Hydrological observations
In this article we use two sources of field data, one derived from the
compilation of previous studies of our own and other teams working in
the region, and the other originated in a specific long term monitoring
program in the El Morro catchment motivated by the last and most intense
pulse of stream incisions in the area, that took place in February of
2015. We complemented these data sources with several remote sensing
tools used to characterize structural and functional changes in
ecosystems.
To characterize regional decadal changes in groundwater level we took
advantage of an existing synthesis of well surveys (BRS, 2002) and
additional records by local farmers. We identified 23 sites in the
catchment with observations between 1966-1980 (most measured in 1975),
and 4 additional sites measured in 1995. Some of these sites had more
than one record through time. For these 27 sites we obtained a water
table depth estimate for recent times (1999 to 2020, mean 2007) based on
(i) direct measurements or records by farmers (11 sites) and (ii)
signals of surface waterlogging or ponding by rising water tables
obtained through visual inspection of high resolution imagery from
Google Earth as described below in the section about ecological
observation (16 sites). In the case of wetlands that were incised we
consider water table depths before and after the incision considering
the stream bed position below the surrounding surface as the new
position. We also took advantage of existing isolated base flow records
obtained in the developing streams by our team between 2008 and 2011
(Contreras et al., 2013) and after the last incision episode in 2015 and
by others in 2008 (Barbeito, 2008). Combining the earliest groundwater
level measurements available with our more recent estimates, together
with the wetland and incision emergence mapping described above and
available vertical electrical soundings (Barbeito, 2008), indicating
depth to the crystalline bedrock; we generated a 2D, 17 km-long
vertical, WSW-ENE-oriented cut representing ecohydrological changes at
the high-intermediate belt of the Quebrachal and Río Nuevo sub
catchments over the last 45 years (Figure 2a), for this purpose we used
all available observation within a range of 600 m at the sides of the
transect line.
In April of 2017, 26 months after the last and most intense incision
episode, we initiated a cycle of periodic water table depth and stream
flow measurements mainly focused in the Río Nuevo sub catchment but
complemented with observation in La Guardia and Quebrachal sub
catchments. Aimed to follow the behavior of the groundwater and stream
system to new sapping episodes, the period covered up to the present has
been drier than the long term average for the meteorological station of
Villa Mercedes (496 vs. 608 mm y-1), offering a good
opportunity to explore the mechanisms highlighted above as ecosystems
become more sensitive to groundwater contributions and streams spend
most time under their base flow regime.
We installed a full meteorological station (Davis Instruments) at Site A
to complement an existing network maintained by a provincial public
agency which has three operating stations within or close to the
catchment (Villa Mercedes, Coronel Alzogaray, La Esquina; Figure 2a).
All these stations provided hourly precipitation, temperature, wind
direction and velocity and relative humidity data. We obtained an
integrated averaged daily precipitation series after accounting for gaps
(<5% of the data). We established a network of 16 monitoring
groundwater wells (Figure 2a), that included pairs of wetlands with
their adjacent croplands in the higher (sites A and B) and intermediate
belts (site D), the largest sediment deposit and its adjacent cropland
(site E) in the lower belt, a transect running 1.2 km away from the
deepest Río Nuevo incision along paired forest-cropland stands with
three sites in each vegetation type (site C) and two additional wells
sampling a cropland adjacent to the Quebrachal incision and a suburban
area in the lowest segment of the Río Nuevo. All these wells were hand
augered at least 1.5 m below the water table and cased with cribbed PVC
pipes. The bottom of the wells ranged from 2 to 16 m of depth. These
wells were monitored three times a year manually and sampled for
chemical and stable isotopic composition (data not shown here) after
purging. Several wells have to be deepened as the water table levels
retreated. With different degrees of completeness, water table levels
were also recorded hourly with pressure transducers connected to data
loggers (Campbell Scientific Instruments).
Streamflow was measured monthly during the first year and three times
per year thereafter at 6 locations (Figure 2b). These included the
Quebrachal stream close to its terminal zone (Site 1), two tributaries
of the Río Nuevo (Sites 2 and 3) in the higher catchment and the same
stream at the end of the lower catchment (Site 4). At this same point we
gaged the La Guardia stream just before its convergence with the Río
Nuevo (Site 5) and then the two merged streams after the flow through a
5 km-long rectified tract (Site 6). We could complement base flow series
with data obtained before (Sites 1 to 4 and 6) and shortly after the
incision episode of 2015 (Sites 1,3 and 5). We also included in the
analysis older data for other sites, such as an active tributary of the
Río Nuevo that dried after 2015 (Site 7), the higher and oldest segment
of Quebrachal (Site 8) and the oldest segment of the Río Nuevo before it
was deepened in 2015 (Site 9). In all cases flow was gaged using an
electromagnetic velocity sensor (Marsh-Mc Birney Flo-Mate 2000) at 10 to
25 positions across the section of the stream. At sites 5 and 6 we
performed several attempts to obtain continuous flow gaging that were
hampered by the unstable nature of the streambed and high sediment and
plant debris transport. Stage records obtained with pressure transducers
in the stream bed first, and with radar distance meters under bridges or
culverts next, proved to be unreliable, yet they were useful to
qualitatively identify peak flow events and qualitatively sort them into
minor and major ones. These provided the context for manual peak flow
measurements performed during two of these events and used to obtain a
maximum boundary estimate of the contribution of peak flow to stream
discharge during the study period.