HESSHydrology and Earth System SciencesHESSHydrol. Earth Syst. Sci.1607-7938Copernicus PublicationsGöttingen, Germany10.5194/hess-20-3831-2016The influence of riparian evapotranspiration on stream hydrology and nitrogen retention in a subhumid Mediterranean catchmentLuponAnnaalupon@ub.eduBernalSusanaPobladorSílviaMartíEugèniaSabaterFrancescDepartment d'Ecologia, Universitat de Barcelona, Av. Diagonal 643, 08028 Barcelona, SpainIntegrative Freshwater Ecology Group, Center for Advanced Studies of Blanes (CEAB-CSIC), Accés a la Cala Sant Francesc 14, 17300 Blanes, SpainCREAF, Campus de Bellaterra (UAB) Edifici C, 08193 Cerdanyola del Vallès, SpainAnna Lupon (alupon@ub.edu)14September2016209383138423February20168March201625August201626August2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://hess.copernicus.org/articles/20/3831/2016/hess-20-3831-2016.htmlThe full text article is available as a PDF file from https://hess.copernicus.org/articles/20/3831/2016/hess-20-3831-2016.pdf
Riparian evapotranspiration (ET) can influence stream hydrology at catchment
scale by promoting the net loss of water from the stream towards the riparian
zone (i.e., stream hydrological retention). However, the consequences of
stream hydrological retention on nitrogen dynamics are not well understood.
To fill this gap of knowledge, we investigated changes in riparian ET, stream
discharge, and nutrient chemistry in two contiguous reaches (headwater and
valley) with contrasted riparian forest size in a small forested
Mediterranean catchment. Additionally, riparian groundwater level (hgw)
was measured at the valley reach. The temporal pattern of
riparian ET was similar between reaches, and was positively correlated with
hgw (ρ= 0.60) and negatively correlated with net
riparian groundwater inputs (ρ<-0.55). During the vegetative
period, stream hydrological retention occurred mostly at the valley reach
(59 % of the time), and was accompanied by in-stream nitrate release and
ammonium uptake. During the dormant period, when the stream gained water from
riparian groundwater, results showed small influences of riparian ET on
stream hydrology and nitrogen concentrations. Despite being a small component
of annual water budgets (4.5 %), our results highlight that riparian ET
drives stream and groundwater hydrology in this Mediterranean catchment and,
furthermore, question the potential of the riparian zone as a natural filter
of nitrogen loads.
Introduction
The study of riparian zones has been of growing interest during the last decades
because they can reduce the pervasive effects of excessive anthropogenic
nitrogen (N) inputs in forested, agricultural, and urban ecosystems across
the globe (Hill, 1996; Pert et al., 2010). Since they can affect both the timing and magnitude of N
delivery to downstream ecosystems, riparian zones are currently considered
hot spots of N removal within catchments (McClain et
al., 2003; Vidon et al., 2010). The high capacity of riparian zones to
reduce terrestrial N inputs stems from the biogeochemical conditions at
their unique interface location between upland and streams, which favors
ammonium (NH4+) and nitrate (NO3-) biological uptake
from shallow groundwater via plant assimilation and microbial
denitrification (Clément et al., 2003; Vidon et al., 2010).
The capacity of riparian zones to diminish inorganic N loads critically
relies on the hydrological connectivity between upland, riparian, and stream
ecosystems because it directly influences water flow paths, and thereby
whether groundwater N interacts with organic-rich soils (Mayer et al., 2007; Pinay et al.,
2000). During wet conditions, the N retention in riparian zones is high
because continuous upland groundwater inputs and the rising water table in
flat riparian areas can promote the contact of groundwater with shallow
riparian soils (Ranalli and Macalady, 2010; Vidon and Hill, 2004). However, little is known about the efficiency
of riparian zones to diminish N inputs during dry conditions, when the
hydrological connectivity between uplands and riparian zones tends to
decrease at the valley bottom of catchments (Covino and McGlynn, 2007; Detty and
McGuire, 2010; Jencso et al., 2009; Ocampo et al., 2006). Low or zero water
inputs from uplands can drop the riparian groundwater level far below the
organic-rich and rhizosphere soil layers and, consequently, diminish the
capacity of riparian zones for removing groundwater N (Burt et al.,
2002; Hefting et al., 2004). Conversely, hydrological disconnection between
uplands and riparian zones can favor the lateral movement of water from the
stream toward the riparian aquifer (defined here as stream hydrological
retention), which can enhance denitrification and biological uptake of
stream nitrate at the stream–riparian edge (Duval and Hill,
2007; Martí et al., 1997; Rassam et al., 2006; Schade et al., 2005).
The riparian groundwater level and the hydrological exchange between the
stream and riparian groundwater can be directly influenced by the activity
of riparian trees, which can consume high amounts of water during the
vegetative period. Riparian evapotranspiration (ET) can drive diel
fluctuations of stream discharge and seasonal patterns of the riparian
groundwater table and soil moisture (Brooks et
al., 2009; Burt et al., 2002; Gribovszki et al., 2010). Thus, riparian trees
could affect the strength, location, and duration of the predominant flow
path, and consequently, influence the capacity of riparian zones to reduce N
not only from upland groundwater inputs, but also from stream water. In this
line of thought, previous studies have reported decreases in stream N
concentration along losing stream reaches attributed to N uptake at the
stream–riparian edge (Bernal and Sabater,
2012; Dent et al., 2007; Rassam et al., 2006). Yet, there has been little
research focused on the influence of riparian ET on upland–riparian–stream
hydrological exchange and its potential to promote variations in stream N
concentrations and fluxes.
