Journal of Environmental Quality
TECHNICAL REPORTS
SURFACE WATER QUALITY
Antecedent Moisture Controls on Stream Nitrate Flux
in an Agricultural Watershed
Caroline A. Davis,* Adam S. Ward, Amy J. Burgin, Terrance D. Loecke, Diego A. Riveros-Iregui,
Douglas J. Schnoebelen, Craig L. Just, Steven A. Thomas, Larry J. Weber, and Martin A. St. Clair
N
itrate-nitrogen (N) loading in the Upper
Mississippi River Basin is largely responsible for the
annual Gulf of Mexico hypoxia zone (Turner and
Rabalais, 1994; Rabalais et al., 1996; Goolsby and Battaglin,
2001). The midwestern United States is the primary source
of agricultural-derived nitrate-N transported by the Upper
Mississippi River (David and Gentry, 2000; Alexander et al.,
2008) due to extensive agricultural activity. Dramatic increases in
nitrate-N loading over the last half century are widely attributed
to increased application rates of nitrogenous fertilizer (Kanwar
et al., 1985; Burkart and James, 1999; Tilman et al., 2001) and
extensive installation of artificial drainage systems (Zucker and
Brown, 1998; Schilling and Helmers, 2008). Although the
regional source area for the excess nitrate-N transported by the
Upper Mississippi River is well established, the factors controlling nitrate-N loading to surface waters draining agricultural
landscapes are complex, interdependent, spatiotemporally variable, and poorly documented.
The fate of nitrate-N in a watershed is largely determined
by the physicochemical interaction of the intrinsic watershed
properties (e.g., geology, hydrology, biology) and by
anthropogenic (e.g., land cover, fertilizer application) and
hydrometeorologic (e.g., climate, precipitation) variables
(Randall and Mulla, 2001). The ability to quantify the relative
contribution of these variables in relation to nitrate-N dynamics
is a necessary step toward predicting watershed-specific nitrate-N
export and thus to targeting conservation and land management
activities (Dinnes et al., 2002; Gassman et al., 2010). In the
absence of excessive changes to the intrinsic and anthropogenic
characteristics of a watershed, as observed over the past half
century and on substantially shorter timescales (e.g., <10 yr),
the effects of dynamic hydrological processes may dominate
nitrate-N mobilization and export (Donner and Scavia, 2007).
Of continued and growing interest is the flushing of agricultural
Abstract
Evaluating nitrate-N fluxes from agricultural landscapes is
inherently complex due to the wide range of intrinsic and
dynamic controlling variables. In this study, we investigate the
influence of contrasting antecedent moisture conditions on
nitrate-N flux magnitude and dynamics in a single agricultural
watershed on intra-annual and rainfall-event temporal scales.
High temporal resolution discharge and nitrate concentration
data were collected to evaluate nitrate-N flux magnitude
associated with wet (2009) and dry (2012) conditions. Analysis
of individual rainfall events revealed a marked and consistent
difference in nitrate-N flux response attributed to wet/dry
cycles. Large-magnitude dilutions (up to 10 mg N L 1) persisted
during the wet antecedent conditions (2009), consistent with a
dominant baseflow contribution and excess groundwater release
in relation to precipitation volume (discharge > > precipitation).
Smaller-magnitude concentrations (<7 mg N L 1) were observed
during the drought conditions of 2012, consistent with a
quickflow-dominated response to rain events and infiltration/
storage of precipitation resulting in discharge < precipitation.
Nitrate-N loads and yields from the watershed were much higher
(up to an order of magnitude) in the wet year vs. the dry year.
Our results suggest that the response of nitrate-N loading to
rain events is highly dependent on intra-annual antecedent
moisture conditions and subsurface hydrologic connectivity,
which together dictate the dominant hydrologic pathways for
stream recharge. Additionally, the results of our study indicate
that continued pronounced wet/dry cycles may become more
dominant as the short-term driver of future nitrate-N exports.
C.A. Davis, D.J. Schnoebelen, and L.J. Weber, Lucille A. Carver Mississippi Riverside
Environmental Research Station, IIHR-Hydroscience & Engineering, Univ. of
Iowa, 3388 Highway 22, Muscatine, IA 52761; A.S. Ward, School of Public and
Environmental Affairs, Indiana Univ., 1315 Tenth St., Bloomington, IN 47405; A.S.
Ward, Dep. of Earth & Environmental Sciences, Univ. of Iowa, 121 Trowbridge Hall,
Iowa City, IA 52242; A.J. Burgin, T.D. Loecke, and S.A. Thomas, School of Natural
Resources, Univ. of Nebraska–Lincoln, 412 Hardin Hall, Lincoln, NE 68583; D.A.
Riveros-Iregui, Dep. of Geography, Univ. of North Carolina, 327 Saunders Hall,
Chapel Hill, NC 27599; C.L. Just, Dep. of Civil & Environmental Engineering, Univ. of
Iowa, 4111 Seamans Center, Iowa City, IA 52242; M.A. St. Clair, Dep. of Chemistry,
Coe College, 1220 1st Ave. NE, Cedar Rapids, IA 52402. Assigned to Associate Editor
Patrick Inglett.
