Water samples and DOC analysis
Water samples from 08 rainfall events in 2016 and 2017 were analyzed. In
each rainfall event, we collected samples from bulk precipitation (BP)
(n=1 for the hillslope) and different forest hydrological compartments:
throughfall (TF) (n=5); under-litter (UL) (n=5) from each hillslope
position. In the SMT and VBT areas, soil solution (SS) samples were
collected at two depths: 50 cm (n=1) and 100 cm (n=1). The stream water
(SW) from Archer River (n=3) complete the each-event collection.
Thus, a total of 384 water samples were analyzed about their DOC
concentrations, distributed as follows among the forest compartments: BP
(n=8); TF (n=160); UL (n=144 – because in EV5 little water was obtained
in each collector and the analyzes were made with samples composed by
the 05 collectors of each area); SS (n=32 – 16 in each depth); and from
the Archer River (n=24).
The BP samples were collected with a handcrafted PVC (polyvinyl
chloride) rain gauge, placed in the no-canopy area: administrative
office of the Tijuca National Park. This rain gauge had a funnel
positioned at the upper opening of the PVC duct, where a “ping-pong”
ball was positioned to allow water to enter and make evaporation losses
difficult (figure 3).
The TF samples were collected with rain gauges identical to the cited,
positioned below the forest canopy (± 1.4 m above ground) and one in
each corner and one in the center, in a previously delimited 300
m2 rectangular (12 x 25 m) plot in each area. With the
same distribution in each hillslope position, the mini-chute collectors
were installed between the litter stock and the soil mineral horizon for
UL samples. The mini-chute was handcrafted using a PP (polypropylene)
plastic dustpan, which drained water flows into an LDPE (Low-density
polyethylene) bottle through a PVC drainage tube (ø 6 mm).
Soil solution samples were collected with handcrafted suction lysimeters
consisting of a PVC pipe (1/2”) with a porous ceramic capsule glued to
the lower tip (positioned inside the soil) and the upper opening sealed
with a pierced rubber stopper through which a PVC tube (ø 4 mm) passed
connecting the inside-bottom of the lysimeter to the outside
environment. At the time of collection in the other compartments, a -68
kPa pressure was applied inside the lysimeters with a manual vacuum pump
connected with the PVC tube. The tube was closed by a clip for keeping
the vacuum within the lysimeter. Approximately 24 hours later, new
tension was applied to remove accumulated water within the lysimeter.
Water samples from Archer River were always collected between 24 and 48
hours after rainfall ended, with it has already returned to baseflow.
The Archer River flow level was monitored by the Alert-Rio System
website (spate risk monitoring) where real-time water level data is
exposed, acquired by automated equipment positioned 650 m downstream of
the collecting areas (Mayrink Chapel Station), maintained by the
Environmental State Institute (INEA-RJ).
Except for lysimeters, all sampling instruments were washed with
deionized water after each collection and did not remain in the field at
all times. The field positioning of the collectors was based on rainfall
forecasts to reduce organic dust or debris accumulation. In the
laboratory, all samples were filtered with Macharey-Nagel 0.45 µm pore
size PES (Polyethersulfone) syringe-filters, kept refrigerated (05° C)
and analyzed within 5 days from the collection, using a Shimadzu TOC-L
equipment, which uses the acid oxidation method.
Intended to approach the leaching from each forest compartment, a second
data set for TF and UL was presented (alternative to field-values),
obtained by subtracting the mean values of previous compartment DOC
concentration from field-values and denominated: canopy leachate (CL)
and litter leachate (LL), respectively. From each TF field sample
values, the same value (BP DOC) was subtracted to all hillslope
positions at each event (CL obtained), while from each UL field sample
values, the mean values of TF from each respective hillslope position
was subtracted (LL obtained).
The study was based on a spatial analysis given on a comparison between
the different hillslope positions and a temporal analysis based on the
comparison between the rainfall events. For the temporal analysis, the
hillslope compartment DOC data (TF mean between hillslope positions, n =
20 per event for TF-CL and UL-LL) obtained in the different events were
tested as a dependent variable against characteristics of rainfall
events and the antecedent rainfall context. The (independent) variables
used were: rainfall volume (RfV, mm); Rainfall intensity (RfI,
mm.hour-1); previous drought days (PdD, number of
days); and antecedent rainfall (AcR, mm) for three time-ranges (10, 15
and 30 days). These data were obtained from the Mayrink Chapel Weather
Station.