Map of the Font del Regàs catchment (Montseny Natural
Park, northeastern Spain). The location of the three sampling sites (black circles),
tributaries (white circles), and the riparian plot where tree transpiration
and groundwater level were measured (black square) are shown. The headwater
reach is comprised between the up- and midstream sampling sites, while the
valley reach is comprised between the mid- and downstream sampling sites.
This study aims to investigate the influence of riparian ET on stream
hydrological retention, and its consequences on stream N concentrations in a
small forested Mediterranean catchment. To do so, we compared riparian tree
ET between a headwater reach with limited riparian forest and a contiguous
valley reach with a well-developed riparian forest. First, we expected higher
riparian ET, and thus, higher stream hydrological retention at the valley
reach, especially during the vegetative period. Second, we expected that
differences in stream N concentration between the headwater and valley reach
will reflect differences in riparian N cycling coupled to the dominant
direction of water flow between the riparian zone and the stream. Based on
longitudinal changes observed in semiarid streams (Bernal and Sabater, 2012; Dent et al.,
2007), we expected decreases in N concentration along the two reaches, but
especially at the valley reach because of higher stream hydrological
retention. The results of this study contribute to our understanding of the
interaction between riparian ET and fluxes of water and nutrients at the
stream–riparian edge. This knowledge could have implications for water
resource management, as well as for anticipating how riparian zones and
stream-water chemistry could respond to decreases in water availability
induced by climate change.
Study site
The Font del Regàs catchment is located in the Montseny Natural Park,
northeastern Spain (41∘50′ N, 2∘30′ E). The climate is
subhumid Mediterranean, with mild winters, wet springs, and dry summers.
Annual precipitation is 925 ± 151 mm, with < 1 % of annual
precipitation falling as snow. Mean annual temperature averages 12.1 ± 2.5 ∘C
(mean ± SD, period 1940–2000, Catalan
Metereologic Service). Atmospheric inorganic N deposition ranges from 15 to
30 kg ha-1 yr-1 and does not show any temporal trend (period 1983–2007;
Àvila and Rodà, 2012).
The catchment area is 14.2 km2 and its altitude ranges from 500 to
1500 m a.s.l. (above the sea level) (Fig. 1). The catchment is dominated by
biotitic granite and it has steep slopes (28 %) (Institut
Cartogràfic de Catalunya, 2010). Evergreen oak (Quercus ilex)
and European beech (Fagus sylvatica) forests cover 54 % and
38 % of the catchment, respectively (Fig. 1). Upland soils
(pH ∼ 6) are sandy, with a 3 cm deep O horizon followed by a 5 to
15 cm deep A horizon. There is no snowpack in hillslope areas and upland
soils are generally > 0 ∘C. The riparian forest
covers the 6 % of the catchment area and it is almost flat (slope
perpendicular to stream < 10 %). Riparian width increases from
6 to 28 m along the catchment and the total basal area of riparian trees
increases by 12-fold. Note that by total basal area we are referring to the
sum of individual tree basal area as defined later in the text. Black alder
(Alnus glutinosa), black locust (Robinea pseudoacacia), sycamore
(Platanus x hispanica), European ash (Fraxinus excelsior),
and black poplar (Populus nigra) are the most abundant tree species
in the riparian forest. Riparian soils (pH ∼ 7) are
sandy loam, with a 5 cm deep organic layer followed by a 30 cm deep A horizon.
For this study, we selected two contiguous stream reaches with contrasting
riparian forest (i.e., the headwater and valley reach) (Fig. 1). The
headwater reach (750–550 m a.s.l.) is 1760 m long and drains 6.74 km2
(Table 1). The reach is flanked by a 5–15 m wide riparian forest that covers
∼ 5 % of the drainage area. A. glutinosa,
F. excelsior, and P. nigra represent 51, 26, and
23 % of the total basal area, respectively. The valley reach (550–500 m a.s.l.)
is 1160 m long and drains an additional area of 4.42 km2
(i.e., total catchment area at this reach is 11.16 km2). The reach is flanked
by a 10–25 m wide riparian forest that covers ∼ 10 % of the
drainage area. A. glutinosa, F. excelsior, P. nigra,
and R. pseudoacacia represents 53, 27, 11, and
9 % of the total basal area, respectively. The two stream reaches show
well-preserved channel morphology, with a riffle-run structure and low
slopes (< 5 %) along the reaches. The streambed is mainly composed
by rock (∼ 30 %), cobbles (∼ 25 %), and
gravel (∼ 15 %) at the headwater reach, whereas rock
(∼ 25 %), cobbles (∼ 30 %), and sand
(∼ 30 %) are the dominant substrates at the valley reach.
The stream channel is, on average, 2 and 3 m wide for the headwater and the
valley reach, respectively. During the study period, riparian groundwater
(< 1.5 m from the stream channel) flowed well below the soil surface
in the two reaches (0.5 ± 0.1 m; averaged from 14 wells, 7 by reach;
n= 82) (Bernal et al., 2015).
