the expected range for such
grazing lands (Table V; Correll, 1996).
The cropland watershed (Figure 3B; Table II), in exceptionally wet (but ob-
served) years, exported 28 times more nitrate per ha than forest and 7.6 times
more nitrate than grazed land. Thirty six percent of this 16 ha watershed was veg-
etated with deciduous hardwood riparian forest, while the rest was in continuous
corn production. The riparian forest formed a continuous buffer around the first
order stream draining the watershed and has been shown to remove most of the
nitrate entering it from the fields in overland storm flows and groundwater during
all seasons (Peterjohn and Correll, 1984; Correll and Weller, 1989). It has also
been shown to remove significant amounts of ammonium and particulate organic-N
from overland storm flows (Peterjohn and Correll, 1984). Fluxes of N-parameters,
especially nitrate, from this watershed would probably have been much higher in
the absence of this buffer. In 1991, a year of just slightly below average precipita-
tion, six other Maryland Coastal Plain and six Maryland Piedmont watersheds with
similar overall land use composition had average total-N fluxes of 11.2 and 16.2 kg
N ha?1 yr, respectively (Table V; Jordan et al., 1997a, b). That yr the Rhode River
cropland watershed had a flux of 5.53 kg N ha?1 yr and our statistical model (Table
II) predicts a flux of 5.68 kg N ha?1 yr for a yr with average precipitation. Thus,
this cropland watershed had lower annual fluxes of total-N than many others in
the region, perhaps because of its well buffered riparian zone. One might conclude
from these data that the cropland watershed was a highly disturbed ecosystem, un-
able to efficiently retain N. The low-intensity grazed watershed was less disturbed,
but still behaved quite differently from mature native forest.
4.2. SEASONAL FLUXES
In the winter season of the study years, precipitation extended from almost 2 SD
below the mean to well over 2 SD above the mean of long-term precipitation.
Winter fluxes of N-parameters increased with volume of precipitation, but the
relationship was usually fairly linear. The regression slope for the overall Rhode
River watershed in winter was lowest for total ammonium and highest for nitrate
(Figure 4). In extremely wet winters cropland discharged 40 and 3.2 times more
nitrate per ha than forest and grazed lands, respectively (Table III). In the spring
572 D. L. CORRELL ET AL.
season, precipitation volumes during the period of study ranged from nearly 2 SD
below the mean to over 2 SD above the mean. Nitrogen parameter fluxes were
highly correlated with volume of precipitation and increased substantially (Figure
5). Power functions explained more of the variance than linear functions. In ex-
tremely wet springs nitrate fluxes from cropland were 12 and 14 times higher than
from forest and grazed lands, respectively (Table IIIB). Therefore, even a modest
proportion of cropland in this landscape can dominate the release of nitrate in the
winter and spring.
Our statistical model predicts the highest seasonal fluxes of all N-parameters
from the overall watershed and the cropland watershed in exceptionally wet sum-
mers, but for forest the nitrate flux in the summer is lower than in the spring and for
grazed land all fluxes are lower in the summer than in the winter or spring (Table
III, Figure 6). Summer precipitation during the study ranged from almost 2 SD
below the mean to almost 2 SD above the mean. The high N-fluxes in very wet
years may reflect the importance of tropical storms in the summers and the high
erosion rates from croplands and forests associated with intense summer storms.
In a summer of average precipitation all N-fluxes from all watersheds were lower
than in the spring. Fall N-fluxes were uniformly very low even in exceptionally
wet years, but did increase with volume of precipitation (Figure 7). No fall seasons
observed were more than 1 SD below the mean and only one fall was more than 2
SD above the mean. Thus predictions for exceptionally dry years (i.e. > 1SD below
the mean) in Table IIID should be viewed with caution.
The regressions we derived from the relationships of N-fluxes with precipitation
and air temperature do not explain all of the variance in the data. The fact that
many of the regressions of N-flux versus precipitation are highly significant reflects
the large number of data points and strongly indicates that there is a relationship.
