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 (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 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 (mechanism 3 ,
Figure 1) was confirmed in the El Morro catchment. With water table
depths >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-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 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
(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
(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 Tamarix sp . start dominating wetlands
(Rios, 2020) or where native forest relicts consume groundwater with
>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).