Materials and methodsField sampling and chemical water analysis
To characterize the riparian forest, we inventoried 14 riparian forest plots
of 30 m long × riparian width (5–20 m) (seven plots by reach, ∼ 5 %
of the riparian area). In each plot, we identified each individual tree at
species level and measured its diameter breast height (DBH; in cm)
and its basal area (BA =π (DBH/2)2, in cm2). For each tree species i, we calculated the
area-specific BA (BAsp,i; in m2 of BA
per km2 of riparian area) by dividing the total BA for a given
species by the total area of the inventoried riparian plots, either for the
headwater (0.23 km2) or valley (0.21 km2) reach.
Reach length, catchment drainage area, percentage of riparian area,
width of the riparian zone, and total basal area of riparian trees for the
headwater and valley reaches.
Riparian zone Reach characteristics characteristics LengthDrainageAreaMeanTotal(m)area(%)widthbasal(km2)(m)area(m2 BA)Headwater17606.744.912822Valley11614.429.9191354
During 2 consecutive water years (from September 2010 to August 2012), we
monitored three stream sampling sites (up-, mid-, and downstream sites),
which constituted the top and the bottom of the headwater and valley
reaches. Stream-water level was recorded at 15 min intervals at each
sampling site with a water pressure transducer (HOBO U20-001-04).
Fortnightly, stream discharge (Q; in L s-1) was
measured using the “slug” chloride addition technique
(Gordon et al., 1992). We used the regression between
discharge and stream-water level measurements to infer Q values
at 15 min intervals during the study period (n= 57, 60,
and 61 for up-, mid- and downstream sites, respectively; in all cases:
R2> 0.97; Fig. S1 in the Supplement). In order to compare stream
discharge among the three sites, we calculated area-specific stream
discharge (Q′; in mm day-1) by dividing Q by drainage
area. Riparian groundwater level (hgw; in cm b.s.s. (below soil
surface) was recorded at 15 min intervals with a water pressure
transducer (HOBO U20-001-04) in a 1.8 m long PVC (polyvinyl chloride) well (3 cm ø) placed
∼ 3 m from the stream channel edge at the downstream site (Fig. 1).
Stream-water samples were collected daily (at noon) from each sampling site
with an auto-sampler (Teledyne Isco Model 1612) and taken to the laboratory
every 10 days. Auto-samplers were installed about 1 m below ground to keep
water samples fresh and prevent biogeochemical transformations (Fig. S2).
From August 2010 to December 2011, discharge and water chemistry was
measured every 2 months at the three permanent tributaries discharging to
Font del Regàs stream (Fig. 1). We used pre-acid-washed polyethylene
bottles to collect water samples after triple rinsing them with stream
water. All water samples were filtered (Whatman GF/F, 0.7 µm pore ø)
and kept cold (< 4 ∘C) until laboratory analysis
(< 24 h after collection). Water samples were analyzed for dissolved
inorganic N (DIN; NO3- and NH4+) and chloride (Cl-), which was used as hydrological tracer
(Kirchner et al., 2001). Cl- was
analyzed by ionic chromatography (Compact IC-761, Methrom). NO3-
was analyzed by the cadmium reduction method (Keeney and
Nelson, 1982) using a Technicon Autoanalyzer (Technicon, 1976).
NH4+ was manually analyzed by the salicilate/nitropruside method
(Baethgen and Alley, 1989) using a spectrophotometer (PharmaSpec UV-1700 SHIMADZU).
Riparian evapotranspiration
From September 2010 to August 2012, we calculated diel variations in stream
discharge at the up-, mid-, and downstream sites (Qlost, in
m3 day-1) by subtracting daily Q to the stream discharge
obtained by linearly interpolating maxima Q (measured
between 00:00 and 03:00 LT) between two consecutive days. We used only stream
discharge during base-flow conditions (i.e., changes in Q< 10 % in
24 h) to avoid any confounding effect associated with storm
events. During the vegetative period, we attributed Qlost to
water withdrawal by riparian tree roots from either the riparian aquifer or
directly from the stream channel (Cadol et al., 2012).
Given that there was no snowpack in the study catchment, Qlost
during the dormant period was attributed to water withdrawal by riparian
understory vegetation (Roberts, 1983) and/or by upland evergreen trees
(Savé et al., 1999). Furthermore, we estimated riparian ET along each
reach as the difference in Qlost measured at the bottom and at
the top of the reach and by assuming that Qlost measured at
each particular site integrated the riparian ET upstream from that point.
Riparian ET (ΔQlost, in m3 m-1 day-1)
was weighted by stream length for comparison purposes. For the
valley reach, we compared ΔQlost values with diel
variations in hgw to explore the influence of riparian ET on the riparian groundwater level.
To explore the relation between diel cycles in stream discharge and the
activity of riparian trees, we compared ΔQlost with an
independent estimate of riparian transpiration based on mean monthly sap-flow measurements of the dominant riparian trees (8 individuals of
A. glutinosa, 5 individuals of F. excelsior, 5 individuals
of P. nigra, and 12 individuals of R. pseudoacacia). Sap
flow was measured using constant thermal dissipation sensors (Granier,
1985). Each sensor consisted of two probes (10–20 mm long) inserted in the
north side of the trunk at breast height 10 cm apart. The upper probe was
heated at a constant temperature. The thermal difference between probes was
scanned at 10 s intervals and recorded as a 15 min average with a data logger
(CR1000, Campbell Inc.). Then, thermal differences were related to sap flux
density (in dm3 of water per m2 of BA and minute)
following the original calibration of Granier (1985). More details can be
found in Nadal-Sala et al. (2013).