Copyright © American Society of Agronomy, Crop Science Society of America,
and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA.
All rights reserved. No part of this periodical may be reproduced or transmitted
in any form or by any means, electronic or mechanical, including photocopying,
recording, or any information storage and retrieval system, without permission in
writing from the publisher.
J. Environ. Qual.
doi:10.2134/jeq2013.11.0438
Received 3 Nov. 2013
*Corresponding author ([email protected]).
1
nitrate-N in response to rainfall events ( Jiang et al., 2010) and
their relation to the annual variability in hypoxic extent in the
Gulf of Mexico (Donner and Scavia, 2007).
Nitrate-N flux and loads in surface waters are determined
by stream discharge and mobilized nitrate dynamics as well
as nitrate-N storage on the landscape. The relationship
between stream nitrate-N concentration and discharge is not
straightforward, however, displaying both positive and inverse
relationships between different stream segments spatially and
temporally in terms of seasonality (Arheimer et al., 1996; Bohlke
et al., 2007) and scale of investigation (Turgeon and Courchesne,
2008). Studies in agricultural watersheds have documented
increasing nitrate-N concentration with increasing stream
discharge on a seasonal timescale (Tomer et al., 2003; Tiemeyer
et al., 2006), although the magnitude of this relationship is
highly variable. On the timescale of individual precipitation
events (hours to days), a positive relationship between discharge
and nitrate-N is often referred to as a “concentration” pattern,
whereas the inverse is referred to as a “dilution” pattern (Poor and
McDonnell, 2007). Hydrologic connectivity, defined here as the
degree of spatial linkage between surface–subsurface water flow,
and the dominant hydrologic flow pathways for water recharging
a stream segment may be responsible for differences observed
in the concentration or dilution signal (Green et al., 2007;
Tesoriero et al., 2009). A dilution signal may also be observed
in a nitrate-N source-limited system, whereas a concentration
signal may indicate an “infinite” nitrate-N source (Poor and
McDonnell, 2007; Murphy et al., 2013).
Rainfall-driven variations in stream nitrate-N concentrations
and discharge reflect the degree of hydrologic connectivity of a
watershed (Biron et al., 1999; Mitchell et al., 2006). Pre-event
conditions influence the connectivity of subsurface hydrologic
flow pathways contributing water to streams as well as the
distribution and availability of nutrients for leaching (Soulsby,
1995; Biron et al., 1999; Stieglitz et al., 2003; Freeman et al.,
2007). Under wet antecedent conditions, catchments are in
a spatially connected or active state where pre-event water
contributions may dominate (Grayson et al., 1997; Ali and
Roy, 2010). Under dry antecedent conditions, the catchment
is in a disconnected or disorganized state that promotes new
water infiltration (Grayson et al., 1997; Ali and Roy, 2010). The
quantitative measure of hydrologic connectivity and antecedent
moisture conditions is complicated, requiring the use of
surrogates or proxies (see review by Ali and Roy [2010]). Further,
the magnitude and dynamics of nitrate-N flux associated with
differing antecedent moisture conditions over variable timescales
and spatial scales is not well understood, limiting our ability to
predict nitrate-N flux and load response to rainfall events.
We investigated the influence of contrasting antecedent
moisture conditions on nitrate-N flux on intra-annual (weeks
to months) and rainfall event (hours to days) temporal scales
in a single agricultural watershed. To evaluate nitrate-N flux
magnitude associated with intra-annual wet/dry cycles, we
analyzed the nitrate-N flux–discharge relationship for the wet
conditions of 2009 (the 11th wettest year in 141 yr of record)
(Iowa Department of Agriculture and Land Stewardship, 2010)
and drought conditions observed during 2012 (the 19th driest
year in 141 yr of record) (Iowa Department of Agriculture
and Land Stewardship, 2013). To quantify nitrate-N exports
associated with individual rainfall events within each wet/dry
cycle, we calculated nitrate-N loads and yields in relationship
to event discharge and precipitation volume. We discuss the
dominant hydrologic processes driving nitrate-N export under
contrasting antecedent conditions and how those may relate
to short-term climate extremes or pronounced wet/dry cycles,
which may affect future nitrate-N mobilization and export.
Materials and Methods
Study Site Description
The study was conducted in the Clear Creek watershed
(HUC-10: 0708020904) within the Lower Iowa River
watershed, located in eastern Iowa (Fig. 1a). Clear Creek drains
approximately 267 km2, discharging into the Iowa River near
Coralville, IA. Land use in the watershed is predominantly
agricultural (Table 1), with growing urbanization near the outlet
of the watershed. The geologic setting is in the Southern Iowa
Drift Plain (Prior, 1991), dominated by glacial deposits atop
Devonian age bedrock and characterized by rolling hills and
uplands dissected by tributary streams (Schwob, 1964). Soils in
the watershed are dominantly fine-grained, organic-rich, siltyclay loam to silty loam of the Tama-Downs, Fayette-Downs,
and Colo-Nevin-Nodaway (Dideriksen et al., 2007). Intensive
agricultural activities combined with highly erodible soil has
led to significant rates of erosion in the watershed (Abaci and
Papanicolaou, 2009; Loperfido et al., 2010). The climate in the
study area is characterized by marked seasonal variations, with
warm summer, cold winter, and wet spring seasons, reflective of
Iowa’s mid-latitude and interior continental location (Highland
and Dideriksen, 1967). Mean annual temperature is about 9°C,
whereas mean annual precipitation is 889 mm. Nearly 75% of
the bulk annual precipitation occurs during the growing season
(Apr.–Sept.; National Oceanic and Atmospheric Administration
[2013a]).
