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\begin{document}

\title{Plants vs. Streams: Their groundwater-mediated competition at ``El
Morro'', a developing catchment in the dry plains of Argentina}



\author[1]{Esteban Jobbágy}%
\author[2]{Santiago Lorenzo}%
\author[1]{Ricardo Páez}%
\author[3]{Nicolás Buono}%
\author[1]{Yésica Díaz}%
\author[1]{Victoria Marchesini}%
\author[4]{Marcelo Nosetto}%
\affil[1]{Instituto de Matematica Aplicada San Luis}%
\affil[2]{Instituto Nacional de Tecnología Agropecuaria}%
\affil[3]{Universidad Nacional de la Plata}%
\affil[4]{Instituto de Matemática Aplicada San Luis}%


\vspace{-1em}



  
  \date{June 18, 2024}


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\selectlanguage{english}
\begin{abstract}
Our understanding of the mechanisms routing precipitation inputs to
evapotranspiration and streamflow in catchments is still very
fragmented, particularly in the case of saturated flows. Here we explore
five mechanisms by which plants and streams compete with each other for
water, based on multiple scales of observations in a flat semiarid
sedimentary catchment of central Argentina subject to abrupt
hydrological transformations. Since the 80s, the ``El Morro'' catchment
(1334 km2, -33.64\selectlanguage{ngerman}°, -65.36°) experienced a fast expansion of crops over
native forests and grasslands, rapid water table level rises
(\textasciitilde{}0.3 m y-1), spontaneous expansion of wetlands and
permanent streams by groundwater sapping. Based on episodic and
continuous groundwater level, stream flow, and remote sensing data we
show that plants not only take away water from streams by drying the
unsaturated zone (mechanism 1), but by tapping the saturated zone in the
expanding waterlogged environments (mechanism 2) and in the upland
environments that remain uncultivated and display increasing tree cover
(mechanism 3). Conversely, streams take away water from plants through
pulsed bed-deepening and water table depression (mechanism 4), and
riparian and wetland zones burying with fresh sediments (mechanism 5).
While earlier work established widespread support for mechanisms 1
preventing stream formation, diurnal and seasonal fluctuations of water
table levels and base streamflow records in this study proved the
importance of mechanisms 2 and 3 under the current high-water table
conditions. These data together with remotely-sensed greenness showed a
growing but localized relevance of mechanism 4 and 5 as the stream
network developed. The distinction of recharge- vs.
topography-controlled groundwater systems is useful to organize the
interplay of these concurrent mechanisms. Findings point to the
unsaturated-saturated contact zone as a crucial and dynamic hub for
water partition and for ecological, geomorphological, and hydrological
knowledge integration.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


\textbf{Introduction}

Although the critical role of water inputs supporting the growth of
plants and the flow of streams is indisputable, the way in which these
two major fates of precipitation may compete with each other on a given
territory is not always obvious and represents, in fact, a very active
front of scientific inquiry. Moreover, this quest is perhaps one of the
main attractors of the convergence of Ecology and Hydrology over the
last decades. How any given~ piece of land, and more operatively, each
catchment, partitions the water that it receives from precipitation
between evaporative losses (among which plant transpiration is usually
dominant) and liquid losses (among which stream flow is generally the
main protagonist), is something that has been quantified for more than a
century using multiple approaches and that has sparked key integrative
theoretical developments such as the Budyko curve (Budyko, 1974; Gentine
et al., 2012). It is notable, however, that our knowledge about the
mechanisms dictating this partition is still very fragmented (McDonnell
et al., 2007). In which ways plants can ''capture'' water that could
eventually reach streams or, conversely, what are the mechanisms that
allow streams to ``capture'' water that may be potentially usable by
plants? Particularly relevant but disconnected are our representations
of the role of saturated flows dictating a ``push and pull'' dialog
between plants and groundwater (Fan, Miguez-Macho, Jobb\selectlanguage{ngerman}ágy, Jackson, \&
Otero-Casal, 2017) and eventually, streams. In this contribution we
explore conceptually how the competition for water between plants and
streams may operate in a dry basin, proposing a list of key mechanisms
that involve groundwater-mediated interactions. Plant-stream competition
should be particularly evident in dry regions (PET \textgreater{} PPT),
where potentially all water inputs can get evaporated (no energy
limitation) and the outcomes of the ``push and pull'' process should
leave a strong imprint both on plants, which would increase their growth
as their capture of limiting water resources becomes more exhaustive,
and on streams, which can potentially have a total suppression of their
flow. In this contribution, we illustrate the proposed mechanisms and
evaluate their relevance both for land ecosystems and hydrology with
data from a semi-arid sedimentary basin subject to strong vegetation
changes and, mostly as their consequence, to an extraordinary rapid
process of novel stream formation through sapping and surface erosion.

A simplified view of the atmosphere-soil-plant/stream continuum can
assume that all the water consumed by vegetation comes from the
unsaturated zone and that any by-pass or displacement flow escaping root
access becomes free to rich the saturated zone and is no longer
accessible to plants. This is still the prevalent representation of many
models that can successfully describe the partition of
evapotranspiration and streamflow in many catchments (Rahtjens, Oppelt,
Bosch, Arnold, \& Volk, 2015) based on what we can define as the single
most relevant mechanism that plants have to take away water from streams
(i.e. unsaturated uptake, mechanism 1, figure 1). In dry regions where
evapotranspiration is water limited, flat and loose sedimentary
substrates occupied by deep-rooted vegetation often display an
exhaustive exploration of the wettable soil volume, leading to the total
suppression of groundwater recharge through ``mechanism 1''. In these
settings any streamflow is limited to storm run-off episodes and there
is no observable base flow (Scanlon et al. 2006; Santoni, Contreras, \&
Jobbágy, 2010). While simple and powerful enough to explain the behavior
of many dry catchments, unsaturated uptake runs short to explain many
biological and hydrological phenomena such as the presence of
exceptionally green vegetation patches in deserts or the fluctuating
daily or seasonal reduction of stream base flow during periods of high
water demand and no rainfall inputs.~ Plant roots access saturated water
directly, as in the obvious case of riparian or wetland environments
were specific vegetation types adapted to the anoxic conditions of the
substrate thrive (Lowry, Loheide, Moore, \& Lundquist, 2011), or
indirectly by tapping the capillary fringe fed by the saturated zone, as
seen (not as easily) with phreatophytic ecosystems (Naumburg,
Mata-González, Hunter, \& Martin, 2005; Jobbágy, Nosetto, Villagra, \&
Jackson, 2011). Therefore, saturated uptake needs to be included as a
``second chance'' that plants have to take away water from streams with
the unique contribution of lateral connectivity allowing vegetation in
one place to use water that plants did not use in another. The most
acknowledged component of saturated uptake is the consumption of ground
or stream water by wetland and riparian communities in the small
fraction of landscapes occupied by topographic low positions and water
body shores (i.e. focalized saturated uptake, mechanism 2, figure
1)(Bond et al., 2002). Yet, saturated uptake can be significant in the
matrix of the landscape as well, when a more widespread contact between
roots and the capillary fringe takes place (i.e. distributed saturated
uptake, mechanism 3, figure 1), something that appears to be
reciprocally and dynamically modulated by the vertical location of roots
and water tables and highly sensitive to vegetation, climate and water
managements shifts (Jobbágy \& Jackson, 2004; Nosetto, Jobbágy, Jackson,
\& Sznaider, 2009, Fan et al., 2017). While being increasingly
considered in catchment models (Immerzeel \& Droogers, 2008, Doble \&
Crosbie, 2017), saturated water uptake by plants introduces a novel
possibility for streams to take away water from plants.