For each reach, we calculated the transpiration of the riparian tree
community (Trip, in m3 m-1 day-1) with
Trip=∑i=1nTi×BAsp,i×A/x,
where Ti is monthly mean daily transpiration (in dm3 of
water per m2 of BA and day) and BAsp,i is the area-specific basal area (in
m2 BA km-2) of each tree species i, A is the riparian
area (in km2), and x is the reach length (in m). Values of mean
monthly T were recorded at the valley of the catchment from January to
August 2012 (Nadal-Sala et al., 2013).
Mass balance calculationsNet riparian groundwater inputs to stream
To examine the temporal and spatial pattern of stream hydrological retention, we measured the
hydrological exchange between riparian groundwater and stream-water bodies
at reach scale. The contribution of mean daily net riparian groundwater
inputs to stream discharge (Qgw) was estimated with
Qgw=Qbot-Qtop-Qtrib,
where Qtop and Qbot are mean daily
discharge measured at the top and at the bottom of the reach, respectively,
and Qtrib is mean daily discharge at the permanent tributaries
(all in L s-1). For the headwater reach, Qtop and
Qbot were the discharge at the up- and midstream sites,
respectively; while we used the discharge at the mid- and downstream sites
for the valley reach. For each stream site, mean daily discharge was the
average of Q for each day. To estimate mean daily discharge at each
tributary, we used the best-fit model (logarithmic model) between Q
measured at each tributary and at the upstream site within the same day
(for each of the three tributaries: R2> 0.97, n= 11,
p< 0.001; Fig. S3). Values of Qgw> 0
indicate the movement of water from the riparian zone to the stream
(i.e., net gaining stream), whereas values of Qgw< 0 indicate
a net loss of water from the stream towards the riparian zone. Therefore,
Qgw< 0 was used as an indicator of stream hydrological
retention (Covino et al., 2010).
Chemical signature of riparian groundwater and stream water
We used a mass balance approach to investigate whether changes in stream-water
Cl-, NO3-, and NH4+ concentrations along the
valley reach could be explained by hydrological mixing between riparian
groundwater and stream water. The mass balance was focused at the valley
reach, where water and N retention were expected to be the highest. Only
discharge and solute concentrations during base-flow conditions were used
for the mass balance approach. For each day, we calculated a predicted
concentration for the downstream site with the following mass balance:
Qbot×Cbot=Qtop×Ctop+Qgw×Cgw+Qtrib×Ctrib,
where Qtop, Qbot, Qtrib,
and Qgw are as in Eq. (2) (all in L s-1).
Ctop and Cbot are daily
solute concentrations measured at the top and at the bottom of the reach,
respectively (in mg L-1). Ctrib is
daily solute concentration at the tributaries (in mg L-1), which was
estimated by fitting the best-fit model (logarithmic model) between solute
concentration measured at each tributary and at the upstream site within
the same day (for each of the three tributaries and for the three solute:
R2> 0.78; in all cases: n= 11, p< 0.001;
Fig. S3). Although this may be a rough estimation of solute concentrations
at the tributaries, it was a useful procedure for inferring riparian
groundwater chemistry at daily time steps. Finally, Cgw is
daily solute concentration in riparian groundwater (in mg L-1). For
periods of Qgw< 0, we considered that
Cgw equaled Ctop. For periods of
Qgw> 0, we assumed similar riparian groundwater
chemistry between the headwater and valley reaches. In this case,
Cgw at the headwater reach was inferred from Eq. (3) by assuming
that there was no biological reactivity within the stream channel. The
predicted Cgw showed a good match with the concentrations
measured at seven wells installed along the headwater reach (< 2 m from
the stream), with median Cgw differing < 5, 7,
and 10 % for Cl-, NO3-, and NH4+, respectively
(Bernal et al., 2015) (Table S1 in the Supplement).
For each day, we calculated the ratio between observed and predicted solute
concentrations (Obs : Pred ratio). For Cl- (hydrological tracer), we
expected Obs : Pred ratios close to 1 if there are no additional water sources
contributing to stream discharge at the valley reach. For NO3- and
NH4+, Obs : Pred < 1 and Qgw< 0 was
interpreted as in-stream biological N retention via assimilatory uptake (for
NO3- and NH4+), nitrification (for NH4+),
and/or denitrification (for NO3-). We interpreted Obs : Pred > 1
and Qgw< 0 as either in-stream
mineralization (for NH4+) or nitrification (for NO3-).
For Qgw> 0 (net gaining stream), Obs : Pred ≠ 1
was interpreted as differences in riparian groundwater nutrient
concentration between the headwater and the valley reaches. We used the
relative difference between measured and predicted Cgw at the
headwater reach as a threshold to determine when observed and predicted
concentrations differed significantly from each other (±1.05,
±1.07, and ±1.1 for Cl-, NO3-, and NH4+
concentrations, respectively).
Statistical analysis
To investigate the influence of riparian ET on stream discharge and stream-water chemistry, we split the data set into vegetative and dormant periods.