In Iowa, the recommended rate of fertilizer application is
105 to 195 lb N acre 1 (11,768–21,856 kg km 2), depending
on the soybean–corn rotation schedule (Iowa State University
For proofing purposes only
© ASA, CSSA, SSSA
2
Fig. 1. Maps showing (a) location of Clear Creek watershed (dark gray
shaded) within the Lower Iowa River watershed in eastern Iowa and
(b) location of nutrient monitoring and rain gauge sites. Discharge (Q)
was measured at two USGS stream gaging stations located at Oxford
(MS-OXF) (USGS 05454220) and Coralville (DS-COR) (USGS 05454300).
US-HST, Homestead; US-SAC, South Amana.
Journal of Environmental Quality
Extension, 2007). Anderson and Downing (2006) measured
atmospheric nutrient deposition at several sites across the state of
Iowa in 2003 and estimated the annual atmospheric deposition
of nitrate-N as 7.7 kg ha 1 yr 1 (770 kg km 2 yr 1).
were obtained from the National Oceanic and Atmospheric
Administration’s observed precipitation coverage (4 km × 4 km
resolution; National Oceanic and Atmospheric Administration,
2013b) and are presented as daily and hourly precipitation
estimates for the US-HST, MS-OXF, and DS-COR 2012
monitoring sites.
Monitoring Methods
Two monitoring campaigns were conducted in the Clear
Creek watershed in 2009 (Putney, 2010) and 2012. Both
campaigns used in situ continuous monitoring nitrate sensors at
three locations within the watershed (Fig. 1b). The 2009 and 2012
monitoring studies differ in the location of the monitoring for
the uppermost site in the watershed; the 2009 study monitored
at the South Amana site, whereas the 2012 study monitored at
the Homestead site (Fig. 1b). Site-specific characteristics of the
upstream South Amana (US-SAC) and Homestead (US-HST),
middle Oxford (MS-OXF), and downstream Coralville (DSCOR) sites are shown in Table 1. Stream nitrate concentration
(C) was measured as nitrate + nitrite as N (mg N L 1) using a
Nitratax sc plus (5 mm; Hach Co.) with a measurement range
of 0.1 to 25 mg N L 1 and 0.1 mg N L 1 resolution. Data were
collected using a CR1000 datalogger (Campbell Scientific
Inc.) logging continuously at 15-min intervals, except for the
2009 South Amana site, which measured at 1-h intervals.
Gaps in the nitrate dataset reflect brief power outages or other
operational issues with the equipment. Each sensor was serviced
and calibrated before field deployment by the Hach Service
Department as per the manufacturer’s specifications.
Discharge (Q) was measured at two U.S. Geological Survey
(USGS) stream gaging stations located at MS-OXF (USGS
05454220) and DS-COR (USGS 05454300), and 15-min Q
data were obtained from the USGS National Water Information
System. Annual mean Q as measured at the USGS stream gauge
near the outlet of Clear Creek for 1980–2012 was 2.35 ±
1.78 m3 s 1 (USGS 05454300). Q at the ungaged locations was
estimated by using a variation of the “Flow Anywhere” method
developed by Linhart et al. (2012), where the drainage-arearatio method is modified to regionalize equations for Iowa. The
equation developed for aggregated region 3 was used to estimate
Q at the US-SAC and US-HST and the USGS Oxford location
as the reference gauge. Daily and hourly precipitation data were
obtained from rain gages operated by the Iowa Flood Center
(http://ifis.iowafloodcenter.org/ifis/en/) during the 2009
monitoring campaign at the US-SAC and OXF01 locations
(Fig. 1b). The OXF01 rain gauge location was used to estimate
rainfall at the MS-OXF and DS-COR monitoring sites for the
2009 study. Precipitation data for the 2012 monitoring campaign
Antecedent Conditions and Selection of Rainfall Events
Antecedent moisture conditions for each of the sites,
preceding each of the selected rainfall events, are described here
by antecedent precipitation (APx) and Palmer Drought Index
(PDI) values. The APx values represent the total precipitation
(in mm) for the preceding x days and were calculated for 7 and
30 d before the rainfall event using the nearest rainfall estimate
location. The PDI values describe long-term meteorological
conditions and were obtained from the National Climatic Data
Center (National Climatic Data Center, 2013). The National
Climatic Data Center’s calculated weekly PDI values represent
moisture conditions ranging from extreme drought to extremely
moist.