Far from passively receiving the hydrological left-over of the three
main water consumption mechanisms pathways highlighted above, streams,
as active geomorphological agents, can take away water from plants when
they remove and deposit sediments. By deepening their beds, streams can
increase their groundwater capture into base flow, transiently enhancing
hydraulic gradients but more permanently by impairing mechanisms 2 and 3
as water tables plunge to reach a new equilibrium and zones where plant
roots can reach it shrink or disappear (i.e. streambed deepening,
mechanism 4, figure 1) (Bravard et al., 1998; Wurster, Cooper, \&
Sanford, 2003; Schilling, Zhang, \& Drobney, 2004). Another way in which
streams can actively restrict plant water consumption is by burying land
ecosystems with new sediment deposits, which are more likely to affect
active riparian and wetland zones with higher water consumption than any
other landscape component (i.e. sediment deposit, mechanism 5, figure 1)
(Kui \& Stella, 2016). The relative timing of ecological succession and
geomorphological changes is critical defining the resulting balance of
mechanisms 1,2 and 3 vs. 4 and 5. Ecological changes such as the
establishment of riparian plants with deeper roots where stream
incisions have gone deeper or the recovery of buried trees where streams
have deposited fresh sediments (Rivaes et al., 2013; Kui \& Stella,
2016), may outpace the slower work of streams, yet landscapes in which
their erosive power get rapidly unleashed as result of climate or land
use changes can match their forces offering an ideal observatory to
explore their interaction.

\textbf{The birth of a catchment}

In its subtropical and temperate belt, South America hosts a vast
aeolian plain covered by Quaternary loessic deposits and sandy mantles
spanning semiarid to subhumid climatic conditions along a 900 km
west-east extent (Zárate \& Tripaldi, 2012). The few rivers crossing the
driest western half of this region have their sources in adjacent
mountain systems and loose water as they traverse it (Marchesini,
Nosetto, Houspanossian, \& Jobbágy, 2020; Poca, Nosetto, Ballesteros,
Castellanos, \& Jobbágy, 2020). A surprising exception emerged in recent
decades, however, when an extremely abrupt groundwater sapping process
started carving and connecting permanent stream segments that now
configure what has been named the ``El Morro'' catchment (Contreras,
Santoni, \& Jobbágy, 2013). Located in the transition from the isolated
rock outcrop of the ``El Morro'' caldera (Sruoga, Ibañes, Japas, \&
Urbina, 2017) and the adjacent aeolian plain, this topographically
well-defined basin had its first active surface water outlet opened less
than four decades ago and continues to gain and connect tributaries
after discrete but increasingly intense sapping episodes caused by
steadily rising groundwater levels. The deepest incision, where the Río
Nuevo stream flows permanently, is now 30 km long with the largest
cross-sections reaching 20 m of depth and 80 m of width. In its exposed
walls, \textasciitilde{}5 m-deep aolian Holocene material laying over
pre-Holocene fluvial beds suggests that the emerging streams are not
only a new feature of this landscape in historical times, but throughout
the last interglacial period (Tripaldi \& Forman, 2012). Where exposed,
the crystalline basement or its overlaying late Tertiary calcretes show
signs of fluvial erosion suggesting that a paleo catchment buried with
aeolian material is being reactivated in recent years.

Originally covered by grasslands and open woodlands (Oyarzabal et al.,
2018), the El Morro watershed has been progressively cultivated since
the beginning of the twentieth century and today more than half its area
is under dryland agriculture with native caldén (\emph{Prosopis
caldenia} ) forests covering less than 10\% of the area (Contreras et
al., 2013). Accompanying these changes, an increasing number of low
areas within croplands and to less extent native vegetation become
waterlogged by raising water tables, initiating a spontaneous succession
towards wetland communities (Contreras et al., 2013; Díaz, Jobbágy, \&
Marchesini, 2018). While increasing rainfall trends, seismic activity
and land use changes have been pointed as possible converging causes for
the novel and rapid hydrological transformation of the El Morro
catchment (Contreras et al., 2013), observations in this region and
other semiarid areas of the plain point to the onset of groundwater
recharge following the conversion of native vegetation to dryland
agriculture as the most determinant factor (Amdan, Aragón, Jobbágy,
Volante, \& Paruelo, 2013; Contreras et al., 2013; Giménez, Mercau,
Nosetto, Paez, \& Jobbágy, 2016; Marchesini, Giménez, Nosetto, \&
Jobbágy, 2017). The rapid shifts in groundwater level, the unusually
fast development of a stream network, and the semiarid climate making
both plants and streams highly sensitive to water supply, offer a unique
setting to explore the interplay of the five mechanisms described above.
Stimulated by this opportunity and the urgency to understand the causes
and possible adaptation or mitigation actions to cope with its negative
effects on people and nature, we initiated a long-term observation
program in the El Morro catchment. In this paper we present hydrological
and ecological observations derived from this program together with
previous observations and different sources of remote sensing
information to characterize groundwater level, stream flow, and
vegetation dynamics in three subcatchments of the El Morro basin at two
temporal scales (long-term: 20-50 years, short term: following the last
sapping event of 2015). We link these observations with the five
mechanisms described above to offer a preliminary quantification of
their relative weight shaping the current and projected partition of
water inputs between plants and streams in this semiarid context.

\textbf{Materials and Methods}

\emph{Study catchments}

The study area, at the western dry edge of the sedimentary plains of
central Argentina, sits in the ecotone of Espinal forests and Pampas
grasslands (Figure 2a). Its temperate semiarid climate shows a mean
annual temperature of 15.7 C with the coldest (July) and warmest
(January) months, having a mean minimum temperature of 1.0 C° and a mean
maximum temperature of 28.6 C°, respectively. Mean annual precipitation
(1903--2019, Villa Mercedes; -33.65, -65.42; 525 m)~ was 601 mm~
y\textsuperscript{-1}. Rainfall is concentrated in the warm season (70\%
between November and March) originating mainly in convective storms with
high spatial variability. A-type tank evaporation is 1640 mm
y\textsuperscript{-1} (2006-2009), reaching maximum values of 11 mm
d\textsuperscript{-1} during summer.

We run periodic observations in three adjacent sub-catchments of the El
Morro Basin that cover together 1334 km\textsuperscript{-2} and range
from 1100 to 500 m of elevation along 50 km in a NNW-SSE direction
(Figure 2a). These sub catchments show an east-west gradient of
developing age with La Guardia having completed its connection to the
main pre-existing collector (Quinto river) in 1986, followed by Río
Nuevo, connected in 2009 but still developing tributaries, and
Quebrachal, which is currently an isolated segment that infiltrates
without reaching the collector (Figure 2b). Río Nuevo has the deepest
and widest incision (20 x 80 m in 2020) and has generated the largest
sediment deposit in its lower section (8 km\textsuperscript{-2}deposited
in 2015), while La Guardia displays the shallowest incision and the
largest wetland area (Table 1). The most recent incision episodes took
place in September 2001, January-February 2008, December 2009 and
February 2015, following periods of several months of high rainfall and
affecting mainly the Río Nuevo and Quebrachal sub catchments (Contreras
et al., 2013; Buono, Menéndez, Cáceres, Jobbágy, \& Nosetto, 2018). The
landscape of these sub catchments includes a higher section (1100-700 m
of elevation, 2\% slope) with isolated rock outcrops and dissected
surfaces in its high end and the initiation of most incisions in
wetlands on its low end, an intermediate section (700-550 m of
elevation, 0.7\% slope) with a transverse dune surface and the greatest
depth to the crystalline basement (\textgreater{}50 m), associated to a
local tectonic depression (Barbeito, 2008), and the largest incisions;
and the lowest section (550-500 m of elevation 0.5\% slope) with a flat
sandy mantle that has hosted most of the recent fluvial deposits (Ríos
2020). Soils are \emph{Entic Haplustolls} in the higher catchment and
\emph{Typic Ustipsamments} and \emph{Typic Ustorthents} in the
intermediate and lower catchment (Galván \& Collado, 2009).