We considered that the vegetative period was compressed between the onset
(April) and offset (October) of riparian tree evapotranspiration (Nadal-Sala et al., 2013).
For each reach, we investigated differences in Q′,
Qgw, mean daily hgw, and stream solute
concentrations between the two periods with a Wilcoxon rank sum test
(Zar, 2010). For each period, the occurrence of stream
hydrological retention was calculated by counting the number of days with
Qgw< 0. For each reach, we further explored the
relationship between Trip, ΔQlost,
and Qgw with a Spearman correlation. A Spearman correlation
was also used to analyze the relationship between ΔQlost
and mean daily hgw at the valley reach.
To explore whether stream hydrological retention influenced stream
NO3- and NH4+ concentrations at the valley reach, we
examined the relationship between Qgw and Obs : Pred
ratios measured at the downstream site with Spearman correlations.
For each solute, we further compared the Obs : Pred ratio between days with
Qgw> 0 and Qgw< 0 with a Wilcoxon rank sum test (Zar, 2010).
All the statistical analyses were carried out with the R 2.15.1 statistical
software (R Core Team, 2012). We chose non-parametric statistical
tests because the residuals of both stream discharge and solute
concentrations were not normally distributed (Shapiro test, p< 0.05).
In all cases, differences were considered statistically significant when p< 0.01.
ResultsSeasonal and diel patterns of stream discharge and whole-reach riparian ET
During the study period, median annual Q was 15.9, 53.9, and 62.4 L s-1
at the up-, mid-, and downstream sites, respectively. The three
sites showed the same seasonal pattern, characterized by a strong
decline in Q during the vegetative period (Fig. 2a). As expressed
by catchment area, median annual Q′ was 0.65, 0.53, and 0.41 mm day-1
at the up-, mid-, and downstream sites, respectively. In
all sites, Q′ was significantly higher during the dormant than
during the vegetative period (Wilcoxon test, p< 0.01).
Diel variations in stream discharge occurred during the whole year, with
maxima in early morning (03:00–06:00 LT) and minima in early afternoon (14:00–17:00 LT).
During the dormant period, diel discharge variations were relatively small
at the three sites (Qlost< 2 % of mean daily Q). Values of Qlost increased during the
vegetative period and showed a marked longitudinal pattern, median values
being 36, 219, and 340 m3 day-1 at the up-, mid-, and downstream
sites, respectively. At the three sites, Qlost increased from
April to June, peaked in summer (July–August), and then decreased until
November. In the summer peak, Qlost accounted for the 7,
15, and 19 % of mean daily Q at the up-, mid-, and
downstream sites, respectively. This seasonal pattern of
Qlost was consistent for the 2 studied water years.
During the vegetative period, riparian ET was lower at the headwater than at
the valley reach as indicated by ΔQlost (0.12 vs. 0.17 m3 m-1 day-1)
and Trip (0.31 vs. 0.49 m3 m-1 day-1).
There was a strong and positive relationship between
Trip and ΔQlost for both the
headwater and valley reach (Fig. 3a). Both Trip and
ΔQlost peaked in summer (July–August) and showed minima in
winter (January–March). At the valley reach, there was a positive
relationship between ΔQlost and diel variations in
hgw (Spearman coefficient [ρ] = 0.58, p< 0.001, n= 277).
Temporal pattern for the period 2010–2012 of (a) stream
discharge (Q) at the up- (light gray), mid- (dark gray), and
downstream (black) sites, (b) riparian evapotranspiration (ΔQlost)
estimated as the difference in the diel variation in
discharge between the top and the bottom of the headwater (white) and valley
(black) reaches, (c) daily net riparian groundwater inputs (Qgw)
for the headwater (white) and valley (black) reaches,
and (d) groundwater table fluctuation (hgw) at the valley
bottom. In (c), the Qgw= 0 line is shown as a
reference of nil net riparian to stream-water inputs; Qgw> 0
and < 0 indicates when the stream reach was net
gaining and net losing water, respectively. In (d), the mean soil
depth of the A horizon is indicated. V: vegetative period, D: dormant period.
Net riparian groundwater inputs and groundwater table elevation
Median annual Qgw was positive at the headwater reach (11.2 L s-1),
but negative at the valley reach (-0.5 L s-1). The two
reaches showed lower Qgw values during the vegetative period
compared to the dormant period, though differences were larger at the valley
reach (Table 2, Fig. 2c). The two reaches showed a negative correlation
between Qgw and ΔQlost (headwater: ρ=-0.57,
p< 0.001, n= 273; valley: ρ=-0.79, p< 0.001, n= 286) (Fig. 3b).
Stream hydrological retention (Qgw< 0) was more
frequent at the valley reach compared to the headwater reach (27 %
vs. 4 % of the time on an annual basis). During the vegetative period,
Qgw< 0 occurred from May to September (59 % of the
time) at the valley reach, while it occurred only in July and August at the
headwater reach (15 % of the time). During the dormant period, days with
Qgw< 0 were infrequent (< 3 % of the time)
for the valley reach and nil for the headwater reach.