Rainfall events were chosen using the following criteria: (i)
>10 mm rainfall measured or estimated at each of the three rain
estimate locations within the watershed, (ii) Q and C data available
for at least two of the in situ monitoring stations, and (iii) no
measurable rainfall at the monitoring stations for 24 h preceding
the start of the event window (after Inamdar et al. [2004]).
Rainfall event window timeframes were selected to encompass
the inflection point (i) of the Q hydrograph, maximum or peak
deviation (max), and final (f ) return of the hydrograph to near
that of the inflection point. In instances where the hydrograph
does not return to near that of the inflection point (e.g., if rain
occurs before the hydrograph returning to initial values), the
end of the window is considered to be the final point (Qf ) in our
analysis.
A total of 23 rainfall events measuring >10 mm at OXF01
occurred during the 2009 intra-annual window, with 13 meeting
the above criteria. In contrast, 11 events measuring >10 mm at
MS-OXF occurred during the same time frame in 2012, with
seven events meeting the criteria for analysis. Four rainfall events,
distributed within each intra-annual window, were selected for
analysis in this study and are representative of the dynamics
observed in the complete data set.
For proofing purposes only
© ASA, CSSA, SSSA
Analysis of Nitrate-N, Discharge, and Rainfall Data
Solute–discharge relationships are commonly used to
investigate watershed-scale dynamics, chemostatic behavior, and
Table 1. Characteristics of nutrient monitoring field stations.
Station
Drainage area
Elevation
Corn
Homestead (US-HST)
South Amana (US-SAC)
Oxford (MS-OXF)
Coralville (DS-COR)
km2
21.2
26.2
157.5
254.3
m
226–271
231–278
213–278
164–278
Land cover†
Row crop (2009/2012)
Soybean
Total
Other‡
————————————————— % —————————————————
35/47
38/25
73/72
27/28
54/60
29/22
82/82
18/18
37/41
26/22
63/63
37/37
30/35
23/19
54/55
46/45
† Land cover percentages determined using National Land Cover Data for 2009 and 2012.
‡ Includes the following National Land Cover Data values: 24, 27, 28, 36, 37, 58, 59, 60, 62, 63, 111, 121, 122, 123, 124, 131, 141, 142, 152, 171, 181, 190,
195, 236.
XXXBHSPOPNZPSHtXXXDSPQTPSHtXXXTPJMTPSH
3
water source areas (Godsey et al., 2009; Guan et al., 2011). To
evaluate the relationship between area-weighted N flux (FAW )
and Q between the two contrasting wet/dry years, we used a
power law relationship (FAW = aQb; after Tomer et al. [2003]),
where a is the intercept value relating to nitrate-N concentration,
and b is the slope of the function. Tomer et al. (2003) describe
the possible reasoning for differences in values for the resulting
slope as (i) b = 1 if concentration does not change, (ii) b > 1 for
high flows exhibiting increased concentration or more effective
removal at low flows, and (iii) b < 1 associated with dominant
baseflow. Tomer et al. (2003) further suggest that in different
locations within a watershed, or with different land use patterns,
high flows may result in dilutions and b < 1.
Nitrate-N load (kg) and yield (kg km 2) were calculated
for each of the selected rainfall events. We also calculated total
rainfall volume (Pv) occurring within the drainage area and total
discharge volume (Qv) at each monitoring station for each of the
selected rainfall events.
are highly variable spatially between sites for the same rainfall
event and temporally between different events for a single site.
Thus, the calculated FAW values are also highly variable (Fig.
3c) spatially and temporally. The three highest-magnitude FAW
variations are noted in late June (20 June 2009), mid-July (11 July
2009), and late August (28 Aug. 2009) and correspond to peak
variations in the measured Q and C (Fig. 3a–c). The FAW values
calculated for the US-SAC location for the three aforementioned
high-magnitude events range from 213 to 391 kg d 1 km2 1. In
contrast, the calculated FAW values for the MS-OXF site range
from 124 to 166 kg d 1 km2 1.
For the dry year (2012), measured Q and C values (Fig.
3d–f ) display a dilution response to rain events, similar to
those observed in 2009, through mid-June 2012. During the
month of July 2012, however, background or baseflow Q (Fig.
3d) and C (Fig. 3e) values steadily decrease, up to an order of
magnitude below that of the early June values. Beginning in late
July, C values (Fig. 3e) begin to show a concentration response
to rainfall events, which persists through the end of the intraannual window. Baseflow C measured in 2012 (Fig. 3e) was
consistently lower than in 2009 (Fig. 3a). Similar to the 2009
results, the 2012 C response varied spatially between sites for
the same rainfall event and temporally between different events
at a single site. The calculated 2012 FAW values (Fig. 3f ) display
similar trends as the C values (Fig. 3e), albeit several orders of
magnitude lower than the corresponding time frame in 2009.
After July 2012, the US-HST displayed an FAW range of 0.5 to
1.2 kg d 1 km2 1 (Fig. 3f ) for the three-highest magnitude events
(4 Aug. 2012, 7 Sept. 2012, and 12 Nov. 2012). Similar yet lower
FAW values were calculated for the MS-OXF and DS-COR sites.