Dryland agriculture is the dominant activity followed by cattle raising
on pastures and native vegetation. Dominant crops are soybeans and
maize, growing in a spring-summer season that starts slightly after the
onset of rains between October and December and ends in April, similarly
to what is seen throughout the western plains of Argentina (Gimenez et
al., 2020). No till prevails and soils remain free of weeds during the
fallow periods. These agricultural systems have progressively replaced
not only native vegetation but perennial pastures of the exotic
grass\emph{Eragrostis curvula} , which where the basis of the livestock
production systems that prevailed until the end of the twentieth century
(Viglizzo, Roberto, Lértora, Lopez Gay, \& Bernardos, 1997), and are
still present as minor component of the landscape. Native vegetation
includes perennial tussock grasslands and forests dominated by the
leguminous trees \emph{Prosopis caldenia} (caldén) and \emph{Geoffrea
decorticans} (chañar) (Oyarzabal et al., 2018, Cabido et al., 2018).~
Wetlands have expanded in the area as a result of raising water table
levels (Contreras et al., 2013). Two types of spontaneous communities
are found, one related to freshwater environments in the
higher-intermediate belt of catchments, dominated by \emph{Cortaderia
selloana} tussocks and \emph{Typha sp.,} and the other associated to
saltier environments in the lower belt of catchments, dominated
by\emph{Tamarix ramosissima} (Natale, Zalba, Oggero, \& Reinoso, 2010;
Diaz et al., 2018).

\emph{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\textsuperscript{-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
(\textless{}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.

\emph{Catchment integration}

In order to estimate base water yields for different areas we considered
that base flows were generated either by the full watershed as defined
by its surface topography (i.e. topographically defined catchment) or,
alternatively, by a more restricted zone that was defined considering
the uniform spacing among the relatively parallel stream lines observed
in the region and recognized as the typical fishbone structures of
sapping erosion regimes (Grau Galofre \& Jellinek, 2017). We observed
distinctive regular spacings of 2.6 and 12.5 km in the
higher-intermediate and intermediate-lower belts of the catchment
(respectively upstream and downstream of the rock basement level drop
line, Figure 2A), and used those widths combined with the length of
streams crossing each zone to calculate their contributing area (i.e.
geometrically defined catchment)

\emph{Ecological observations}

We used complementary sources of remote sensing to characterize
structural and functional vegetation changes in the catchment. In first
place, we combined our field surveys of the region with detailed
satellite imagery from Google Earth (2000-2020) and aerial photographs
(1962, see Bogino \& Jobbágy, 2011) to describe the advent of new
wetlands, lagoons, sediment deposits, and incisions/streams in the
landscape. All these features were easily identified with high spatial
detail imagery but given its associated poor temporal resolution (up to
10 years in some cases), we narrowed down their timing with a
complementary analysis of Landsat imagery. In the case of wetlands, and
in order to obtain a single date for their advent to be used in the
regional water table depth analysis described above, we used the mean
between the latest date without an earliest date with evident surface
signals, as the time of the observation. In the case of stream
incisions, we adjusted this date to the most likely year of occurrence
based on the precipitation series and local observations by farmers.
Based on our experience observing wetland vegetation and groundwater
levels in the field we were able to distinguish sites with \textless{}0
(surface water) and 0-1 m of water table depth in the images, using
these ranges as an estimate for the long-term regional analysis
described above.

To obtain relative estimates of water uptake trends for the different
types of vegetation or vegetation change trajectories we used greenness
indexes. Although real evapotranspiration estimates are available from
platforms like MODIS, they tend to have very negatively biased estimates
which end being extremely dependent on surface greenness (Normalized and
Enhanced Vegetation Indexes, i.e. NDVI and EVI). In previous studies
across neighboring regions we found that the direct use of these indexes
provides better estimate of relative transpiration rates for cultivated
and native vegetation (Contreras, Jobbágy, Villagra, Nosetto, \&
Puigdefabregas, 2011; Nosetto, Paez, Ballesteros, \& Jobbágy, 2015).
Using 16-day and 250 m resolution NDVI MODIS data and a supervised
classification of the current (year 2018) vegetation into forests,
grasslands and pastures, croplands and wetlands together with
field/ground truth control points, we estimated greenness trends from
2000 to 2020 for all the pixels corresponding to each one of this cover
types. We used two criteria to illustrate greenness trends. In the first
one we obtained an average decadal trend comparing the first and the
second decade mean values for each pixel. In the second one, we obtained
linear regressions of mean annual NDVI in response to the number years
elapsed since 2000 for each individual pixel, computing the proportion
of them that showed significant trends (p\textless{}0.05) and averaging
the slope of that subgroup for each vegetation type. Since the low
spatial resolution of MODIS imagery (250 x 250 m) did not allow for a
precise description of wetlands and sediment deposits and comparisons
with their adjacent cropland or forest stands, we performed a
complementary analysis using LANDSAT 8 images with a monthly and a
higher spatial resolution (30 x 30 m) for the years before and after the
last erosion episode (2013-2020). We analyzed three stable vegetation
types (forests, croplands and wetlands) and three vegetation transitions
during that period (croplands to wetlands, croplands to deposits,
incised wetlands). In all cases we had at least 6 stands except in the
case of incised wetlands where a single site was considered.~
Statistical greenness differences among the first five cases within each
year of analysis were evaluated using ANOVA.

\textbf{Results}

\emph{Groundwater dynamics}

Within an overall context of raising groundwater levels, observations
showed different water table behaviors depending on the temporal scales
considered and the locations of observation points within the whole
catchment and, more locally, regarding landscape features and vegetation
types. Below we introduce long-term trends and their manifestation in a
representative vertical section of the landscape, focusing afterwards on
the more detailed dynamics captured after three years with periodic
observations and, finally, selecting three illustrative cases of
seasonal, daily and hourly level fluctuations.

Over the last four decades the El Morro catchment experienced a dramatic
rise of groundwater levels leading to an expansion of the areas occupied
by wetlands, lagoons and seepage zones. Water table levels raised on
average 3.06 meters per decade across 27 wells where groundwater depth
was measured in the past (most commonly in 1975) and measured or
estimated again over the last decade (Figure 3). Noticeably, while the
majority of those points showed level raises throughout one or two
measuring intervals, those located within or very close (\textless{}
500m) to stream incisions (6 sites) showed absolute water table depth
declines that were largest in the intermediate belt of the catchment. In
the lower belt, a site that received the thickest deposit of fluvial
sediments showed a relative decline of its water table level as the
surface raised (Figure 3). The observed groundwater level raises were
highest in the higher belt of the catchment where the most extreme case
approached a 37 m level ascent and levels climbed on average 4.83 meters
per decade (initial mean depth of 15.33 m)(Figure 3). That area was
followed by the intermediate and lowest (terminal plain) belts where,
ignoring the incision cases, decadal raises averaged 3.50 and 3.77
meters per decade, respectively (initial mean depths of 11.55 and 7.85
m, respectively)(Figure 3). The smallest changes were observed in the
lower belt of the catchment, where the average ascent was 0.97 meters
per decade, reflecting the fact that water tables were already shallow
at the beginning of the study period there (initial mean depth of 3.6
m)(Figure 3).

Based on the higher availability of data and fast changes observed in
the contact zone between the higher and intermediate belts of the
catchment we reconstructed a transverse vertical cut depicting major
landscape and groundwater shifts (Figure 4). By the year 2000 the water
table in this section of the catchment (covering the full sub catchments
of Quebrachal and Río Nuevo streams), approached the surface in multiple
low positions that were disconnected from groundwater in the late 70s.
Incisions formed in many of the local low positions in west to east
temporal sequences (Figure 4). A relatively uniform lateral distribution
of incisions or wetlands with permanently running surface wetlands can
be appreciated along the transect with distances of 3.2, 3.2, 2.6, 1.9,
and 2.1 km (2.6 km on average) among them. A similarly regular but
larger spacing is observed in the intermediate and lower catchment,
averaging 12.5 km between the main collectors of each sub catchment.