Relationship between (a) the monthly mean of daily
riparian transpiration estimated from sap-flow data (Trip) and
riparian evapotranspiration estimated as the difference in diel discharge
variation between the top and the bottom of each stream reach (ΔQlost),
and (b)ΔQlost and daily net
riparian groundwater inputs (Qgw) for the headwater (white)
and valley (black) reaches. Data are shown separately for the vegetative
(circles) and dormant (squares) period. The Spearman coefficients are
indicated in (a) (in both cases: p< 0.01, n= 8). In (b), the
Qgw= 0 line is shown as a reference of nil net riparian to
stream-water inputs; Qgw> 0 and < 0 indicates when the stream
reach was net gaining and net losing water, respectively.
At the downstream site, median annual hgw was 70 cm b.s.s. and
showed higher values (i.e., lower water table levels) during the
vegetative period compared to the dormant period (Fig. 2d, Table 2). There
was a moderate positive correlation between mean daily hgw and
ΔQlost (ρ= 0.60, p< 0.001, n= 277).
Net groundwater inputs to stream discharge (Qgw),
number of days with stream hydrological retention (Qgw< 0) and
groundwater depth (hgw) for the vegetative and dormant period,
respectively. The number of cases is shown in parentheses for each group.
For Qgw and hgw, data are shown as median ± interquartile
range [25th, 75th], and the asterisks indicate statistically significant differences
between the two periods (Wilcoxon rank sum test, *p< 0.01).
Stream Cl- concentration was lower at the upstream than at the mid- and
downstream sites for both the vegetative and dormant periods (Table 3). The
upstream site showed no differences in stream Cl- concentration
between the two periods, while the mid- and downstream sites showed lower
Cl- concentration during the dormant than during the vegetative period
(Table 3). The highest stream NO3- concentration was observed at
the upstream site and the lowest at the midstream site (Table 3). Stream
NO3- concentration was higher during the dormant than during the
vegetative period at the up- and midstream sites, while no seasonal pattern
was observed at the downstream site (Table 3). Stream NH4+
concentration was higher at the upstream than at the downstream site. The
three sites showed higher stream NH4+ concentration during the
vegetative than during the dormant period (Table 3).
Comparison between observed and predicted stream solute concentrations at the downstream site
During the study period, there was a good match between observed stream
Cl- concentrations at the downstream site and those predicted by
hydrological mixing as indicated by Obs : Pred ratios ∼ 1
(Fig. 4a). For NO3-, Obs : Pred ratios were close to 1
during the dormant period, while increased up to 1.95 during the vegetative
period (Fig. 4b). For NH4+, Obs : Pred ratios were higher during
the dormant period (∼ 1.15) than during the vegetative period
(from 0.29 to 0.87) (Fig. 4c).
Temporal pattern of the ratio between observed stream
solute concentrations at the bottom of the valley reach (downstream site)
and those predicted from hydrological mixing for (a) chloride,
(b) nitrate,
and (c) ammonium during the period 2010–2012. Bold lines indicate the
running median (the half-window is 7 days). The Obs : Pred = 1 line is
indicated as a reference. V: vegetative period, D: dormant period.
The relationship between Obs : Pred ratios and Qgw was
nil for Cl- (ρ= 0.2, p> 0.05), negative for
NO3-, and positive for NH4+ (Fig. 5). For
NO3-, Obs : Pred ratios were significantly higher for
Qgw< 0 than for Qgw> 0, while the opposite
pattern was observed for NH4+ (for the two solutes: Wilcoxon test,
Z>Z0.05, p< 0.01).
Median and interquartile range [25th, 75th] of stream solute
concentrations at each sampling site for the vegetative and dormant
periods. The number of cases is shown in parentheses for each group. The
asterisks indicate statistically significant differences between the two
periods (Wilcoxon rank sum test, *p< 0.01).
Relationship between mean daily net groundwater inputs (Qgw)
and the ratio between stream concentrations observed at
the bottom of the valley reach (downstream site) and those predicted from
hydrological mixing for (a) chloride, (b) nitrate and
(c) ammonium. Data are shown separately for the vegetative (circles) and dormant (squares) period.
The Spearman coefficient is shown in each case. The solid line indicates no
differences between observed and predicted concentrations, and the dashed
lines indicate the uncertainty associated with the zero line as explained in
the material and methods section.