Increased Q led to increased FAW for all three monitoring
sites in 2009 and 2012; however, the slope of the relationship
was smaller in 2009 (b < 1) than in 2012 (b > 1) (Fig. 4). The
results of the 2009 monitoring campaign for the combined
three sites have a higher relative Q and FAW (approximately
two orders of magnitude) compared with that of the 2012
monitoring. The a value is higher for the 2009 relationship
than for 2012, owing to the consistently higher background
magnitude of C measured in 2009. For both relationships, the
correlation coefficient is near 0.8.
Characteristics of the selected rainfall events, including
antecedent conditions preceding each rain event, are summarized
For proofing purposes only
Results
The antecedent conditions for 2009 and 2012 are shown
in Fig. 2 as cumulative rainfall since 1 May and as weekly PDI
values for each intra-annual temporal window. Cumulative rain
measured in 2009 far surpassed rainfall measured in 2012 by at
least 600 mm during the same time frame (Fig. 2). The PDI values
for 2009 range from +3 to +4, indicating very moist to extremely
moist conditions. In contrast, the 2012 values range from 0 to
4, suggesting midrange to extreme drought conditions.
Variability in the temporal Q, C, and FAW for the wet year
(2009) monitoring campaign are shown in Fig. 3a–c. A general
decreasing trend is observed in the background or non-event C
values through the 2009 time series (Fig. 3b); the highest value
of 14.5 mg N L 1 was observed at the US-SAC location in June,
and the lowest background value of 5.2 mg N L 1 was noted
at the DS-COR site in late September. A relatively consistent
spatial gradient in the background C values occurs along the
flow gradient (Fig. 3b), ranging from 9.3 to 14.5 mg N L 1 at
US-SAC, from 6.9 to 11.0 mg N L 1 at MS-OXF, and from 5.2
to 9.3 mg N L 1 at DS-COR. There is a dilution response of the
C values (Fig. 3b) associated with rain events (Fig. 3a) that is
approximately coincident with peaks in the Q hydrograph (Fig.
3a), at all sites during 2009. The magnitude of the C dilutions
© ASA, CSSA, SSSA
Fig. 2. Time-series showing cumulative rain since May 1 and associated weekly Palmer Drought Index (PDI) values for 2009 and 2012.
4
Journal of Environmental Quality
For proofing purposes only
Fig. 3. Time-series showing (a, d) discharge, (b, e) nitrate concentration, and (c, f) area-weighted nitrate-N flux for the 2009 and 2012 monitoring
campaigns, respectively. Daily rainfall data are presented for the Oxford rain gauge site. Numbers 1 through 4 shown in (a) and (d) represent
individual rainfall events analyzed for each intra-annual window. DS-COR, Coralville; MS-OXF, Oxford; US-HST, Homestead; US-SAC, South Amana.
in Table 2. Each of the rainfall events is unique in duration,
coverage, total rain volume, and event discharge volume when
comparing between events, sites, and years (2009 or 2012).
However, consistent differences in antecedent moisture
conditions between the 2 yr are observed, especially in the AP30
and PDI values. The AP30 values range from 97 to 214 mm,
and the PDI values are increasingly positive (greater than +3)
preceding 2009 events, whereas calculated values of AP30 and
PDI for 2012 are 10 to 86 mm and increasingly negative (less
than 1), respectively.
of nitrate-N exported between the 2 yr of investigation. Most
notably, the magnitude of the temporal FAW was appreciably
higher in the wet year (2009) (Fig. 3b,c), when PDI values were
dominantly positive (Fig. 2). The higher FAW magnitude observed
in 2009 was largely driven by the higher relative Q as opposed to
the contribution by C magnitude, as evidenced by the qualitative
similarity in the duration of the individual Q and FAW peaks and
the dilution of C in response to increased Q.
Previous studies describe seasonal differences in C patterns,
which are widely attributed to seasonal variations in Q. For
example, Poor and McDonnell (2007) describe seasonal
© ASA, CSSA, SSSA
Discussion
Antecedent Moisture Controls on Intra-Annual
Nitrate-N Dynamics
A watershed’s capacity to mobilize nitrate-N is dependent on
numerous interacting properties and processes. One difficulty in
assessing landscape-wide nitrate-N mobilization is in our ability
to separate, characterize, and quantify the relative contributions
of the interdependent and often competing properties and
processes. We monitored a single watershed over a relatively
short time frame (<4 yr), in which we assume little measurable
change in the intrinsic (e.g., geology, geomorphology) or
anthropogenic (e.g., land use, fertilizer application) properties of
the landscape. The primary difference between the two individual
years is in the antecedent moisture conditions due to short-term
hydrometeorologic variability (Fig. 2). Our intra-annual results
(Fig. 3 and 4) showed marked differences in the magnitude
XXXBHSPOPNZPSHtXXXDSPQTPSHtXXXTPJMTPSH
Fig. 4. Area-weighted nitrate-N flux versus discharge shown as
aggregated data for all three monitoring sites during the 2009 and
2012 monitoring campaigns. FAW, area-weighted nitrogen flux.