Water table depth observations in the network of monitoring wells
reflected the dry conditions of the 3-year study period with an overall
deepening trend (Figure 5). However, important contrasts were found
between vegetation types and landscape positions. At site A, the
wetland, which appeared in 2000 and has the crystalline basement close
to the surface (see Figures 3 and S1), showed the most stable levels,
always less than 15 cm below the surface, likely as a result of
sustained seepage at that site (Figure 5a). The neighboring cropland
instead, showed seasonal oscillations (summer declines, winter raises)
over imposed to the general declining trend, suggesting that water
consumption during the period of high vegetation activity and
evapotranspirative demand, which is also the rainy season, outpaced
recharge fluxes during the study years (Figure 5a, d, e). Wells at site
B showed a more extreme, steady and parallel decline during the first
year (Figure 5b) (they had to be deepened to continue measurements
afterwards), revealing the likely effects of the recently carved or
deepened incisions flanking the site (Upper Rio Nuevo and Uke
tributaries), drying a wetland that emerged in the late 90s and
interrupting its stream (site 7 in Figures 2b and 6).~ A particular
aspect of this level decline is its constant rate in the wet and dry
season of the first year, which suggests drainage from below rather than
consumption from above as the likely cause of this behavior. Located by
the deepest incision of the catchment (enlarged from 9 to 20 m of depth
and 28 to 60 m of width in 2015), site C represented the deepest water
table situation showing a strong contrast for its three pairs of
forest-cropland wells (Figure 5c). There, the forest, similarly to the
cropland and wetland sites with shallower water tables, displayed
seasonal oscillations suggesting a pulsed consumption of groundwater by
the dominant winter-deciduous trees whose active roots were found
growing around sensors at 9 m of depth. Croplands, only 600 m away at
this site, showed a steady decline with very subtle seasonal rates of
change. Differences along the transect at this site suggest that wells
at position 1, \textasciitilde{}100 m away from the deepened incision
are closer to rich there a new equilibrium while those at positions 2
and 3 may still be yielding water towards the stream (Figure 5c).~ Two
additional monitoring wells not shown in the figure reflected similar
patterns with Site F hosting a cropland with deep water tables close to
the Quebrachal incision. That site followed croplands in site C until no
saturated conditions could be detected above the crystalline bedrock at
14 m of depth. Site G, in a suburban area of the low catchment affected
by periodic waterlogging followed the same pattern of the wetland at
site E.

Continuous monitoring of groundwater depth with pressure transducers
offered a more precise perspective on the recharge and discharge
dynamics of the study sites (Figures 6 and 7). At Site A we highlight
contrasting periods of net water gains (summer-fall 2018) and losses
(spring-summer 2019-2020), showing the contrasting behavior of the
wetland and the cropland. In the first of these periods, the wetland
displayed high daily level fluctuations that amplified progressively
following rainfall events (Figure 6, left panels), suggesting an
increasing reliance on groundwater as the rainfall pulse was consumed.
No fluctuations were observed in the cropland, that at that time was in
fallow stage prior to the establishment of a pasture (see NDVI in figure
6, left panel). A single and intense rainfall event in this period (74
mm on April 1 measured at the precise site) switched the wetland to a
flooded condition reducing but not eliminating fluctuations, likely as a
result of the shifting groundwater yield-depth relationship under
flooding. The same event and the following two triggered direct recharge
episodes under the fallow with level rises of 2.7 -- 3.8 mm per mm of
rainfall (or a specific yield of 0.26-0.37)(Figure 6, left panels). In
the second period, which followed three consecutive dry growing seasons,
the wetland displayed wider diurnal fluctuations with the same pulsed
reduction and gradual amplification in response to rainfall events
(Figure 6, right panels). These fluctuations averaged 15.2 cm
d\textsuperscript{-1} (daily maximum -- minimum level) and, based on
Loheide, Butler and Gorelick (2005) methodology, generated an average
groundwater level depletion of 16.6 cm d\textsuperscript{-1}.
Noticeably, in spite of its apparent reliance on groundwater, the
wetland greenness increased after rainfall events (see NDVI in figure 6,
right panel). The cropland in this period was covered by an active (and
typically deep-rooted) alfalfa pasture which was likely responsible for
sustained water table fluctuations averaging 1.6 cm
d\textsuperscript{-1} with an estimated mean depletion of 4.9 cm
d\textsuperscript{-1}. The largest rainfall pulse of this period (125 mm
in November 17-20), triggered a slow 36 cm level raise of 10 days in the
cropland which had a much lower velocity than the recharge pulses of the
previous study period (Figure 6, right vs. left panel), suggesting that
the level recovery resulted from the interruption of groundwater
consumption.

Deep water table levels at site C, likely favored by its neighboring
incision, provided an opportunity to explore vegetation-groundwater
interactions under conditions that could have been more common for the
catchment in the past (Figure 7). Sensor data provided here a more
detailed description of the seasonal departure of water tables at forest
stands with regard to their neighboring croplands (Figure 7 vs. Figure
5), showing that forest trees produce similar diurnal fluctuation,
seasonal depression and slow level recoveries after rainfall events than
those described for the alfalfa pasture above, yet they do so with water
tables that are 8-10 m deep (Figure 7). Groundwater uptake, as indicated
by daily water table fluctuations, started a few days after the
deciduous tree species leafed out in mid-October (see NDVI cycles in
Figure 7) and levels initiated their sustained and asymptotic recovery
when the trees gradually lost their leaves in late May, approaching
equilibrium with the surrounding cropland matrix throughout the dry (by
ecologically inactive) season (Figure 7). Diurnal water table level
fluctuations during the active period of the forest (October 15 - April
15) averaged 1.7 cm d\textsuperscript{-1} with an estimated mean
depletion of 3.1 cm d\textsuperscript{-1}. In the three consecutive
years of observations, levels dropped/recovered -89/47, -71/55 and
-79/57 cm in the forest site. The same net drop of -80 cm over the
three-year measurement period was observed in the cropland but without
seasonal fluctuations. No direct recharge episodes were documented at
any of these wells (Figure 7), yet slow and partial level recoveries in
response to rainfall pulses were observed in the forest, as depicted in
detail for a series of events in March 2019 (Figure 7, bottom left
panel) and recorded in November 2019 and January 2020 when of 15
cm-recoveries took place along 8 and 17 days following rainfall pulses
of 90 and 77 mm, respectively (Figure 7).

\emph{Stream flow and yield}

Periodic base flow measurements obtained during the monitoring program
together with previous data revealed contrasting seasonal and
multiannual trends across different streams in El Morro catchment
(Figure 8). During the three years of frequent measurements, a clear
seasonal pattern with higher flow in the dry (and cold) season and lower
flow in the rainy (and warm) season was evident at all sites~ (-31 to
-50\% in summer compared to the rest of the year, Table 1) except Site
2. Matching the seasonal patterns of water table depths (Figure 5),
these fluctuations illustrate the dominant effect of groundwater uptake
rather than precipitation inputs controlling stream base flow.
Noticeably, the only site with no seasonal fluctuation corresponded to a
segment of the Río Nuevo with deep incisions down to the gaging point
(riparian consumption less likely), whereas most intense fluctuation
were seen at site 3 and 5 (running dry in February 2020), which had
shallow incisions and a large fractions of their trajectory surrounded
by wetlands (Table 1). Older base flow data obtained before and shortly
after the last erosion episode show how sites 1, 2,~ and 3 gained flow
while site 7, which was flanked by the first two, got completely dried
out after being a high-yielding stream before (Figure 8, Table 1).
Matching water table observation at site B, this switch in stream
dominance suggests competition of neighboring incisions capturing
groundwater as they deepen. While showing no seasonal trends, the deeply
incised segment of Río Nuevo gaged at site 2 showed a slight but
significant decline in its base flow with time between (-23 l
s\textsuperscript{-1} per year between November 2016 and August 2020,
r2=0.60, p\textless{}0.01).