However, this should not be misinterpreted. There are other independent variables
which could cause N-fluxes to be increased or decreased. A few examples should
help to illustrate this point. On cropland watershed 109, if the crop fertilizer is
applied just prior to a large storm, more N-discharge than expected will occur that
spring. If the precipitation in a given spring is composed of a few, very large storms
instead of many small storms, more N-discharge than expected will occur from
all of the watersheds that spring. There are many such possible explanations of
residual variances in N-discharges. However, our statistical models can be used
to predict annual and seasonal N-fluxes from Rhode River watersheds and other
similar watersheds in this region with a fair degree of reliability.
4.3. NITROGEN CONCENTRATIONS
The observed changes in N-fluxes with precipitation were not solely due to changes
in water discharge. Sometimes N-concentrations were significantly correlated with
volume of precipitation. For example, the mean concentration of nitrate discharged
in the winter from the Rhode River watershed ranged from 199 to 720 g N L?1
WEATHER EFFECTS ON NITROGEN DISCHARGES 573
and volume of precipitation explained 61% of this variation (Figure 8). Since the
volume of water discharged from a watershed is obviously somewhat correlated
with volume of precipitation and since nutrient fluxes are the product of the volume
and composition of water discharges, one would expect lower correlations between
N-concentrations and precipitation volumes than between N-fluxes and precipita-
tion volumes. That is what we found. However, water discharge is not as highly
correlated with precipitation volume as one might suppose. For example, Rhode
River water discharge and precipitation volume had coefficients of determination
of 0.68, 0.51, 0.76, 0.56, and 0.53 for annual, winter, spring, summer, and fall, re-
spectively. Correlations between N-concentrations and volume of water discharged
were sometimes more interesting than for precipitation. Thus, for example, mean
annual concentration of total-N discharged from the overall watershed correlated
highly with water discharge in the spring, moderately in the winter and not at all
in the summer and fall. Observed mean spring concentrations of total-N from the
overall watershed increased with higher precipitation from 728 to 2290 g N L?1,
over three-fold. The relationship of nitrate concentrations to seasonal and annual
precipitation volume is different from what is found on short-term or storm event
temporal scales. During individual storm events nitrate concentration often reaches
a minimum at peak stream discharge (e.g. Correll et al., 1987; McDiffet et al.,
1989). This minimum is due, in part, to the lower concentration of nitrate in over-
land flows than in groundwater. We believe that the higher nitrate concentrations
found in high rainfall seasons or years is due to more efficient leaching of nitrate
from surface soils by more frequent precipitation events and the more efficient
transport of groundwater nitrate to the stream, especially in the riparian buffer zone
(Correll and Weller, 1989).
Only in the spring were organic-N concentrations discharged from the overall
watershed correlated with precipitation. For the cropland and forested watersheds
precipitation explained 73 and 54%, respectively, of the variation in spring organic-
N concentrations. For the cropland, observed mean spring organic-N concentra-
tions increased from 314 to 2388 g N L?1 (7.6 fold). Similarly observed mean
spring organic-N concentrations from forest increased from 155 to 1095 g N L?1
(7.1 fold).
Total ammonium concentrations were unique in that they often declined with
precipitation and water discharge. Total ammonium concentrations from the overll
watershed increased with precipitation in the spring, but decreased in the winter
and summer, while in the fall there was very little correlation. Ammonium con-
centrations often had more correlation with interannual variations in mean summer
temperatures than with precipitation or water discharge. The correlation with tem-
perature was positive for the overall watershed and cropped watershed, but negative
for the grazed watershed. One would like to think that these results were brought
about by unusually warm summer temperatures causing higher rates of organic-N
mineralization in the soils, but why not in the grazed land?
574 D. L. CORRELL ET AL.
Acknowledgments
This research was supported by a series of grants from the Smithsonian Environ-
mental Research Program, the National Science Foundation, the Environmental
Protection Agency, and the National Oceanic and Atmospheric Agency?s Coastal
Oceanic Program.
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