5
differences in C patterns in an agricultural watershed shifting
from a C dilution response in the fall and winter to a C
concentration response in spring and a decreasing trend in
nitrate-N export associated with a wet period. Our results
show a consistent C dilution response throughout the wet year
(2009) and a shift from C dilutions in early summer 2012 to C
concentrations after July and through the end of the study (Fig.
3). Although previous studies attribute the shift in C patterns
to seasonal characteristics (Webb and Walling, 1985; Poor
and McDonnell, 2007), our results suggest that antecedent
conditions can hold a primary control on C behavior and seasonal
variability. The consistent dilution response observed during
the 2009 monitoring campaign is likely the result of relatively
unchanging antecedent moisture conditions and consistently
positive PDI values. We attribute the apparent shift or threshold
behavior in the 2012 C response (Fig. 3e) to the rapid depletion
of soil moisture due to lack of rain, as evidenced by the sharp
decline in weekly PDI values (Fig. 2) during June and July.
Furthermore, we hypothesize that this threshold behavior is the
result of a decrease in catchment connectivity, where the soil
profile is increasingly disconnected from the stream channel
and deeper alluvial groundwater sources. It is likely that this
threshold behavior appears concurrent with seasonal patterns, as
previous studies have suggested (Poor and McDonnell, 2007);
however, the lack of a threshold response in the wet year (2009)
provides additional evidence that such a threshold may be driven
by distinct changes in antecedent moisture conditions.
At the timescales of individual rainfall events and intraannual dynamics, we find that nitrate-N loading to streams
and dynamics between C and Q are highly variable. Our study
adds to previous works that suggest nitrate-N dynamics occur
at rapid timescales spanning minutes (event-based) to weeks
(intra-annual). For example, weekly surface water sampling
for nitrate-N and averaging of daily stream discharge may be
acceptable for evaluating nitrate-N flux on a seasonal or yearly
scale; however, this approach is not sufficient to capture individual
or sequenced event-related nitrate-N flux (Sebestyen et al., 2008;
Raymond and Saiers, 2010; Pellerin et al., 2012). Furthermore,
previous research established that a single storm event or flood
period imparts a large amount of the cumulative yearly nitrate-N
flux from a watershed (Goolsby et al., 1993; Hubbard et al.,
2011). In addition to the importance of the temporal scale of
investigation, the selection of an appropriate temporal frequency
of measurement of the hydrologic and hydrochemical variables
For proofing purposes only
Table 2. Characteristics of selected rainfall events and antecedent moisture conditions preceding each event.
Event
no.
Event window
1
23 June 2009, 08:00
26 June 2009, 16:00
2
3 July 2009, 16:00
7 July 2009, 16:00
3
Aug. 2009, 16:00
30 Aug. 2009, 16:00
4
24 Sept. 2009, 00:00
28 Sept. 2009, 00:00
1
2
Station†
Event Total event
duration
rain
h
mm
US-SAC
MS-OXF
DS-COR#
US-SAC
MS-OXF
DS-COR#
US-SAC
MS-OXF
DS-COR#
US-SAC
MS-OXF
DS-COR#
3
3
3
10
9
9
24
20
20
8
5
5
11
26
26
42
35
35
143
109
109
26
21
21
US-HST
MS-OXF
DS-COR
US-HST
MS-OXF
DS-COR
US-HST
MS-OXF
DS-COR
US-HST
MS-OXF
DS-COR
6
6
5
1
1
2
3
2
2
8
8
7
28
30
15
20
24
24
16
14
17
42
34
23
Event rain
volume
Event discharge
volume (Qi to Qf )‡
————— m3 —————
2009
284,027
2,087,783
3,692,749
14,886,933
6,200,546
20,818,419
1,109,607
1,991,805
5,712,366
10,736,474
9,098,617
13,795,964
3,746,744
13,403,599
18,027,125
127,346,486
28,533,204
164,225,783
677,724
862,779
3,479,398
4,000,683
5,540,591
6,369,436
2012
598,781
280,677
4,751,175
1,432,910
6,198,921
2,271,507
431,554
76,644
3,718,866
290,971
6,049,982
950,395
345,243
14,366
2,317,630
41,204
3,912,604
395,885
884,686
315,595
5,452,319
1,208,371
7,709,821
2,985,822
AP7§
AP30§
——— mm ———
118
60
121
0
0
0
25
24
33
10
7
17
185
120
214
191
106
194
140
120
135
164
97
145
+3 to +4
+3
3
+3 to +4
4
4
4
4
12
4
8
5
5
7
6
10
6
5
6
4
43
36
38
14
10
12
48
46
68
59
67
86
0
0
© ASA, CSSA, SSSA
30 May 2012, 16:00
2 June 2012, 00:00
4 Aug. 2012, 00:00
7 Aug. 2012, 08:00
3
22 Oct. 2012, 00:00
25 Oct. 2012, 08:00
4
10 Nov. 2012, 00:00
25 Nov. 2012, 00:00
PDI range¶
3 wk
1 wk
3 to 4
4
4 to 3
3
3
3 to 4
† DS-COR, Coralville; MS-OXF, Oxford; US-HST, Homestead; US-SAC, South Amana.