Base flow water yield estimates were obtained considering a topographic
(larger) and geometric (smaller) definition of the feeding catchment
area for each site (Table 1). These two criteria suggested specific
yields ranging from 4 to 56 and 8 to 159 mm y\textsuperscript{-1},
respectively, with the highest values corresponding to the upper Río
Nuevo sub catchment (Site 2) and the lowest to the Quebrachal sub
catchment (Site 1).~ Along the Río Nuevo catchment, water yields appear
to be highest in the higher belt (Sites 2 and 3), declining but
maintaining a positive contribution in the intermediate belt (site 4 and
``site 4 - (sites 2 + 3)'' in Table 1). In the final segment where the
merged~ La Guardia and Río Nuevo streams enter the terminal plain,
negative yields were consistently observed (site 6 and ``site 6 -
(sites~ 4 + 5)'' in Table 1). At this closure point, the total specific
yield of the catchment was 19/29 mm~
y\textsuperscript{-1}(topographic/geometric area definition)(Table 1).
While not accurately measured, a high-end estimate of peak flow
contributions achieved at this site suggested that it would add less
than 11\% to the total yield of the catchment (Supplementary material).

\emph{Ecosystem activity}

Vegetation activity as captured by NDVI from MODIS and LANDSAT images
showed divergent temporal trends for croplands, native vegetation and
wetlands (Table 2 and Figure 9). The long-term analysis of the whole El
Morro catchment based on MODIS data revealed a decreasing greenness
across the forests \textgreater{} grassland/pasture \textgreater{}
cropland (all differences being significant with p\textless{}0.01, Table
2). An increased vegetation activity with time, and likely water
transpiration, was observed comparing the second decade (2011-2020) with
the first (2000-2010) with average greenings of 5.5 and 3.5 \% for
forests and grasslands/pastures, respectively. In the same temporal
comparison, croplands and sediment deposits experienced a -1.1 and -6.7
\% drops, respectively (Table 2). The catchment as whole, displayed a
small decadal greenness increase of 1.3\% as the previous contrasting
trends compensated out, particularly between the dominant components of
grassland/pastures vs. croplands (Table 2).

With the higher spatial resolution of LANDSAT imagery, we targeted
vegetation types and change trajectories more precisely focusing on the
shorter period that encompassed the last erosion episode and our field
observations (Figure 9). Forest sustained the highest greenness
throughout the whole period while croplands the most temporally
variable.~ Pre existing wetlands and new wetlands emerging in croplands,
displayed a sustained and stable greenness raise (Figure 9). Deposits
landed on croplands in 2015 showed a sharp greenness decline followed by
a steady recovery that was still not completed but close to wetlands and
croplands four years later. The single dried wetland location (site B),
had a high greenness that matched that of forests until one year after
the new incisions drained the area. After that time its greenness
dropped steadily for three consecutive years approaching levels that are
close to those in the recovering sediment deposits (Figure 9).

\textbf{Discussion}

\emph{Ecohydrological changes at multiple scales}

The El Morro catchment underwent intense and interconnected
hydrological, ecological and geomorphological transformations over the
last 45 years. In the longer term (1975-present) we documented a
widespread increase in groundwater storage, accompanied by the expansion
of wetlands and the extension of a network of permanent streams opened
by episodic sapping events (Figure 2b, 3 and 4). Raising groundwater
levels and increased flooding have been reported for the same period in
different locations across the sedimentary plains of Argentina during
periods of higher than normal precipitation likely in connection with
long term shifts from native forests, grasslands and planted pastures to
continuous annual crop cultivation (Nosetto et al., 2015; Kuppel,
Houspanossian, Nosetto, \& Jobbágy, 2015; Gimenez et al., 2016, Giménez
et al. 2020; Alsina, Nosetto, \& Jobbágy, 2020). Similar transitions
have been documented in dry farmlands of North America, Australia and
Africa (George, McFarlane, \& Nulsen, 1997; Scanlon, Reedy, Stonestrom,
Pruderic, \& Dennehy, 2005; Leblanc et al., 2008). The peculiar context
of the El Morro basin, with higher slopes and closer depths to the rock
basement than most of the central plains of Argentina (Jobbágy et al.,
2008), is that this hydrological change has triggered an unusually rapid
and pronounced sapping process that seems to have no precedents at the
site for the whole Holocene (as suggested by sediment dating by Tripaldi
and Forman et al 2016). Under a similar context of deforested farmlands,
sapping and piping processes have been recently documented in a piedmont
loessic plains 600 km north of our site (Pereyra, Fernández, Marcial, \&
Puchulu, 2020), and massive sapping episodes may have accompanied the
long-dated expansion of cultivation in the loessic plains of China (Zhu,
2012).

In the mid-term (2000-present) the process of stream formation expanded
exponentially with episodes of increasing magnitude in 2001, 2008, 2009
and 2015 (Figure 2b and 4, Supplementary material). Meanwhile, two
opposite and coexisting trends of vegetation activity were observed. On
one hand, currently cultivated areas decreased their greenness, likely
in response to the replacement of pastures by continuous agriculture
(Contreras et al., 2013), but more so to the adoption of no-till
cultivation and the narrowing and delaying of the active growing season,
which are part of a general drought avoidance strategy (Gimenez et al.,
2020)(see Figure 7, top panel). On the other hand, remaining
uncultivated areas including forests and grasslands increased their
greenness, likely in response to rising water table levels favoring the
expansion of wetlands and the growth of forests (see Gonzalez Roglich,
Swenson, Villarreal, Jobbágy, \& Jackson, 2015 for similar forest trends
in other areas of the caldén belt). In the short-term (last erosion
episode to the present) our study shows that despite the relative low
precipitation of the last three years (Figure 9), wetlands kept
expanding and greening and streams flowing (Figure 8). Groundwater
uptake by plants appeared to be widespread in forests but limited to
shallow water table depths (\textless{} 2 m) in croplands (Figures 5, 6
and 7 and Supplementary material). Mean water yields for the three years
of periodic observations approached 20 mm~ y\textsuperscript{-1} (3.3\%
of long-term average rainfall inputs) with higher yields in the most
deeply incised sub catchment sections of Río Nuevo (30-50 mm~
y\textsuperscript{-1}). This output was considerably stable for a dry
region, with a seasonality governed by water demand rather than
rainfall, since lowest flows occurred in summer when rainfall was
highest but water uptake too (Figures 5,6, 7 and 8, Table 1). The
observed water yields at the outlet of the catchment are still lower
than those predicted by the most widely used global empirical model
(Zhang, Dawes, Walker, 2001), which under pure grassland or forest cover
predicts yields of 83 and 33 mm~ y\textsuperscript{-1}, respectively,
based on the mean climatic conditions of our site.

\emph{Mechanisms of plants vs. streams competition}

The unusual temporal match between the vegetation change (typically
faster) and stream expansion processes (typically slower) at the El
Morro catchment combined with its aridity offer a unique opportunity to
assess the five mechanisms by which plants and streams may compete for
water. Unsaturated water uptake (\textbf{mechanism 1,} Figure 1), has
been likely the dominant source of transpired water under the deep
groundwater table conditions before the 70s (Figure 4). Diverse sources
of evidence point to the absolute prevalence of mechanism 1 and its
capacity to fully inhibit stream flow in dry sedimentary plains in
Argentina and elsewhere, including geoelectrical profiles and deep
sediment cores showing dry vadose zones with an uninterrupted buildup of
atmospheric chloride accumulation under native vegetation (Santoni et
al., 2010; Jayawickreme, Santoni, Kim,Jobbágy, \& Jackson, 2011; Amdan
et al., 2011 in our study region; Scanlon et al., 2006 for a global
synthesis).

