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.