‡ Qf, final discharge value; Qi, initial discharge value.
§ Antecedent precipitation at 7 d (AP7) and 30 d (AP30) preceding the event.
¶ Palmer Drought Index range for 3 wk preceding the event and for the week of the event.
# Rainfall estimates from OXF01.
6
Journal of Environmental Quality
affects interpretation of the data. Solute–discharge relationships
can be highly dependent on scale of measurement (Arheimer et
al., 1996; Turgeon and Courchesne, 2008).
Consistently high C values observed during 2009 baseflow
suggest that the mobile nitrate pool is not source or transport
limited but may be attributed to “biogeochemical stationarity,”
as discussed by Basu et al. (2010). Biogeochemical stationarity
postulates that legacy accumulations of nutrients within a
landscape result in a buffering effect to the expected variability
of C. Evidence of stationarity is not apparent in 2012, where a
transition to lower baseflow C values occurs concurrently with a
decline in PDI values and thus soil moisture. We suspect that the
decrease in baseflow C is the result of the subsurface becoming
increasingly disconnected from the legacy stores, and thus both
transport and source limited. At low baseflow, there is greater
contact between the water source and the stream channel and
higher residence times, which can promote denitrification in the
subsurface and uptake by in-stream aquatic vegetation (Alexander
et al., 2000; Tomer et al., 2003), hence a lower relative N.
Table 3. Event-related nitrate load and yield calculated for the selected
rainfall events during the 2009 and 2012 monitoring campaigns.
Event no.
1
2
3
Station†
US-SAC
MS-OXF
DS-COR
US-SAC
MS-OXF
DS-COR
US-SAC
MS-OXF
DS-COR
US-SAC
MS-OXF
DS-COR
Event load
(Fi to Ff )‡
kg
2009
122
602
1280
299
5136
5464
1584
23,262
25,647
79
–
1148
2012
–
270
267
3
11
41
0.1
0.5
6.1
134
409
643
Event yield
(Fi to Ff )
kg km
2
4.7
3.8
5.0
11.4
32.6
21.5
60.6
147.7
100.9
3.0
–
4.5
For proofing purposes only
Antecedent Controls on Event-Related N Flux
How in-stream N concentrations respond to rainfall events
is highly dependent on intra-annual antecedent moisture
conditions. Although each rainfall event is unique in its own
characteristics (e.g., intensity, duration), we observed a marked
and consistent difference in the character and magnitude of the C
response to precipitation, with large dilutions (up to 10 mg L−1)
dominant during the wet year rainfall events (2009) and small
concentration peaks (<7 mg L−1) persisting during the drought
conditions of 2012. The calculated event N load and yield were
appreciably higher during the wet year (2009) (Table 3), primarily
driven by the larger event Q measured in 2009. Additionally, we
found a large (up to two orders of magnitude) difference between
the total Qv and Pv between the two contrasting wet/dry years
(Fig. 5). For 2009, we observe a Qv in excess of Pv, which suggests
that excess water is released from the subsurface beyond that added
by the rain event. Iqbal (2002) explained this as groundwater
being released to the stream due to excess vertical fluid pressure
from infiltrating rain water, creating increased horizontal flow. In
contrast, we observe the storage of event water in 2012, as shown
by the negative Qv-Pv values in Fig. 5.
A clear difference is observed in the relationship between FAW
and QAW (Fig. 4) and in Qv-Pv (Fig. 5) for the two contrasting
years, suggesting that different hydrologic processes dominate. In
2009, baseflow contributions likely dominate during rain events
because b < 1, C dilutions are observed (Tomer et al., 2003), and
excess water volume is released during 2009 rain events (Fig. 5).
In contrast during the drought conditions of 2012, quickflow or
overland flow likely dominate the hydrograph signal, resulting
in C concentrations, b > 1 in the relationship between FAW and
QAW (Fig. 4), and a water deficit in Qv-Pv (Fig. 5). Quickflow can
be generated regardless of soil moisture conditions, although
in a wet year the in-stream nitrate-N concentration response is
likely overwhelmed by the much larger-magnitude groundwater
delivery response.
In addition to antecedent controls on rain event water
storage/release and nitrate-N flux magnitude, nitrate-N stores in
soils are a function of antecedent moisture conditions and inputs
to the landscape. We suggest that nitrate-N stores are depleted
4
1
2
3
4
US-HST
MS-OXF
DS-COR
US-HST
MS-OXF
DS-COR
US-HST
MS-OXF
DS-COR
US-HST
MS-OXF
DS-COR
–
1.7
1.0
0.1
0.1
0.2
0.006
0.003
0.024
6.3
2.6
2.5
† DS-COR, Coralville; MS-OXF, Oxford; US-HST, Homestead; US-SAC,
South Amana.
‡ Ff, final nitrate N flux value; Fi, initial nitrate N flux value.
by flushing during the wet year (2009) and that nitrate-N
accumulates during the dry year (2012).