At the El Morro catchment, like in many semiarid sedimentary regions
subject to intense cultivation, mechanism 1 has been relaxed thanks to
the reduced capacity of annual crops to exhaustively use of vadose
moisture (Peck \& Williamson, 1987; George et al., 1997; Santoni et al.,
2010; Giménez et al., 2016). As a result, water table levels have
raised, likely following pulses of high rainfall (Giménez et al., 2016),
creating the opportunity for a more widespread contact between plant
roots and groundwater. While in first place, such hydrological shifts
may favor localized groundwater use in lowland areas or incipient
streams by waterlogging-tolerant species (mechanism 2), contact zones
can be widespread enough to favor a more distributed use of groundwater
in the whole catchment (mechanism 3). At our study catchment, the
progressive appearance of wetland communities displaying an intense
groundwater consumption was manifested in water table depth and
greenness patterns (Figures 4, 6 and 9), providing qualitative evidence
for \textbf{mechanism 2} (Figure 1). Complementary, base streamflow
provided more quantitative evidence, with stream segments having shallow
incisions and greater proportions of their shores flanked by wetlands
showing stronger summer flow drops than the deeply incised upper Río
Nuevo segment that was seasonally invariant (Table 1). Assuming that
summer baseflow drops reflect the effect of mechanism 2, it would have
accounted for 48 and 20\% water yield reduction in summer and the whole
year, respectively, at the closure of the catchment (Table 1). Mechanism
2 may not only capture groundwater feeding streams but streamflow
directly as well, yet, disentangling these two sources has proven
difficult (Dawson \& Ehrlinger 1991; Bowling, Schulze, \& Hall, 2017).

Distributed use of groundwater by vegetation (\textbf{mechanism 3} ,
Figure 1) was confirmed in the El Morro catchment. With water table
depths \textgreater{}7 m caldén forests used groundwater (Figure 7), as
documented earlier for forest relicts north of our site (Gimenez et al.,
2016). More generally, across the semiarid and subhumid loessic plains
of Argentina, Hungary and China tree stands within herbaceous matrices
have often shown intense groundwater use (Jobbágy \& Jackson, 2004;
Nosetto,~ et al., 2013; Yasuda et al., 2013; Toth et al., 2014).
Mechanism 3 could have massive effects on streamflow if a large fraction
of the El Morro catchment gets reforested. A simple comparison of the
trajectories of water table depths at site C (Figure 5) suggest that the
mean drop of 93 cm during three years in the forest (69 cm in the
croplands) would have get magnified to 196 cm if winter level recoveries
would not have taken place (something expectable in full forest coverage
scenario). The more precise estimate of groundwater depletion rates
derived from diurnal water table depth fluctuations suggests potential
depression of 4-5 meters per year (5 active at 3.1 cm~
d\textsuperscript{-1}, Figure 7). These rough estimates suggest that the
water table raises that expanded wetlands and created new streams could
revert in less than a decade with full reforestation. Paired catchment
experiments have shown how streams in dry regions can fully vanish
following massive afforestation, yet the relative weight of mechanism 1,
2 and 3 explaining these ecohydrological transitions was not
disentangled (Farley, Jobbágy, \& Jackson 2005; Jobbágy et al., 2013).
Mechanism 3 opens the possibility for opposing feedbacks, as hinted by
tree ring observations for the same \emph{Prosopis} species at another
site (Bogino \& Jobbágy, 2011), where raising water tables boosted tree
growth first (negative feedback), but caused their massive waterlogging
die-off afterwards (positive feedback). Croplands consumed groundwater
too at our catchment, but their shallow rooting depth limited the extent
of this effect (Figure 6, Supplementary material)(Nosetto et al., 2009).
Rather than assuming rigid and additive effects of vegetation types in
the catchment, the fact that mechanism 3 can compensate the relaxation
of mechanism 1 creates more opportunities for plants to take away water
from streams. The notable match between rooting depths and water table
depths globally (Fan et al., 2017), suggests that the dynamic
coexistence of these two mechanisms may be widespread.

While the previous discussion focused on the mechanisms by which plants
can take water away from streams, our study illustrated how streams had
the reciprocal effect on plants (Figure 1). By deepening their incisions
(\textbf{mechanism 4} , Figure 1), streams not only created the
hydraulic gradients needed to capture groundwater (comparison of
positions 1 to 3 in site C in Figure 5), but hampered mechanism 2
(wetland at site B in Figure 5, drying wetland in Figure 9) and, likely,
mechanism 3 (cropland at the same site, Figure 5), favoring their own
baseflow (positive feedback). While in the flat areas of the sedimentary
plains of Argentina raising groundwater recharge and levels lead to
flooding (Kuppel et al., 2015, Gimenez et al., 2020), the more tilted
and shallow sedimentary system of the El Morro has likely favored the
observed abrupt sapping process that tend to decoupled water table
levels from the surface, as they were in the 70s (Figure 4). An
approaching saturation of stream density is hinted by their regular
spacing and the observed episode of rapid ``stream piracy'' (Calvache \&
Viseras, 1997) where they may be too close (effect of the upper Río
Nuevo and Uke tributaries on the stream that they flanked, Figure 2b and
8). Theoretical approaches explaining the switch of groundwater surfaces
from topography- to recharge-dominated show that increases in recharge
can couple water tables to the surface (linear effect) while stream
length and depth growth can decouple them (quadratic and linear effect,
respectively) (Haitjema \& Mitchell-Bruker, 2005; Gleeson, Marklund,
Smith, \& Manning, 2011). A simple exercise with one of these models
suggests that the stream network is reestablishing a recharge-controlled
equilibrium in the La Guardia and Río Nuevo sub catchments, but is still
far in the Quebrachal sub catchment (Table 2, supplementary material).

The final mechanism by which streams can take away water from plants
involves the burial of riparian and wetland areas with fluvial sediments
(\textbf{mechanism 5} , Figure 1). In the El Morro catchment this
mechanism accounted for a relatively small fraction of the area (Table
2) but had a very strong effect on vegetation activity that lasted at
least 4 years (Figure 9). While likely negligible in terms of its
contribution to baseflow yields, this mechanism can create ideal
conditions for repeated sapping erosion events as it tends to sustain
saturated conditions along the present and future stream bed for longer
periods, increasing the chances of successive high precipitation years
accumulating their effects. Dry sedimentary regions experiencing deep
recharge and water flushing for the first time in geological scales may
have salinization as an additional mechanism that favors the partition
of water to streams. Increasing salinity in soils and groundwater has
the capacity to reduce or even eliminate completely plant transpiration
fluxes, as found already in the El Morro catchment (Jobbágy et al 2020)
and in deforested areas of the plains of Argentina and Australia
(Marchesini, Fernández, \& Jobbágy, 2013; Marchesini et al. 2017). While
the ``winner'' flux under salinization is direct evaporation, the
restriction of transpiration would hamper mechanisms 2 and 3 favoring
water table level raises and, as a result, sapping or surface processes
due to the reduced unsaturated thickness of the catchment.~ To some
extent natural ecosystems adjust to increasing salinity when
salt-tolerant species like \emph{Tamarix sp} . start dominating wetlands
(Rios, 2020) or where native forest relicts consume groundwater with
\textgreater{}20 dS/m of electric conductivity (Gimenez et al., 2016, at
site C in this study). Currently cultivated species in our study system
(soybean, maize, alfalfa) however do not show this adjustment capacity
(Ashraf \& Wu, 2011, Jobbágy et al., 2020).