DiscussionInfluence of riparian ET on stream and riparian groundwater hydrology
Our results revealed that riparian ET can influence stream and riparian
groundwater hydrology, though its relevance varies depending on the timescale considered. On a sub-daily basis, the strong relationship between
Trip, diel variation in hgw, and ΔQlost suggests that riparian vegetation drives diel
fluctuations in stream discharge likely by taking up water from the riparian
aquifer (Cadol et al., 2012; Gribovszki et
al., 2010; Lundquist and Cayan, 2002). However, the fact that ΔQlost
values were lower than those of Trip
suggest that riparian trees fed also on soil water. This result concurs with
previous studies showing that riparian tree species can obtain between
30 and 90 % of their water requirements from the surface soil (0–50 cm depth)
(Brooks et al., 2009; Sánchez-Pérez et al., 2008; Snyder and Williams,
2000). On a seasonal basis, riparian ET influenced the temporal pattern of
both stream and groundwater hydrology because ΔQlost
was negatively related to Qgw, and positively related to
mean daily hgw. In agreement, previous studies have reported
that riparian water demand (0.5–5 mm day-1) can severely drop the
groundwater table (Sabater and Bernal, 2011; Schilling,
2007) and decrease the amount of groundwater entering streams by
30–100 % (Dahm et al., 2002; Folch and Ferrer, 2015; Kellogg et al., 2008). On an annual basis,
riparian transpiration at the study site (350–450 mm yr-1) was small
compared to published values of ET for other riparian forests worldwide
(400–1300 mm yr-1) (Scott et al., 2008) as
well as compared to oak and beech upland forests (600–900 mm yr-1)
(Àvila et al., 1996; Llorens and Domingo, 2007). These low ET values could partially be explained
by the low radiation reaching the riparian canopy (36 ± 18 W m-2 day-1)
compared to the radiation reaching non-shaded areas of the
catchment (270 ± 70 W m-2 day-1; unpublished data), a
phenomenon already described in the literature (Aguilar et al., 2010). The
relatively low ET values, together with the fact that the riparian forest
occupied a small area of the catchment (6 %), resulted in a minimal
contribution (4.5 %) of riparian transpiration to the annual water budget
for this catchment. This estimate is similar to values reported for tropical
(Cadol et al., 2012), temperate (e.g., Petrone et al., 2007; Salemi et al.,
2012), and Mediterranean (e.g., Bernal and Sabater, 2012; Folch and Ferrer, 2015; Wine and Zou, 2012)
systems, while being several folds lower than values reported for semiarid
and dry land regions (Contreras et al., 2011; Dahm et al., 2002; Doble et al., 2006) (Fig. 6). Together,
these results suggest that the relative contribution of riparian ET to
catchment water depletion across biomes could be explained by differences in
water availability (Fig. 6 and Table S2). Therefore, the potential of
riparian forests to control catchment and stream hydrology at both large and
fine timescales could dramatically increase in regions experiencing some
degree of water limitation (P/PET < 1).
In concordance with our expectations, the influence of riparian ET on stream
hydrology varied along the stream continuum, likely due to changes in the
balance between water availability and water demand. At the upstream site,
maxima Qlost values (7 % of mean daily Q) were
similar to values reported for systems with no water limitation
(Bond et al., 2002; Cadol et al., 2012), while
maxima Qlost values for the downstream site (19 % of mean
daily Q) were close to those reported for water-limited systems
(Lundquist and Cayan, 2002). Stream hydrological retention
occurred mostly at the valley reach, where riparian forest was well
developed, thus suggesting higher riparian water requirements at the valley
bottom (Bernal and Sabater, 2012; Covino and
McGlynn, 2007; Montreuil et al., 2011). Yet, the increase in stream
hydrological retention along the stream could be favored by additional
factors such as longitudinal changes in channel geomorphology, riparian
topography, upland–riparian hydrological connectivity, or the hydraulic
gradient between the riparian aquifer and the stream (Covino et al., 2010;
Detty and McGuire, 2010; Duval and Hill, 2006; Jencso et al., 2009; Vidon
and Hill, 2004). Overall, our results suggest that, despite being
insignificant for catchment water budgets, riparian ET exerted a strong
influence on diel and seasonal patterns of riparian groundwater table and
stream discharge likely due to the proximity and strong hydrological
connectivity between these two water bodies.
Relationship between the relative contribution of
riparian evapotranspiration (ET) to annual catchment water depletion and the
ratio between annual precipitation and potential evapotranspiration (P / PET)
for a set of catchments worldwide (n= 15). Total water output fluxes from
the catchment are stream discharge, catchment evapotranspiration, riparian
evapotranspiration, and anthropogenic extraction (if applies). The Font del
Regàs catchment (present study) is indicated with a gray circle. More
information and references of the study sites are in the Supplement (Table S2).
Influence of stream hydrological retention on stream N concentrations
In contrast to our expectations, the prevalence of stream hydrological
retention during the vegetative period at the valley reach was accompanied
by an increase of stream NO3- concentrations (Obs : Pred > 1).
This result suggests NO3- release within the
stream channel, which conflicts with previous studies reporting
NO3- uptake at the stream–riparian edge in net losing reaches
(Bernal and Sabater, 2012; Duval and Hill, 2007;
Rassam et al., 2006). Biological NO3- uptake at the
stream–riparian edge typically occurs when a large volume of water flows
directly or remains a long time in anoxic zones within the rhizosphere and/or
the organic-rich soils flanking the stream channel (Duval
and Hill, 2007; Schade et al., 2005). At Font del Regàs, however, there
was a permanent disconnection between riparian groundwater and surface soil
layers, which may have limited the occurrence of microbial denitrification
and plant NO3- uptake during periods of stream hydrological
retention (Burt et al., 2002; Hefting et al., 2004).
Furthermore, in-stream NO3- release was accompanied by
NH4+ uptake (Obs : Pred < 1), suggesting that in-stream
nitrification prevailed at the valley reach. Previous studies have reported
sustained in-stream nitrification in well-oxygenated, slow water flowing,
hyporheic zones (Dent et al., 2007; Jones et al., 1995; Triska et al., 1990), and also when stored
leaf packs are rich in organic N and labile carbon (Mineau et al., 2011; Starry
et al., 2005). The two aforementioned explanations suit Font del
Regàs because the valley reach had inputs of N-rich leaf litter
(Bernal et al., 2015) and a well-oxygenated hyporheic zone
(∼ 7 mg O2 L-1; unpublished data) during periods of
stream hydrological retention. Moreover, in-stream nitrification in summer
could be stimulated by warm water temperatures (Laursen and Seitzinger,
2004) and both low discharge (< 30 L s-1) and stream depth
(< 15 cm), which ultimately could favor the contact between
nutrients and the microbial communities. Alternatively, differences in
NO3- and NH4+ concentrations between the headwater and
the valley reach could be explained by hydrological mixing with unaccounted
water sources, such as deep groundwater (Clément et al., 2003) or
riparian N-rich soils (Hill, 2011). However, these two
explanations were discarded because small mismatches between observed and
predicted Cl- concentrations indicate that the mixing model included
the main water sources contributing to stream discharge. Together, these
results suggest that processes occurring within the stream surface channel
or in the hyporheic zone can overwhelm those occurring at the
stream–riparian edge, especially during periods of high hydrological retention.