A Conceptual Understanding of Hydrologic Connectivity
and Nitrate-N Mobilization
© ASA, CSSA, SSSA
XXXBHSPOPNZPSHtXXXDSPQTPSHtXXXTPJMTPSH
On the basis of our findings, we suggest that the degree of
hydrologic connectivity in relation to antecedent conditions
Fig. 5. Bar charts showing difference between discharge volume
and event precipitation volume for the selected events. DS-COR,
Coralville; MS-OXF, Oxford; US-HST, Homestead; US-SAC, South
Amana.
7
dictates C response, and thus FAW magnitude, in response to
precipitation at intra-annual and individual event timescales.
Increased hydrological connectivity during wet cycles means that
precipitation events rapidly mobilize water to the stream, with
both the baseflow contribution and overland flow diluting the
in-stream C. The higher degree of spatial subsurface connectivity
may result in a dominant pre-event water contribution (Grayson
et al., 1997; Ali and Roy, 2010). Flux of nitrate-N is high due
to excess groundwater released to the stream (Iqbal, 2002) and
encountering the mobile nitrate-N pool.
In a study on connectivity of hillslopes to riparian areas,
Jencso et al. (2009) suggest that a wet, connected state, that is
highly transmissive, is required for flushing of solutes from
the landscape to the stream through groundwater. Similarly,
McGlynn et al. (2004) proposed that landscape elements with
increased hydrologic connectivity link source area flow paths to
streams. As in Jencso et al. (2009) and McGlynn et al. (2004),
the magnitude of the nitrate-N flux response documented
here is likely associated with integrated spatial landscapescale connectivity. In contrast, during dry periods the primary
contribution is flushing of nitrate-N overland or from the
nitrate-N stores in the vadose zone, providing enrichment of
the in-stream nitrate-N. Nitrate-N flux is lower, being transport
limited as opposed to supply limited (Basu et al., 2010). Under
dry antecedent conditions, the catchment is in a disconnected or
disorganized state that promotes new water infiltration (Grayson
et al., 1997; Ali and Roy, 2010), and storage.
In summary, our results suggest marked differences in
nitrate-N flux response attributed to wet/dry cycles and more
specifically the water source compartment that dominates
event recharge to the stream (Fig. 6). In 2009, large-magnitude
dilutions (up to 10 mg N L−1) persisted during wet antecedent
conditions, consistent with a dominant baseflow contribution
and excess groundwater release in relation to precipitation volume
(discharge > > precipitation). Smaller-magnitude concentrations
(<7 mg N L‒1) were observed during the dry conditions of 2012,
consistent with a quickflow dominated response to rain events
and infiltration/storage of precipitation resulting in discharge
< precipitation. Nitrate-N loads and yields from the watershed
were much higher (up to an order of magnitude) in the wet year
vs. the dry year.
Conclusions
We collected high temporal resolution in-stream nitrate-N
data to characterize the response of an agricultural landscape
to intra-annual and rainfall event dynamics. This allowed us
to characterize the variability in nitrate-N flux across spatial
scales in a nested watershed and in response to multiple rainfall
events. We find evidence that antecedent moisture conditions,
in particular short-term climate extremes in the form of wet/
dry cycles, are a primary control on intra-annual nitrate-N
flux character and magnitude. Our study was conducted in a
single watershed with the primary uncontrolled variable being
precipitation and moisture conditions. Results of this study
demonstrate the complexity of climate–landscape interactions in
agricultural watersheds and demonstrate the potential of highresolution nitrate-N monitoring to advance our understanding
of coupled hydrological and biogeochemical systems.
Given that individual precipitation events can be
responsible for a significant portion of total annual load
For proofing purposes only
© ASA, CSSA, SSSA
Fig. 6. Generalized conceptual diagram summarizing the hydrologic and nitrate N observations from this study. PDI, Palmer Drought Index.
8
Journal of Environmental Quality
(Hubbard et al., 2011), it is important to understand the
relationship between short-term hydrological dynamics
and nitrate-N loading to surface waters. Further, due to the
drought conditions of 2012, we speculate that the lack of
rainfall led to enrichment of nitrate-N in soils. The result
of this drought-related nitrate-N retention and reduced
loading was one of the smallest Gulf of Mexico Dead Zones
on record in 2012 (7480 km2; Louisiana Universities Marine
Consortium [2013]). The small size of the 2012 Dead Zone
was clearly linked to the hydrologic control of decreased
nitrogen inputs to the Mississippi River basin.
Acknowledgments
This study was supported by IIHR– Hydroscience & Engineering at
the University of Iowa and by the U.S. National Science Foundation
(DEB-1263559) and EPSCoR (EPS-1101284). The authors thank
Maclaine Putney for permission to use the 2009 nitrate dataset, the
IIHR Model Annex staff for assistance with field site construction and
data collection, and the City of Coralville. The authors also gratefully
acknowledge the support of the Roy J. Carver Charitable Trust. Lastly,
we greatly appreciate the reviews by three anonymous reviewers whose
comments significantly improved the manuscript.
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