\emph{Plants vs. streams: Many aces under their sleeves}

Far from following a simple predictable partition of rainfall inputs
into evapotranspiration and streamflow, the El Morro catchment has shown
over the last decades a rapidly switching balance between them. After an
early stage in which mechanism 1 has kept the system free of streamflow
(and streams), away from the expected water yield suggested by global
empirical models (Zhang et al., 2001), the expansion of cultivation,
aided by a relatively rainy period, has brought the system to a new
stage of higher groundwater storage. Saturated water consumption
(Mechanisms 2 and 3) in this new stage had increased evapotranspiration
locally but was unable to prevent stream formation (Mechanism 4) and the
onset of a stable water yield that approaches empirical global
predictions. Sapping erosion is still taking away water from plants in
the catchment, yet its long-term outcome is hard to predict. Climate
fluctuations, but more importantly, land use changes and spontaneous
vegetations shifts can twist it. Acknowledging the non-additive and
non-linear effects of vegetation and streams on groundwater is crucial
to develop more successful hydrological models for dry sedimentary
catchments. The peculiarities of the El Morro catchment may cast light
on overlooked (and not so peculiar) hydrological mechanisms.

\textbf{Acknowledgements}

Work in the El Morro catchment was only possible with the generous
collaboration of local farmers and ranchers. We are particularly
indebted to Alberto Panza; the Rizzati, Uke, Bosco, Rodriguez, Comelli
and Bridarolli families, and the late Coco Viotto who observed many of
the ongoing changes with a sharp and curious eye (they can be ``met'' at
the ``Río Nuevo'' documentary, available online). People at INTA,
particularly Claudio Saenz, Mario Galván and Juan Cruz Colazo; and at
the local government, specially Emiliano Colazo, helped us with their
field experience. At the GEA lab, George Castellanos, Silvina
Ballesteros, Raul Giménez, María Poca, and Tomás Milani provided
valuable help and insight in the office and the field. Leonardo Ríos
provided his geological experience in the area. Long field days with
Alfonsina Tripaldi, Steve Loheide, Jeff McDonnell, Steve Forman, Coco
Chiesa and Guillermo Ojeda were the source of many of the ideas
presented here. We deeply acknowledge Deolinda Correa´s guidance in the
first steps of this research plan. Grants from the Inter-American
Institute for Global Change Research (IAI) SGP-HW 056 (GovernAgua
Project), from FONCYT-Argentina (PICT2018-01216), CONICET (PUE 2016); as
well as support from the REM AQUA network and Gobierno de la Provincia
de San Luis; financed our work.



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\begin{longtable}[]{@{}lllllllllllll@{}}
\toprule
& & & & & stream configuration & stream configuration & stream
configuration & stream configuration & catchment definition criteria &
catchment definition criteria & catchment definition criteria &
catchment definition criteria\tabularnewline
\midrule
\endhead
& & base flow & base flow & base flow & horizontal & horizontal &
horizontal & vertical & topographic & topographic & geometric &
geometric\tabularnewline
Site ID & Location & mean & CV & summer drop & total & deposits &
wetlands & max. depth & area & yield & area & yield\tabularnewline
& & l s\textsuperscript{-1} & \% & \% & km & km & km & m &
km\textsuperscript{2} & mm y\textsuperscript{-1} & km\textsuperscript{2}
& mm y\textsuperscript{-1}\tabularnewline
\textbf{This study} & & & & & & & & & & & &\tabularnewline
Site 1 & Quebrachal - R33 & 55.1 & 34\% & -42\% & 38.1 & 6.5 & 13.4 & 8
& 414 & 4.2 & 227 & 7.7\tabularnewline
Site 2 & R\selectlanguage{ngerman}ío Nuevo - R33 & 214.3 & 13\% & -7\% & 16.3 & 0 & 0.7 & 10 &
121 & 55.8 & 42 & 159.4\tabularnewline
Site 3 & Uke West - R33 & 40.7 & 32\% & -50\% & 4 & 0 & 1.2 & 3 & 63 &
20.4 & 10 & 123.4\tabularnewline
Site 4 & R\selectlanguage{ngerman}ío Nuevo - R8 & 478.0 & 32\% & -31\% & 44.4 & 5.6 & 1.9 & 20 &
412 & 36.6 & 354 & 42.6\tabularnewline
Site 5 & La Guardia - R8 & 182.0 & 49\% & -43\% & 55.2 & 0 & 21.3 & 3 &
508 & 11.3 & 246 & 23.3\tabularnewline
Site 6 & Rio Nuevo - R7 & 647.9 & 34\% & -48\% & 104.1 & 5.6 & 23.2 & 20
& 1070 & 19.1 & 700 & 29.2\tabularnewline
\textbf{Segmented estimates} & & & & & & & & & & & &\tabularnewline
A, Site 4 - (Sites 2 + 3) & R\selectlanguage{ngerman}ío Nuevo mid-low & 246.3 & 48\% & -44\% &
24.1 & 5.6 & 0 & 20 & 228 & 34.1 & 301 & 25.8\tabularnewline
B, Site 6 - (Sites 4 + 5) & Rio Nuevo lowest & -46.2 & 338\% & .(70\%) &
4.6 & 0 & 0 & 1 & 150 & -9.7 & 99 & -14.7\tabularnewline
\textbf{Archive} & & & & & & & & & & & &\tabularnewline
Site 7 - 2008 (n=1) & R\selectlanguage{ngerman}ío Nuevo - dry branch & 46.2 & - & - & 3.4 & 0 &
1.7 & 0.5 & 90 & 16.2 & 9 & 164.8\tabularnewline
Site 8 - 2008-2009 (n=4) & Quebrachal & 136.5 & 25\% & - & 6.1 & 0 & 2.7
& 1 & 190 & 22.7 & 16 & 271.4\tabularnewline
Site 9 - 2010-2011 (n=12) & R\selectlanguage{ngerman}ío Nuevo & 364.0 & 18\% & 1\% & 14.2 & 0 &
3.1 & 9 & 255 & 45.0 & 118 & 97.2\tabularnewline
\bottomrule
\end{longtable}

Table 1. Synthesis of base flow and yield for the El Morro catchment.
The location of sites is presented in Figure 2b. For each stream base
flow mean, coefficient of variation of all observations and the
proportional flow decline during the warmest season (December to April)
compared to the rest of the year (May to November)~ is indicated as
``summer drop''. Segmented estimates are based on the subtraction of
nested sections of the catchment. All base flow estimates are based on
17 sampling dates obtained between November 2017 and August 2020 except
for archive data whose year and number of repeated measurements is
presented in the table. For each sub catchment or segment the total
length, corresponding to the linear extension of all continuous
permanent streams feeding it is shown. Length of deposits and wetlands
represents the extension of those streams intersecting those features
without prominent incision (\textgreater{} 1 m). The maximum incision
depth corresponds to present conditions. Catchment area and their
associated base yield were defined according to a purely topographic
rule following surface contributing areas to the sampling point and a
geometric rule considering constant supply width of 2.6 and 12.5 km in
the upper-mid and mid-lower catchment, respectively.