During the dormant period, when the two reaches gained water from the
riparian groundwater, Obs : Pred ratios at the downstream site were ≥ 1
for both NO3- and NH4+. This finding does not support
previous studies showing that riparian zones increase their N buffer
capacity from headwaters to valley bottom (Montreuil et al., 2011; Rassam et al.,
2006). For NO3-, this pattern could be explained by limited
riparian denitrification, given that (i) NO3- availability was low
in groundwater arriving from uplands (< 1 mg L-1; unpublished
data), and (ii) groundwater and organic-rich soils were hydrologically
disconnected even during the dormant period. Additionally, high rates of N
mineralization and nitrification in the riparian soil during winter
(0.84 ± 0.23 mg N kg-1 day-1) could promote N export from the
riparian zone to the stream at the valley reach (Lupon et al., 2016).
The influence of in-stream N cycling on N export ultimately depends on water
fluxes and the hydrological exchange between riparian and stream ecosystems,
which vary over the year. During the vegetative period, NO3-
fluxes entering and exiting the valley reach were similar (median = 8.8
and 8.9 mg N s-1, respectively) mostly because the increase in stream
NO3- concentration at the valley reach was counterbalanced by the
loss of water from the stream towards the riparian zone induced by riparian
ET. Otherwise, NO3- export to downstream ecosystems would have
been 15 % higher. Similarly, during the dormant period, there were no
differences between input and output NO3- fluxes at the valley
reach, but in this case discharge and NO3- concentrations were
similar between the top and the bottom of the reach (Q= 110
vs. 113 L s-1 and NO3-= 0.166 vs. 0.168 mg N L-1). These
back-of-the-envelope calculations highlight that riparian ET and
stream–riparian hydrological exchange can substantially influence stream N
fluxes during some time windows of the year, despite it having small
implications for N fluxes at annual scale.
Conclusions
Our study adds to the growing evidence demonstrating that riparian ET is a
key process for understanding temporal patterns of stream discharge and
hydrological processes at the stream–riparian edge in small forested
catchments, despite its modest contribution to annual water budgets
(Folch and Ferrer, 2015; Medici et al., 2008). Riparian ET strongly controlled the temporal pattern of net
groundwater inputs and stream discharge across daily and seasonal scales.
From a network perspective, the influence of riparian ET on stream hydrology
increased along the stream continuum and promoted stream hydrological
retention at the valley reach. In contrast to previous studies, high stream
hydrological retention was accompanied by increases in nitrate
concentrations, likely due to in-stream nitrification enhanced by low stream
flows, large stocks of N-rich leaf litter, warm conditions, and well-oxygenated hyporheic zones. In addition, we found no clear evidence of
riparian effects on stream N dynamics during the dormant period. Our
findings highlight that riparian ET can regulate the spatiotemporal pattern
of stream-water fluxes in Mediterranean regions and question the N buffering
capacity of Mediterranean riparian zones at catchment scale.
Data availability
The data sets used in this paper can be obtained from the authors upon request.
The Supplement related to this article is available online at doi:10.5194/hess-20-3831-2016-supplement.
Anna Lupon, Susana Bernal, and Francesc Sabater designed
the experiment. Anna Lupon, Susana Bernal, and Sílvia Poblador carried
them out. Anna Lupon performed all laboratory analysis. Anna Lupon analyzed
the data set and prepared the manuscript with contributions from
Susana Bernal, Sílvia Poblador, Eugènia Martí, and Francesc Sabater.
Acknowledgements
We are thankful to Ada Pastor and Lídia Cañas for their invaluable
assistance in the field, and to Dani Nadal for providing data on riparian
tree evapotranspiration. Financial supported was provided by the Spanish
Government through the projects MONTES-Consolider (CSD2008-00040-MONTES),
MEDFORESTREAM (CGL2011-30590), and MEDSOUL (CGL2014-59977-C3-2). Anna Lupon was
supported by a FPU PhD fellowship from the Spanish Ministry of Education and
Science (AP-2009-3711) and the MEDSOUL project. Susana Bernal work was funded by the
Spanish Research Council (JAE-DOC027), the Spanish CICT (Juan de la Cierva
contract JCI-2008-177), European Social Funds (FSE), and the NICUS
(CGL-2014-55234-JIN) project. Sílvia Poblador was supported by a FPI PhD fellowship from
the Spanish Ministry of Economy and Competitiveness (BES-2012-054572). We
also thank site cooperators, including Vichy Catalan and the Catalan Water
Agency (ACA) for permission to sample at the Font del Regàs catchment.
Edited by: C. Stamm
Reviewed by: two anonymous referees
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