Table 2. Greenness trends derived from MODIS data for the El Morro
catchment. Vegetation was classified in 2018. For each vegetation type
mean NDVI across pixels was calculated each year and then mean and
standard deviation across years were obtained. NDVI shifts are described
based on the proportional change observed between the average values of
each pixel in 2000-2010 and 2011-2020 and by identifying and selecting
those which displayed significant long linear term trends
(p\textless{}0.05).\selectlanguage{english}
\begin{longtable}[]{@{}lllllllll@{}}
\toprule
& & & & & Positive linear trend & Positive linear trend & Negative
linear trend & Negative linear trend\tabularnewline
\midrule
\endhead
Vegetation Type & NDVI & NDVI & area & decadal NDVI shift & significant
pixels & slope & significant pixels & slope\tabularnewline
& mean & interannual SD & \% & \% change 2010s vs. 2000s & \% & units
10y-1 & \% pixels & units 10y-1\tabularnewline
Forest & 0.489 & 0.028 & 6.2 & 5.47 & \textbf{29.77} & 0.0427 & 0.25 &
NS\tabularnewline
Grassland \& Pasture & 0.412 & 0.020 & 41.6 & 3.49 & \textbf{11.46} &
0.0514 & 1.04 & NS\tabularnewline
Cropland & 0.388 & 0.019 & 50.7 & -1.12 & 4.39 & NS & \textbf{6.34} &
-0.0447\tabularnewline
Deposits & 0.351 & 0.032 & 1.5 & -6.70 & 0.25 & NS & \textbf{8.88} &
-0.0947\tabularnewline
\bottomrule
\end{longtable}

\textbf{Figure Legends}

Figure 1. Schematic representation of five mechanisms involved in the
partition of the water inputs received by catchments towards
transpiration and streamflow. The top panel shows three ways in which
plant uptake and transpiration takes away water from streams. The first
one (mechanism 1) involves the most acknowledged process of uptake from
the unsaturated zone. The second one (mechanism 2) incorporates uptake
from the saturated zone or the capillary fringe above it in focalized
wetland and riparian areas with a closer contact between plant roots and
water tables. The third one (mechanism 3) encompasses distributed uptake
from the saturated zone or the capillary fringe in the matrix of the
catchment. The second panel illustrates processes by which streams can
take away water from plants reducing transpiration and enhancing stream
flow. They include deepening streambeds (mechanism 4), which enhance
hydraulic gradients and limit plant access to water tables in low areas
or even in the matrix of the landscape; and burying riparian and wetland
vegetation with sediment deposits (mechanism 5) which temporarily
reduces transpiration in those highly transpiring zones.

Figure 2. The developing catchment of El Morro in Argentina. A. Map of
the catchment illustrating the location of the major sub catchments as
defined by their topographic limits, active permanent streams, and
potential drainage lines. Groundwater monitoring sites, which in some
cases include several wells, and meteorological stations are indicated.
The dotted white frame shows the limits associated with the schematic
view on the right. B. Scheme of new permanent streams indicating their
period of formation (also referencing vanished or dead-ended segments).
The area occupied by wetlands and sediment deposits is depicted together
with the location of streamflow measurement points corresponding to the
present study or to previously available records. The Quinto river is
the pre-existing permanent water course receiving the new streams with a
mean discharge of 5.5 m3 s\textsuperscript{-1}.

Figure 3. Decadal groundwater level trends in the El Morro basin. The
twenty seven sites are grouped according to their corresponding
elevation belt within the catchment (higher \textgreater{} 700 m,
intermediate 700-550 m, lower 550-500 m, terminal plain \textless{} 500
m). A zero level is assigned at the time of the first measurement in
order to highlight the absolute level shifts starting from that point.
Dotted lines illustrate periods and sites in which incisions were carved
less than 500 m away or where sediment was deposited right on the
location. Mean level changes per decade were calculated excluding the
incision/deposit situations.~ All sites correspond to water wells used
for ranching whose static level was recorded in the past. In 11 of them,
modern levels were measured directly in the same well or in a new ad-hoc
borehole whereas in the rest levels (always \textless{} 1.5 m) were
estimated by the presence of wetlands or lagoons.

Figure 4. Schematic transverse vertical cut of the Quebrachal and R\selectlanguage{ngerman}ío
Nuevo sub catchments. The transect corresponds to the transition between
the higher and intermediate belts (-33.464, -65.475 to -33.409 and
-65.305, see Figure 2). The surface of the terrain corresponds to the
central transect while belowground features were obtained using
observation less than 600 m away from the transect. Depth of the
crystalline basement comes from vertical electrical sounding (Barbeito
2008) and the old water table level from the long-term observation sites
(Figure 3). The new water table level is reconstructed based on those
same sites and landscape features like wetlands, lagoons and incisions.
An aerial photograph together with google earth imagery were used to
indicate the year of the advent of these landscape features.

Figure 5. Water table depth in 14 monitoring wells at the El Morro
catchment. Levels were manually measured in hand augered wells. The
locations of each pair or group of sites and their corresponding
vegetation and relevant neighboring features are shown accompanying each
plot. Dual numbering in the Y axes is color-coded like the markers in
the plot and used to capture depth offsets between wells using an
identical scaling across all plots.

Figure 6. Vegetation greenness, precipitation and water table depth
dynamics for two periods of 84 days with contrasting hydrological
conditions (recharge vs. discharge) at site A. Greenness is represented
by an 8-day MODIS product, precipitation was recorded at the site and
water table levels where measured with pressure transducers. Note the
color-coded numbering in the Y axis matching the line color of each
stand (green=wetland, black=5-day moving average in wetland,
red=cropland). Grey zones represent the subperiod zoomed at the bottom.

Figure 7. Vegetation greenness, precipitation and water table depth
dynamics for the whole study period in neighboring wells at site C
(cropland and forest at position 2). Greenness is represented by an
8-day MODIS product, precipitation was recorded at ``Coronel Alzogaray''
meteorological station, 15 km away, water table levels were measured
with pressure transducers. At the cropland an early gap on sensor data
was filled with manual measurements (red dotted line). Note the
color-coded numbering in the Y axis matching the line color of each
stand (black=forest, red=cropland). Light grey bands depict each growing
season while darker grey bands represent the subperiod zoomed at the
bottom.

Figure 8. Base flow at different locations within the El Morro
catchment. Grey areas depict isolated measurements before the continuing
monitoring program started. Site location and baseflow synthesis are
presented in Figure 2b and Table 1.

Figure 9. Greenness trends at sites with contrasting vegetation
types/trajectories in El Morro catchment. NDVI from Landsat imagery for
areas occupied since the beginning of the period by forests (n=11),
croplands (n=16), and wetlands (n=20) where compared with areas
experiencing the transition from croplands to wetlands (n=14) and
fluvial deposits (n=7). A single case of wetland drying (site B) is also
depicted. Alternate white and grey bars highlight successive growing
seasons and letters indicate significant differences within years based
on ANOVA. The lower panel shows mean monthly precipitation for the whole
catchment and its 12-month moving average. The erosion episode took
place in February 2015.

\textbf{Hosted file}

\verb`Table 1 - BASE FLOW AND YIELD SYNTHESIS.xlsx`  available at \url{https://authorea.com/users/12925/articles/483327-plants-vs-streams-their-groundwater-mediated-competition-at-el-morro-a-developing-catchment-in-the-dry-plains-of-argentina}

\textbf{Hosted file}

\verb`Table 2 - MODIS NDVI.xlsx`  available at \url{https://authorea.com/users/12925/articles/483327-plants-vs-streams-their-groundwater-mediated-competition-at-el-morro-a-developing-catchment-in-the-dry-plains-of-argentina}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-1---MECHANISMS/Figure-1---MECHANISMS}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-2---MAPS/Figure-2---MAPS}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-3---LONG-TERM-water-table/Figure-3---LONG-TERM-water-table}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-4---VERTICAL-CUT/Figure-4---VERTICAL-CUT}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-5---MANUAL-LEVEL/Figure-5---MANUAL-LEVEL}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-6---Sensor-levels-Site-A/Figure-6---Sensor-levels-Site-A}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-7---Sensor-levels-Site-C/Figure-7---Sensor-levels-Site-C}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-8---MANUAL-FLOW/Figure-8---MANUAL-FLOW}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-9----LOCAL-NDVI-2013-2020/Figure-9----LOCAL-NDVI-2013-2020}

\end{center}
\end{figure}

\selectlanguage{english}
\FloatBarrier
\end{document}

